Moving directly on from the last post at the end of January, I thought I’d share a little progress on the exhaust front.
Although I’ve been completely absorbed in doing engines for other people, I’ve been wanting to find some time to go back to basics and map the Gibsons made exhaust that I’ve been using on the Scott racer. Now this isn’t an easy thing to do completely accurately as the exhaust is shaped to wind its way through the frame, but I think I made a reasonable job of measuring it out.
I drew it up with CAD, but it’s actually easier to see on sketch up.
I know you can’t see all the detail but it’s interesting to see the lengths involved and tapers used. A couple of things are interesting, the stinger ID works out to be around 54% of the ID of the start of the header pipe. Now I don’t know much about expansion pipes except what I’ve read and been told, but the stinger ID is a critical dimension and very small dimensional changes can make noticeable differences in the effect of an exhaust. If too small a stinger, excess pressure can cause overheating in the engine. Now back at Cadwell in 2015 I was told that as a rule the percentage difference between the stinger and the header should be around 60%, which would make it ~29.8mm ID. That’s 2.8mm difference, which is not even that small. Also, there’s another thing: this is a 2 into 1 pipe, although I’ve only drawn a single pipe in the example. I’ve a Scott engine behind me on the bench with a similar exhaust duration as mine; mine is around 160° duration and the one behind me is around 157°. If this bike had a pipe for each cylinder, then it would have ~203° of crank rotation from the closure of the exhaust port to the re-opening on the next cycle. Since this is a 2 into 1 pipe, the exhaust has ~23° of crank rotation from the closure of one exhaust to the opening of the other. That’s around 11% of the time at any equivalent RPM or even if there’s a mathematical reason why that’s not completely true, it’s a lot less time to do the same job.
Given that the 60% guideline for the stinger is supposed to be based on a one pipe to one cylinder design, am I to extrapolate the time difference to apply to the cross sectional area of the stinger? Again, I’m sure the maths are well above my pay grade for that, but what’s obvious is that it’s about time and area, and most bikes rev faster than a Scott.
A quick googling leads to me think that an early 70’s TZ 250 would rev to 10,500 rpm, for example. Given that the tuning data from Bell would have been used on actual racing machines in that era, then it’s fair to assume that this was representative of the kind of engine speed that the 60% Stinger ID to Header ID rule of thumb, was worked out for. For sure the exhaust durations are different too, but I need to look at this to make a guess at what the stinger ID should be.
Also, having been able to establish the design and measurements of the Gibson pipe, I found that it’s actually made for an engine that revs quite a bit higher than mine.. like my dad’s! (although it’s even on the high side for that. Working backwards, the dimension between the piston face and the mid point in the deflector cone is 1244.5mm, which according to Bell makes it tuned for a whisker under 5500 rpm. Now Roger’s engine, with his substantially balanced four bearing crank will spin up past 5500rpm, but that’s not a place he likes to ride it anyway. Dobsy used to rev it hard, but even my dads engine is getting a little out of breath at that point.. just a limit to what the ports can flow, even with the differences on his engine. My calculations would suggest that I need a tuned length of around 1530mm for 4450rpm. 5000rpm is about right for max revs, but as I’ve said before, I just don’t think it can breath properly up there. Thats 285mm longer, not insubstantial in terms of getting it to fit on the bike. We might be looking at the muffler the other side… I know that’s been done.
So I bought Bell’s two stroke tuning book as well as John Robinson’s to go with my copy of Jennings and have been working through some possible re-designs. I purchased a TIG welder a few weeks ago and so that’s another step in the right direction. I think hydroforming would be the best way of testing some designs.. the most fun you can have with a pressure washer as far as I can see!
I set this site up mainly to provide a platform to share some of my experiences and endeavours in developing the engine of my racing 1932 Scott Flying Squirrel.
The simple reason that I’ve not been updating is that I’ve not been working on my bike much or racing it either.
I did have a rather memorable meeting at Cadwell park in June, but not for the right reasons.
