July 2015 : an update

I’ve not written much recently but there’s plenty been going on.

My racer is still on the stand, although I now have a new front aluminium mudguard to replace the one damaged in the lowside crash at Anglesey earlier in the year. I also set about the paintwork on the headstock to try and find trace of a frame number, which would help me to register it for the road if I so desired. I found it in the end (but not before I’d removed a decent amount of perfectly decent paint) and amusingly it confirmed that the frame was never a Super Squirrel frame, but a 1932 Flying Squirrel. I have actually called it a Flying Squirrel in the past (and a Sprint Special when I was very young and simply wanted it to be one…) but the idea that it was a Super Squirrel stuck for some reason and I can’t even remember why. It’s not even that the engine type was the same.
So from this moment on I shall call my noble steed by its rightful title: My Flying Squirrel racer.

Although Anglesey was a while ago now, one of the other things that happened at the time was that my dad’s bike was due to be tested the following Tuesday for Classic racer magazine by Steve Plater, a former motorcycle racer and TT winner. I made the journey to Cadwell the day after getting back from Anglesey and we were very interested to see how he got on and what he made of the bike. He’s used to modern machines and I don’t think he’d ever ridden anything like it before. My dad advised caution through Charlies because of it’s tendency to get out of shape on the exit, but apart from that let him work it out himself.
We changed the bars to give him a different position and he seemed to be gaining confidence quite quickly through hall bends, where we were watching. He was certainly moving. However, maybe the confidence was a little premature as he lost it out of Charlies as a result of a tank-slapper that he couldn’t control.
Noticing that he hadn’t come round for another lap, I feared the worst and ran up to the van just in time to intercept the recovery vehicle. I took a deep breath when they opened the back door as the steel Vincent straight handlebars were bent vertically both sides, like bulls horns. I could see the top fork links had bent significantly before I even got it on the stand, and by the time Steve told me that it had gone over a couple of times I already had a mental picture of what, in all honesty, was the worst racing incident it had ever endured in over thirty years.
Of course, we were all relieved that Steve was ok. It could have been very nasty for him. On reflection, I think we were naive to think that even a highly successful professional modern rider might just sit on something with as lowly relative performance as my dad’s Scott and be able to work it out easily. Riding a rigid bike, or more to the point, racing a rigid bike requires a whole skill-set of its own. The feedback to the rider from a rigid chassis with girder (almost rigid) forks has little comparable in the modern motorcycling world. Racing with modern tyre compounds winds up the chassis and causes some instability that you get used to and some you know you can’t. Even though you ride a different line to avoid the ripples or the sudden dip in track surface etc, arse off the saddle.. damping with your knees like a jockey… there are some corners where you don’t want to try to push the line, and the exit of Charlies always has been one.
At the end of last year, Bill Swallow had a ride on my Dad’s bike in one of my races and he got into a tank-slapper coming out of Charlies which certainly caused him to wind it back it little. He knew he couldn’t push any further.
Maybe Steve felt a little under pressure to perform? Possibly, although he is obviously a great rider with a lot of experience of real racing pressure so it’s difficult to believe. I think the truth is, that he just didn’t know what the bike would do and assumed that he’d be able to tame it. It was a sad end to a day which promised some exposure for the British Historic Racing Club, plus a wonderful chance to see what a well respected modern champion might achieve with the bike in the way Paul Dobbs did with such style, ten years ago. Rest in peace, Paul.
In the end, it was a mistake, just unfortunately one which will take a while to sort out. Now stripped, his racer needs to have to frame checked for straightness, the bearings, the wheels etc. The fork blades are bent and most likely other parts of the assembly too.
Of course it will be done, but at 74 Roger has a lot of engine work for customers and it’s just difficult to find the time for minor developments, let alone complete re-alignment and rebuilding work. It won’t be done until next year, for certain. We have to remind ourselves that beyond the feelings of sadness and regret over the incident there must remain one clear point:
This is what racing is.