I have not given up on petrol or AVGAS (as I have have most recently been using) but my experiences with seizures in 2015 made me realise that I had more development work to do before I could confidently go to a racetrack again using that fuel. I have my suspicions that the diameter of either the ‘stinger’ or the muffler outlet pipe on the exhaust is inadequately sized and that excess pressure build up is creating heat at sustained higher revs. I’m not convinced that the exhaust is actually the optimum design anyway (Supposed to be a direct copy of Roger’s, but Gibsons didn’t make it with quite the same dimensions), but it certainly goes well with it on methanol and I simply didn’t have the time (or the TIG) to make test modifications at that (or this) point. However, as I’m sure I reflected on at the time, the problem may be something else entirely or indeed a combination of factors. Certainly, the seizures were not something I managed to replicate on a dyno during short output-measuring runs, although I since discovered that the dyno can indeed be set up to provide a constant load condition to replicate the conditions experienced when flat out for a period of time. It’s not really about measuring power but simply observing what is happening. To make the most of a morning on the dyno doing this, I would need to get some datalogging going to be able to really study the effect. I have thermocouple ports in the headers but I could also do with the temperature in the water jacket being monitored as well. I can switch exhausts to a standard straight-through pipe to make comparisons.The radiator is only small on my bike and may be a significant factor as well. It would likely be a couple of hundred quid for a morning on the dyno so I would have to make it count.
So, with the petrol incarnation of my engine needing time to work through the options, I thought I would return to good old methanol to give me some limited riding in 2016.
I was still hoping to try to squeeze more power out of my engine, and had the combustion chambers to my cylinder head welded up to enable a new profile to be tried. I didn’t have much time for the work on this and Roger kindly did some machining on it for me. He checked the head for flatness, which he thought was fine and I built it up.
The premise is simple. The Scott engine is limited to fairly low revs by several design constraints; you simply have to make the most out of the available range, which is effectively up to ~5000rpm (with peak torque somewhere between 3000 and 4000 rpm). I had done some development work with the cylinder head previously and seen on the dyno the effects of lowering the compression whilst improving the combustion chamber shape; I lost low rev torque whilst the engine revved to higher RPMs. If the engine was capable of breathing at higher revs, then an increase in power may have been the effect but as (according to my calculations and observations) the ports restrict the gas flow required for anything above about 3750rpm, then there’s just no point holding on.
When you also take into account the three speed box the task as I see it is to maximise low rev torque and discount the pursuit of revs entirely.
Now I would like to say at this point that a good strong engine can be made to run very reliably on petrol (of course!) and I raced it without mechanical problems for years like this (with a far less radical pipe, prior to fitting the expansion chamber I received as a wedding present from Roger!) but it is at this point where you are trying to experiment where you have to accept that you might hit trouble. I have built many engines for other people and I wouldn’t build an engine for anyone else with anything but tried and tested clearances and configurations, as we know that these work and work well but I have always wanted to keep experimenting with my own Scott engines and in doing so you commit to the highs and lows associated with that development process.
The Cadwell park meeting in June was a big event. An anniversary meeting for the British Historic Racing Club and also the Scott Owners Club, which had a stand in attendance.
The Friday had given people the opportunity to ride around the circuit on their road bikes, with a full weekends racing ahead.
I had returned to methanol for the meeting, thinking that was the safest option as I had never had problems running on methanol before. I was also quite excited at the prospect of the improvement I may find with the new cylinder head design. That excitement was short lived however as the head gasket was obviously not sealing and the water was being blown out the radiator expansion pipe.
I will not make a long story out of this weekend as it simply consisted of various attempts to seal the head (with associated peaks of hope and troughs of disappointment) aided by several kind and enthusiastic helpers. Kev Bayliss and Alan Noakes deserve a special mention.. many thanks.
Unfortunately, It was not to be. In that moment, having hired a caravan and being accompanied by my wonderful wife and two small children and being surrounded by so many Scott owners who, I’m sure, would have loved to have seen Roger and I trying our best in the vintage races, I was left with the feeling that I simply had to re-assess my approach to racing given the available time and finances available.
I never wanted to simply ride around, and when you feel like you could get more from your engine, it’s hard not to succumb to the urge to tinker. You must have the time to see it through though.
My main plan for 2017 is to finish the rebuild of my lathe and build a workshop to house my other machines; a Thiel 158 universal mill and most recently acquired, my Jones and Shipman 1014 grinder, which is a true universal toolroom machine with surface grinding, cylindrical internal and external as well as tool and cutter grinding capabilities. These are currently in storage and I need to stop playing with bikes for a while and make them a home. This has to be a priority above racing and bike development. I will rebuild the engine but I may do it as a good road engine to run with a normal pipe and finally register it for the road. I’m not certain about it, but I could possibly do some local events, hill climbs, sprints etc and enjoy it in a different way whilst building up the workshop. We’ll see but I quite like the idea. Racing is very expensive and there’s a lot of tooling I can get for the price of a two day meeting.
I have also been rebuilding a couple of engines for other people and it’s obvious that I need to have better facilities in place if I am to continue doing this within any reasonable time-frame as I’ve had to rely too heavily on Roger fitting my work into his already packed work schedule. It’s been a lesson which has contributed to my resolve that this year must be the year of the workshop.
So it may be that in 2017 my posts are less racing oriented. I’m still working on Scott engines anyway so I’ll post about that as well as my machines and any related progress.
We’ll see what happens…
Happy New Year to all who visit and good Scotting!
Roger went over to the continent a number of times during the 1970’s, racing his Scott. On this early foray his Scott was pretty much as it is now, having been rebuilt by me around 10 years ago. By the late 70’s he was thick into development and had commissioned a new frame, tank and radiator and this is where his iconic current racing machine came into existence. The pictures are from Montlery, which was a great experience. One of the French racing contingent was a man called Christian Olivaux, who invited us all (the whole family went) to his apartment in Paris to stay, which we duly did on our return.
Many years have passed since this time, but my dad was sent a picture recently by Christian from Montlery in 1975. He remembers coming third in the race. Others in the picture are ( I think): Geoff Pollard (second in the race on his 1939 Tiger 100) and his son Derek (or Bill.. one was a nickname), Fred Ellis, Tim Maton (won on a Vincent twin). If anyone remembers anyone else let me know…
The picture of the group of them together is from Christian, the others are from the same meeting that Roger had already.
Happy Memories indeed.
It’s been a quiet summer on the Scott racing front. All kinds of responsibilities and activities fill a Summer when you’ve got two very small children and I must admit to at least attempting to lead a balanced life, especially when the sun is out. Also, doing a bit of Scott work for other people does mean that I’m spending time on their engines rather than my own! It’s all good though and I was looking forward to getting to the September meeting at Cadwell park as this is my favourite meeting of all.
The Flying Squirrel (remember, it’s a Flying Squirrel and not a Super Squirrel as I discovered this year..) was still on the bench looking a little forlorn at the beginning of September. I had removed the top end after the seizures at Anglesey because I wanted to check the pistons and bores. The seizures had occurred on both pistons at the rear corners and the left hand piston was a little distorted because of it. I spent quite a bit of time filing the damage out carefully. The bores were fine, although I flex-honed them to freshen them up a bit.
The last time I had assembled the engine, I used a gasket compound called ‘three bond’ which has good gap filling properties and remains flexible. I used a tiny smear on various surfaces that I had scraped flat, just to ensure a good seal. I had a hell of a job getting things apart and in fact the bond was so good that it pulled the devcon epoxy out of some corrosion damage on one of the carburettor flange faces. There’s such a thing as too good! I think I’ll use it where a face isn’t good, but back to a silicone sealant where it’s needed.
So, with the engine back together and the repaired radiator back in (thank you again, Graham Moag), the bike just needed a few bits sorting out to get it ready. The front mudguard needed to be replaced and the L/H footrest needed straightening. A few other little bits as well.
Inevitably, some of these things ended up being left to the last minute, which was OK since I’d arranged to have the day free before I went to Cadwell just to make sure everything was finished. Unfortunately, my wife had a work deadline which meant that I had to look after our children on this day. This was an error in my calculations! Instead of an unhurried day and early afternoon departure to Cadwell, I frenetically worked on the bike from around 5pm till 9.00pm on the Friday night, then packed the van and went to bed at 10.30pm. At 1.00am I arose, drank coffee and set off to Cadwell… over 6 hours away.
The drive wasn’t so bad and the morning was crisp, with a low mist that hugged the warm ground. It was obvious that it would clear and clear it did. It was indeed a beautiful morning welcome from Cadwell Park.
I arrived pretty much as people were starting to get up and got the bike down to Scrutineering early. No problems here and I signed on and started to check the bike over myself.
Practice for solos was called and the bike started immediately. The twin carbs breathing without bellmouths because I knew I needed to make some that actually flowed properly and I hadn’t done testing to base a length on. I figured it would do at this point.
The Scott seemed willing at low revs although I was concentrating on the clutch, given the seizures it had experienced at Anglesey. Sure enough, at the bottom of the park straight it came. I was quick to get the clutch in and coasted to the marshall’s post to wait for the end of the session and the recovery truck.