June 2015 news

I have to admit that I haven’t completely fixed my Scott yet after my low side off at Anglesey. The radiator is fixed and looks beautiful. Thanks go to Graham Moag, who is ‘the’ Scott radiator man for sorting it out so quickly. I’ve still got to sort the front guard, the left hand footrest and just give it a good check over. Engine-wise, I’m going to lift the block to check the top end out and clean things up a little.
I’ve also been trying to get some of the other things done that I need to. The workshop has to have time spent on it from time to time and with a few good weather opportunities, I needed to take advantage of shelf making opportunities. I’ve also been spending quite a lot of time re-scraping the slideways on my Smart and Brown Model M Mk2 toolroom centre lathe. I bought the lathe blind from E-bay last year and although I thought it was a beautiful peace of equipment, it had certainly had a life beyond its initial toolroom existance. The front slideway had almost 0.010″ vertical wear, which I have now reduced to around 0.002″. It’s rough scraping at the moment and now I’m going to start on the rear slideway, just to get close before the more precise fun starts. Roger has machined the saddle up for turcite, which is a very low friction material used commonly in this kind of work. It comes in all kinds of thicknesses and you use a two pack epoxy to glue it on. It’s easy to machine and easy to scrape…apparently.
I suppose re-appraised it a few weeks ago when I saw a Hardinge TL lathe come up on ebay quite close to me. I had been thinking that the Smart and Brown was more of a project than I needed at the moment, so I did a bit of reading on ‘lathes.co.uk’ and thought I’d check it out. It was a lovely little lathe but the more I looked the more issues I saw. The hardened slideways were still worn and the feed shaft had been removed meaning that the only feed was on the main slide axis using the screwcutting lead screw, which was worn. These lathes seem to have been designed specifically to facilitate screwcutting and they have a clever operation which certainly would enable you to do it far quicker than on most lathes but.. I don’t do that much screwcutting, and certainly not enough to merit a lathe only set up for that function. I started to think about what I needed… and came to the conclusion that it was already in my workshop!
I’d already managed to buy a replacement back gear, rack, and feed screws for the cross slide, compound and also half nuts for the lead screw last year. The lead screw can turn around to use the least worn end.. with a little modification. It’s still going to take time but it should be pretty good when I’ve finished.

I also have welcomed a new member of the workshop family, although this will be based at my friends farm: a Thiel 158S duplex universal milling machine. Picked up on a trailer last week, it took me almost three hours to construct a frame to transport it safely, but we made it all the way back to Devon without a problem.

2015-06-11 20.39.06

2015-06-17 12.17.53

Featherston workshop

Roger had a recent email exchange with a gentleman in New Zealand, John Stewart, who has had a long history around Scotts. His grandfather was a photographer and captured a wonderful photograph of his father in his workshop in Featherston, New Zealand. John’s son, Scott, repaired the plate glass image and apart from Yowl (the journal of the Scott Owners club), it’s not been published before. He has kindly allowed me to do so. The copyright belongs to John Stewart and I use the image here with his consent.

Enjoy

The attached photograph of my father’s workshop in Featherston may be of interest. This was almost certainly taken by my grandfather, GT Stewart on his glass plate camera (which we still have), sometime during WW1. The garage was established in 1906 as Stewart and Son, Later Stewarts Imperial Garage. The garage provided a wide range of services including maintenance of steam traction engines, motor cars and motor cycles and during the war, repair and servicing of army vehicles for the nearby Featherston army camp.. Two vehicles in the background are almost certainly army truck chassis.

The man at the back is Dick Rowe who was workshop foreman. The lass sitting on the chassis is Miss Freed secretary and the other figures apprentices and tradesmen a couple of whom appear to be working on Model T Ford engines.

The interesting bit is of course the Scott on the right. This I believe is a 1914 model and was the machine that my father, H.H.Stewart raced on the grass track in Featherston with some success. The family left Featherston in the mid 1920’s and dad kept the engine and two speed gear out of the Scott along with the remains of an 1898 De Dion Tricycle and a 1900 Locomobile steam car. He carted these parts round the country during a number of moves until finally settling in Auckland in 1926. The two speed gear was used as a change speed gear on a turret lathe after WW2. The engine I gave to a friend many years ago who had unearthed the remains of his late brother’s 1914 Scott with no engine and the De Dion has been subjected to full restoration over the last 3 years which I finished in Dec. last. It runs superbly.

Featherston Workshop

Anglesey racing report: May 16th and 17th 2015

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.
Angleseymap2012

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.

damaged radiator and footrest... unfortunate but could have been a lot worse.
damaged radiator and footrest… unfortunate but could have been a lot worse.

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.

A carburation diversion – an atomisation exploration

It’s too long since the last post but there’s been a lot going on. Racing, crashing and children with chicken pox.