Back in the paddock I started to go through the fuel system. A gentleman called Peter, who was interested in the Scott, was kind enough to help as I went through the fuel system checking the flows through the taps, lines and banjos.
It’s dangerous to assume but sometimes you forget when you have assumed. I had the bike on the dyno at the beginning of the year and had thought that this would show up any problems with fuel supply. It wasn’t that I was expecting it to give problems.. and it didn’t. I’d made sure that the fuel lines were all of a descent size and had put the lot together being as careful as possible to avoid flow related issues. The dyno runs were completed without the suspicion of a seizure, so I hadn’t thought there was likely to be a problem.
I did flow tests all the way to the banjos that fit to the bottom of the carbs and was surprised at the results. They were barely able to flow the potential of the main jet. With my new found assistant’s help, I drilled every fitting out and found that I’d increased flow by a couple of hundred cc’s per minute to each carb. Part of the flow improvement was that I removed a connector pipe between the feeds to the two float chambers from my twin taps. I’d thought that this was was a good idea in case one tap became blocked but in effect the T- piece connectors were just another cause of a pressure drop. Amal main jets are rated by cc’s per minute flow (albeit under certain controlled conditions that I wasn’t trying to emulate) but at least now I had almost double the flow specified by each jet. I thought that should cover that as a possibility of seizure.
I was therefore hopeful when I got out in my first race, though aware that I still may be missing something.
It seized at the end of park straight on about the second lap. Bugger.
I started to think about the history of this problem. Roger had presented my future wife and myself with the exhaust as a wedding present in 2011. She was overjoyed of course :). It had been made by Gibsons in the south east and they’d had the bike to fit it to properly. They hadn’t run any calculations but had copied my dad’s pipe which he’d supplied them with for that purpose.
I went to the Prescott hill climb early in 2012 as the first outing with the bike and .. guess what.. it had seized at the first corner. It sound so hard edged and got so hot that I’d ended up fitting another head gasket at the next outing (BHR meeting at Lydden) simply to get it to stop overheating. It didn’t overheat (though it still got very hot) but it was slow. I thought perhaps that all this was a sign that if I wanted the power, it was a case that I’d need to make the engine dissipate heat better. Maybe my radiator, which is very small, was incapable or that my block (cast iron) was too slow in getting that heat away. It started to look very expensive.
That was why I switched to methanol in the first place…that I thought it might provide me with an easy alternative.
Methanol had worked, but was there another problem that it was hiding?
I had been wondering somewhere of course whether the pipe was in fact unsuitable for some reason. Since none of the problems existed before the pipe, it seems like a clear possibility. I talked to Rex Caunt (BSA Bantam tuner) on the Sunday and told him that I wondered whether the stinger outlet was too small and that it was the exhausts inability to get rid of the pressure quick enough that was causing the heat build up. He gave me a ‘rule of thumb’ to work out whether this was the case.
Apparently, the inside diameter of the ‘stinger’ pipe outlet is normally around 60% to 62% of the inside diameter of the beginning of the header pipe (around 50mm).That would make it around 30 to 31mm ID. I couldn’t get to the stinger, but I could measure the silencer outlet and it was around 26.8mm. That’s under 54% of the ID.
When I start to think about it more, I think there’s another reason why it might need to be bigger. It’s a 2 into 1 pipe and although the operation of the phases are separated, there’s not the time to dissipate pressure that you get with a normal 1 pipe for 1 cylinder operation. In fact it makes sense to me that the outlet should be bigger than a standard pipe for this.
So in terms of racing it was a terrible weekend but in terms of development, I feel happy that I’ve got some new direction for the off season. I’m going back to basics with the fuel system and the exhaust and maybe even make some experimental pipes simply for the dyno. I also have started to realise what the dyno may or may not be useful for. I’ll still use the dyno to show what relative power I’ve achieved but I wont assume that it’s all I need for testing.
I’ve got lots of reasons to be excited about this winter. Roger finally took his bike to Motoliner in Maidstone to have the frame and forks checked and straightened after Steve Plater’s crash at the beginning of the year. It will be good to see that make progress.
The Moss Silk Scott needs to start moving to the next level… a dry build to see what kind of tank and seat unit it’s going to need and where the exhausts can go. I really look forward to moving forward with this.
I’ve got my lathe to finish scraping/ re-building, my milling machine to re-commission and tool up for and various other engines and gearboxes to work on in the meantime.
I still have a dream of making my Flying Squirrel the machine to beat in the vintage class and will be yet again edging toward that over the winter. I can but try!
Anglesey is a long way from Devon. This became more and more obvious over my 6 1/2 hour traffic-jam-filled journey to the circuit on an inclement Friday afternoon.
Roger had cancelled because of the flu. He sounded dreadful and had done for some time. His voice had become more and more hoarse and he couldn’t sleep for coughing, propped up on the sofa was the only way he could get any rest. His lack of spleen doesn’t facilitate recovery from viruses as far as I understand, being an important part of the body’s immune response. It has to be bad to miss a race meeting too.
So it was that I found myself erecting my tiny backpacking tent in what I understand now to be good Anglesey conditions: unrelenting high winds but without rain.
The wind is something else. I had heard about it but now I can really imagine what it could be like. The circuit is right on the Atlantic and the drive through Anglesey itself starts to give clues to the general conditions. A lack of trees and the prevalence of very gnarled bushes with what scant tenacious foliage they had swept at an acute angle an indication of life on this coastline.
Saturday was clear and dry. I went through Scrutineering and signed on.
Practice gave me my first taste of what was to come. When you rounded ‘Church’ corner in the middle of the long back straight you hit a wall of wind which made me think that the bike was seizing up. Then, on the third lap, it did seize up, though I caught it pretty quickly and rode back though to the paddock.
I took the needle up a notch and got ready for the first race.
My first race was the ‘up to 63 unlimited’ which is my second choice event to give me another couple of rides. It’s almost collapsed as an event at the moment and there was only one other guy entered in this on a BSA twin. I don’t know what happened but I guess the people have all entered different bikes in different classes or simply stopped. The ‘up to 48’ class is better supported. It’s not a big deal as long as you are sharing the grid with a mixture of other classes and so there were plenty on the grid. All far too powerful for me, but I was starting on the back anyway just to see how it would run.
It was better, but it still nipped up accelerating out of Church corner into the back straight. I nursed it round and went back to the van.
A Scott owner, Bernie Dunmore, offered his assistance. We’d never met before but he spent the following few hours helping me get to the bottom of the seizure issue. It was puzzling because it was theoretically running richer than the optimum needle setting chosen on the dyno, and they seemed to think that I’d be running rich in the real world even at that setting. The wind was a considerable force though and maybe that was causing the engine to have to work so much harder that the engine was getting too hot? The timing had changed also, possibly jumping a tooth on the slightly-under-tensioned belt when it nipped up… the momentum of the rotor forcing the jump. I checked the fuel flow to the carbs and thought I noticed the fuel flow to one of them increase as I moved the feed pipe. Was it possibly an air lock in the flexible hoses? In plumbing, air locks can cause all kinds of problems. Maybe it was this? For good measure, I thought I’d go up in the heat range on my plug. Ken Inwood was there and I bought a couple of NGK’s, heat range 8. That’s two up from where i was. It shouldn’t be this but I needed to cover all the bases.
By this time it was the end of the day with one race left… I got out and it didn’t seize up. It was obvious that the torque I’d lost at the start of my rev range since changing from Methanol was causing me a problem as the engine wasn’t able to pick up in third when shifting up from second at 5000rpm as I rounded Church corner into the wind. It was bogging down and I was losing a lot of time here. I started to learn the track at last though and really enjoyed this last race of the day. The rear tyre, already pretty worn out by the end of last year, danced its last dance and I could feel it drifting though ‘Rocket’ but mostly through ‘Peel’ which is before the drop down the hill toward the left that leads into the corkscrew. I really enjoy racing tyres when they are finished… you can’t really carry speed because they’re not gripping any more but you can feel them drifting far earlier and it’s great fun. Fortunately, I’d arranged for Roger to send me up a spare with another competitor (as well as a new final drive chain from ‘The Chain Man’, Andy Forsdick)
I changed the gearing, just by a tooth on the back, to try and get into third before I hit the wind and then I changed the tyre. Unfortunately I ended up pinching the tube so it was a case of waiting for Ken to return in the morning.
I made a sandwich for my tea, and crawled into my tent.
Sunday morning. Overcast and windy.
I was first to get to Ken and he changed my tube, very graciously not mocking me for making a mess of doing it by myself. He did note however that the tube had been too big for the 90/90 profile of the Avon roadrider, undoubtedly a legacy from the days when we were running 3.25 section GP’s.
I think I had a race first thing, which was the big class. Again I just started from the back and just scrubbed my tyre in, taking it steady.