So, I had the Super Squirrel on the dyno, which showed that I was down on power by almost 2 bhp from the best of my methanol runs. I had some evidence that the reduced compression ratio, combined with an ignition retarded slightly beyond optimum was capable of giving me more revs to play with at the top end but with an ever decreasing power output. The beginning of the curve was not as strong and in fact seemed to not show useful power until a few hundred rpm higher than the dope set-up. I had taken a guess at the needle jet size as 108 which seemed to be ok as we settled on the middle needle position as the optimum position, and the final main jet was 280. I’ll come back to all this later.

So I wasn’t whooping with joy, but neither was I crying in my tea. This is what development is.

After another trip to the airfield for yet more AVGAS, I thought I’d attempt to mask the early season financial hemorrhage by seeing if there were any unwanted bits and pieces I could sell as I’m all out of Kidneys. I put a couple of items on ebay and a guy in a nearby town contacted me to ask about one of them and to see whether I’d got anything else. I called him and it turned out that not only did he used to race (he won a Manx Grand Prix), that also he was a two stroke fanatic and a very recent Scott owner. He also used to run a very well regarded bike dyno cell in the south east and was interested in my dyno work. He asked me a lot of questions about the dyno which I couldn’t answer and gave me cause to doubt my assumptions about the graphs. My basic understanding is that here are different types of rolling road dynos and they can be operated in different ways. The difference between those operating methods should guide the way you interpret the results… all of course mixed in with the operators skill and understanding. So, I felt like I had more questions than answers… and still do. That’s good though, It’s when things aren’t working as well as you want them and you’ve run out of questions that you’ve got problems.

With this in mind, I went back to the dyno charts. I realise that I have taken a very literal view of the charts and assumed that the x axis (revs/time etc) was representative of the position of the throttle, and therefore the position of the needle within the jet. It’s not necessarily a completely wrong assumption, and Steve who runs the dyno was very measured in the way he opened the throttle. However, I don’t absolutely know the extent to which the graphs can be interpreted as a clear representation of engine response to throttle opening and I need to gain a better idea of what I’m looking at. As part of this I also need to understand more about how this dyno works and whether I can gather any other information that will make the graphs more useful.

In the meantime though I had my first race meeting coming up imminently and I had to work out what I could do to try and get back the missing chunk of torque at low revs. I decided to map the carbs and the needles to work out the relationship between the intake aperture and the needle aperture, with the eventual switch to the main jet. I can say 1/8 pilot, 1/4 slide cutaway then up to 3/4 needle and then main jet, but I have no idea what is actually the case in any given machine. How does that relationship actually (and measurably) manifest itself? For a start, I’ve never understood why the needle is a constant taper when the rate of increase of aperture area is not constant for any incremental lift. I expect that it’s simply a compromise born a need for manufacturing simplicity but I don’t know.

I started to look into it and then got carried away…maybe a colossal waste of time but certainly interesting!

I saw pretty quickly (after having made a calculation spreadsheet) that the 1/8,1/4,3/4 guides refer to the aperture area, and not the lift. Obvious that it should be, but I can’t remember ever seeing it written down. Working out the area of a segment when you know the radius of the aperture and the vertical height enabled me to start to put a picture together.

I measured the position of the needle (at #3 position) in relation to the top of the needle jet at the throttle closed position and then measured the corresponding point on the needle at the calculated lifts corresponding to the 1/4,1/2,and 3/4 aperture area throttle positions to give results throughout the needle range and show the transition to the main jet. I accept the possibility that there will be some inaccuracies in my results as I’m only using my eye and a vernier, and using a fine-liner to mark the needle (I don’t have an inspection microscope like my dad!) but I think it’s close enough to show something useful.

The following are the results from my investigations showing the information for one of my 1″ bore type 76 Amal carbs detailing the main jet, needle jet and needle position.

Needle jet fuel info-108-RHMtwin-needlepos4
Needle jet fuel info-108-RHMtwin-needlepos3
Needle jet fuel info-106-RHMtwin-needlepos4
Needle jet fuel info-106-RHMtwin-needlepos3

I also created a set of results for the Standard Scott type 206 carburettor, but these are based on one main assumption: that the relative positions of the needle jet and needle are the same as a type 76. They use the same needle, I think, so I can’t see how they wouldn’t be.
The Scott type 206 in standard setting has a 1 1/16″ aperture, runs a 106 needle jet and a 170 main jet. Some people seem to be increasing the main jet size up to 200 nowadays but I have not any information about the individual experiences that have led to this. I did the calcs for the standard set-up.
So, with my standard disclaimers in place!
Needle jet fuel info-Standard_Scott-106-pos3-170main