Things were better but still it was a struggle to pick up on the back straight. I resolved to just do the best I could, and investigate further when I returned home.
The next race came around, which was my ‘up to 48’ class. I was feeling a bit more like it by now and though I didn’t get a great start, I was suddenly at the back of some of the ‘up to 1983 Japanese 500’ class with whom we share the grid going into the ‘Banking’, the exit of which leads into the first part of the straight that leads to Church. The brakes are good on the Super Squirrel and I passed a couple on the brakes and got through the corner well. People were passing me down the straight at a fair clip, but it did seem that I could make ground going into Church and then also at the end of the straight into the very tight left hander. It was like this for a couple of laps, where I was hauling people in from this corner to the banking and then simply losing it as bikes came past me on the straight like I was standing still. I was especially hunting an RD 400 and a CB 450, I never got to the 450, but the RD 400 was incredibly quick on the straight and went past like I was standing still. I really pushed to keep getting past him on the rest of the circuit and in the end pushed too hard, losing the front end into the tight left at the end of the straight. I saw him later and he said that he’d thought I’d never get round after I’d outbraked him on the inside going into this corner. I said that I couldn’t argue with him because I didn’t make it! My fastest lap was quicker than the 5th placed Japanese class guy (on the CB450)but I would still have been second in my class to Tony Perkin on his Rudge 500 who was 2 1/2 seconds per lap faster than me… absolutely flying.
The radiator was damaged, the front mudguard and the right hand footrest but little else. The radiator was unfortunate as it’s never taken a hit before but sometimes these things just don’t fall in your favour. You have to push when you are trying to race an RD 400 on a vintage Scott.
In all, I really enjoyed Anglesey but left realising that I still had a way to go with the new set-up. I think I need more compression, and some more development to iron out the problems with the fuel system. I was due to take the bike to Roger’s in Leicestershire in readiness to take both bikes to Cadwell Park on Tuesday so that Steve Plater could do some testing. I thought maybe if I got home to Devon I could sort out a repair on Monday morning and drive up to Cadwell on Monday evening to be there for the test.
It would be a lot of driving but I thought I could do it. I said my farewell to Anglesey, resolving to return and headed home.
I really enjoyed taking the Scott to the dyno yesterday. It was the first time I’d ever actually booked a proper set-up rather than a single run. I suppose cost was always an issue but I have realised how useful it is to be able to see the results of the changes you make and a proper session allows you to experiment with many different settings and see how each affects the result. It’s a bloody good way to see what affects your power output, and how. The cost is similar to me going to a trackday, and there’s no way I’d have gained the same information from that.
Of course, you need to have someone who can run the dyno and can interpret results. GT motorcycles of Plymouth have a very interesting division called ‘Performance Engineering‘ that specialise in rebuilding, restoring and tuning Honda racing bikes, and it’s these guys that run the dyno. Steve is highly knowledgeable and experienced in getting the most from an engine on the dyno. I’ve done my previous runs with him, but yesterday he was able to really show me the benefits of a dyno setup. Steves colleague Andy, another two stroke fanatic, was assisting and as the Scott’s engine note became crisper and cleaner they were both obviously enjoying the experience. Ok, so the Scott doesn’t put out the sort of horsepower as an NSR500, but it did sound so very good .. and maybe the 4.5% Castor mixture helps too!
Anyway, I realise that for many people the idea of going on a dyno seems a bit extreme but carburation and ignition set-up problems can persist for years if you don’t seize the opportunity to really get to the bottom of them. I don’t want to spend my time at a race meeting or a track day like the Beezumph constantly messing around with jets , needles and timing just to try and get somewhere close. You’ll always have a certain amount of tweaking.. but at least I feel like I am quite close with what was a completely unknown arrangement.
So, What happened?
I’d fitted the 1″ carbs with 320 main jets for a start. The needle was on the bottom groove, so all the way up. I’d figured that it might be rich but I had no desire to start from the other direction! The timing started at 22BTDC.
It was rich. Our runs followed the following order:
1: down two mainjet sizes to 300
2: down two mainjet sizes to 280
3: drop needle one notch
4: drop needle another notch
5: drop mainjet one size
6: raise mainjet one size
7: change timing to 23.5
You can see the graphs here for the final results compared to the very good curve I produced when I first set up the bike up on methanol.
The exhaust temperature sensors were really useful and we were able to monitor for significant changes with any modifications to setup that we made. The final high was around 530°C. Always increasing by about 30°C to 40°C the moment you shut the throttle off… interesting stuff.
You can see that the Torque curve starts later and then increases at a very high rate. It may be that this lost section (around 400 revs) at the beginning is retrievable by playing with cutaways, but the gain in midrange torque is very good indeed. That may be down to the improved gas flow in the head. It loses to the methanol later in the rev range, about 4000rpm. The methanol torque stays high all the way to 5000rpm where it suddenly falls off a cliff. The new curve hits a higher torque figure but tails off more noticeably. However, the fact that suddenly I started to have more revs was a bonus (as long as I can get the 400 back at the beginning!). I think Steve’s attitude was that the extra ‘over rev’ was extremely useful as the engine was able to keep pulling longer because of it.
As far as I understand, the fact that I increased the head volume, lowering the compression ratio might have created the conditions to encourage the engine to keep revving.
As the torque was lower late in the new curve the maximum power is less than before, but I’m not too concerned about that at the moment. It might be that I could do with the shape that I have created in the combustion chambers, but with a high compression ratio. It would be worth experimenting with heads sometime.. but not now. It may even be that the carb size is a little small, and I’ve never done any further tests with the exhausts but short of changing the slides to try a higher cutaway I’m now looking at actually using it! It felt really smooth and sounded great.
So, this afternoon I posted off my entry to the British Historic racing clubs second meeting of the year at Anglesey on the weekend of the 16th and 17th May. Roger is going too. I’ve never ridden Anglesey before but I’m really looking forward to it.
One of my main aims for this year for my ’32 Scott racer was to set up the carburation properly for petrol again, having decided to move away from Methanol. Methanol worked really well, and the power characteristics really suited the three speed box. The bike was fast and really responsive to ride with the single overbored Amal type 89, but I worked out that the venturi area was less than optimum and resolved to improve that.
With the single carb, the shape and position of the torque curve was pretty much exactly what I’d expected based on the calculations I’d made using Gordon Jennings formulas relating to port time/area relationships. Basically, the you need a certain area of port available to enable adequate gas flow at a certain revs and that area increases as the revs increase as the gas has less time to pass through it. Nothing I have done over this winter should change that torque curve shape too much, as I’ve not changed port timings. Changes in exhaust pulse characteristics relating to the increase in temperature anticipated with the change to petrol will make a difference, and quite possibly not a beneficial one but we’ll see. I do expect that the new fuel system, using a pair of 1″ Amal type 76 carbs on a vee type manifold, should keep that curve from tailing off quite when it does at the moment. I spent hours setting up the float heights and chamber positions so I hope it works. The extra breathing coupled with an attempt to tune the inlet lengths to work better with the slightly extended inlet timing duration I have on this engine should in theory pay off. With the single carb, you could see an inch or so ‘stand off’ of mixture blown out (and sucked back) at low revs. The longer tracts will help eliminate that. I have also spent a significant time working on the head profile to try to allow a more direct route for the flame front to move to the extremities of the combustion chamber but there’s a fair bit of finger in the air stuff… with a bit of borrowed knowledge and the rest; ideas formed through slow but incremental observation of what has already happened.
So, with a completely new fuel system, I need testing to get it right. The most straightforward way for me to do this is by getting onto the dyno that I’ve been using for my other tests so far. This time though, not to do a run but to do a full set up.
So this week, I’ve booked a morning at Alan Jeffery’s dyno in Plymouth, run by Steve from GT motorcycles who seems to spend most of his time tuning NSR 500 Honda GP bikes for people all over the place. A good man to have on-side.
I’ve tried to make sure that I’ll be ready to make the most of it. Roger sent me his block of main jets, so I’ve got changes to make. I’ve fitted the exhaust temperature sensors to enable me to use that information to help establish whether we are too lean. I’ve got a couple of plugs from a different heat range and on top of that… I also picked up some AVGAS as I’ve been convinced of the advantages of this over even the higher octane rated unleaded from the forecourt.
I’ve also ordered a radiator hose connector which allows the fitting of an 1/8″ temperature sensor and at some point I’d like to fit one of these to go with my Scitsu rev counter which (although it probably needs servicing) stopped working altogether when I switched to methanol.
Hopefully, I’ll get a better result that the first dyno run I did last year. That gave a maximum torque figure of around 38ft/lbs at near to 4000 rpm and a maximum horsepower of 33 at 5000rpm.
I really want to get out to do more racing this year and I really want it to be competitive.
After I decided to return to running petrol in the Super Squirrel racer, I thought that I’d re-profile the cylinder head ‘squish’ chamber, given that the compression was probably on the high side and I really wanted to concentrate on trying to create a head shape which made it easier for the flame front to spread. The old shape worked well on methanol but I know that I am more likely to suffer from detonation with petrol and so I thought it prudent to make some changes accordingly. I also asked Alan if he’d braze a couple of fixing points onto my exhaust headers, to allow the installation of exhaust temperature sensors as this information may be useful in determining the correct jetting.It will be interesting to note the changes anyway. I’m also intending to monitor the temperature underneath the spark plugs, to see what that tells me. I’d like to monitor coolant temperature also but I’ve not yet picked up a sensor for that. Whether any of this stuff gives me absolutely necessary information is questionable, but it will certainly be interesting.
I’ve also been working on the detail of the new improved twin carb set-up. I’ve not actually set them up(!) but I am almost at the point when I can put it all together. I’ve spend quite a lot of time matching the carbs to the inlet manifold and also the flow going in. I also have to clean the jet block fuel and air apertures and work out whether I need to increase the size of the pilot jet apertures. I’m not bothered about tickover of course, but the pilot is the only source of fuel when the throttle is shut and I wonder whether, at the end of a long straight, this is necessary for cooling the piston. Maybe it’s not, but nip ups quite often happen when you shut the throttle and it strikes me that this might be related. At any rate, the carbs were orignally destined for something like a 350 single running a one hundred and something main jet. I’m going to be running considerably more than that so I’m surmising that the pilot jet should be appropriately modified too.
I’m also planning to try different lengths of intake tract for the carbs as I extended the intake timing duration a little and it will benefit from some length and velocity on the inlet.
Otherwise, I have to do a lot of cleaning of gritty parts, and then get it back together for testing.
I emailed Colin a couple of days ago to ask him whether he knew of anyone who had cut the bridges out of the transfer successfully and he sent me this. Thanks Colin.
“Just a little more about the pictures. These are the barrels used by Martin Heath in that one glorious season when he won 12 events, including his first win. This was at Cadwell when he was left on the grid as the compression was so high the rear wheel just slid instead of turning the engine over, on the old downhill start by the timing/ commentary box. He managed to start it eventually by vaulting on from the greatest possible height and it fired up. The field were disappearing into Charlies by now, but he caught and passed them all to win. Proper Boys Own Paper stuff.
However, the point is that the barrel, as we had no other, was reclaimed by boring out the broken skirt from a damaged set and pressing in a flanged liner from the top. ( It sat in a machined recess). The machining had broken through into the jacket so good old Loctite sealed it all up and provided what I think is called a ‘wet liner’. We could not bring ourselves to put dividing bars back in the ports so made them elliptical as you see and relieved them to give the rings a softer time. You will also see that we needed to use a ‘detachable’ steel ring for the lower seal – and this located on a small step machined on the liner o/d.
Another simultaneous experiment was to make the liner full length down to the very bottom of the crankwell to see whether preventing any piston ‘rock’ would help. You can see the remains of this in the pics after it was later cut off. The rods had to be scalloped to clear the base of the liner, but we still use them to this day with no trouble.
The heads are a type you are familiar with and the drilled/tapped holes onder the dome on the exhaust side were so that we could use a head steady onto a frame cross tube. ( This was our super lightweight T45 frame, it was more of a frame steady than an engine steady. It was so light it sang like a tuning fork even after the engine was cut).”
I also sent him the pictures of my piston for his interest and to get his thoughts.
“As for the fuel pattern on the piston crown, by using a close profile high compression arrangement I expected that any ‘clean’ area on the side was trapped fuel ‘end gasses’ that got rudely pushed/ sucked out of the way before they had a chance to detonate.”
and of the photo of Martin:
I think it was taken at Mallory but expect Martin will correct me if necessary. Incidentally the silencer shown was our first effort after introduction of silencing regulations. This one is designed on the principle of ‘silencing by controlled leakage’ whereby multiple small outlets are provided under the crankcase all carefully pointed in different directions. From memory this set up has five intentional outlets including the official one. The theory is that one noise meter will have difficulty covering all directions at once. It worked well enough, for we would have been excluded on open pipes, and to my surprise the machine seemed, subjectively, to go just a little better than before.
The virtual oily corkboard of a vintage motorcycle racing family