Assuming (dangerous I know) that this is representative of a correct(?) carburation relationship, what’s interesting to me is the comparison between the annular (or probably crescent shaped if the needle is against the side of the jet) aperture of the needle jet with the needle in it and the single round aperture of the main jet. Looking at the figures you can see that on my setup, the ‘dyno assessed to be appropriate’-main jet aperture is far smaller than the needle jet aperture at 3/4 opening. It was my desire to have a control result that led me to profile the standard Scott setup. On this (needle at#3), with a 170 main jet, the needle jet aperture is larger than the main jet even at 1/2 throttle.

One possibility from all this that the renolds number involved with the greater wall surface area of an annular aperture means that it has to be of a greater surface area than a single aperture to flow the same size. Another is that the needle aperture’s job is different to that of the main jet and that the needle jet/needles job of metering to the airflow at(or near) the top of the emulsion tube, requires a range of surface areas over the throttle aperture changes that are linked to the response of that fuel to a given low pressure area over the emulsion tube. This may mean that the needle/needle jet surface area simply has to be larger than the main jet at throttle apertures where you would expect the main jet to exceed the needle/needle jet aperture simply from looking at its cross sectional area.

It’s probably of limited value to compare the carburettor settings of the Standard Scott to my own (other than for interests sake) since most of the conditions are different. A Standard Scott will would have a different gas speed profile over the rev range through its single 1 1/16″ carb than mine through my twin 1″ carbs. As standard it would probably also change far less throughout the rev range being high already at low revs as the carburettor is far smaller than the inlet port area. A standard iron block I have here shows 6 x inlet ports at 19mm x 14.5mm (sorry about metric but I find it easier for sectional area stuff) which gives ~16.5cm². The throat area of a standard 1 1/16″ 206 is 5.72cm². That makes the carburettor aperture just around 35% of the inlet port area. Even taking into account the flow disturbances of the bridges, that’s a big difference. It’s probably good for low engine speed pick up, though not great for breathing at higher revs. That’s why Scott’s respond so well to inlet work to the carb and inlet tract.

My carb area is actually about 95% of my cylinder inlet port. I’ve a smaller but better flowing inlet port(s) with more carburettor aperture. I would therefore expect my gas speed over the emulsion tube to be lower, at lower revs. I may therefore require a bigger needle jet simply to give more fuel surface area to lift at these revs. However, with a straight taper, would that make me rich further up the range when the gas speed is higher?

I feel glad that I’ve gone into this, even though I know that you can tune effectively simply by changing bits until it’s right and not trying to analyse the workings of a carburettor. Certainly I’ve no solid conclusions to draw and I can’t be certain of my absolute accuracy but lots of interesting relationships and patterns have emerged that I think are valid and I hope will enable me to better develop my Scott.

I’m always open to constructive comment!

Dyno chart analysis

Andy from Performance Engineering has sent me the files from the dyno runs I’ve done from the last couple of years and with software downloaded from Winpep I am able to view all of them to compare.
I’ve been looking forward to this because I wanted to see where I could maybe make gains by modifying the needle. It’s very interesting to see the comparisons and to consider the next steps.

These are the most interesting charts, torque and power:
To be clear, the settings are as follows (a bit mixed up on the graph notes)

Run #17: Twin carb set up (avgas), High comp head (32cc)
Ign set at 22degs, main jet 280, #4 needle jet position (5bottom), 1/4t out on pilot air screw
Run #18: Same as 17 but the second run.
Run# 19: Same as above except needle now raised to #3 position (middle)
Run #28: Same as above except ign set 23.5, needle on middle position #3.

dyno graph comparison chart torque
dyno graph comparison chart power

You can see how 17 and 18 really want to rev further, and that’s with the needle on position #4. You can also see that they must be too rich at the beginning, as there’s no curve taken. I’m guessing Steve must have felt it flat and simply not sampled it at the beginning.
So it looks to me that the needle could be reduced in diameter in the final section to enable a bit more richness at the top end without compromising the bottom. That should give me an extra five hundred revs according to the chart. That should give me the best of the start of #19 and the end of #17/#18. I can work the amount out simply by checking the difference in the needle within the pitch of a needle adjustment notch.
The timing change seems to have a decent effect in increasing peak torque across the range, so I’ll leave that where it is.
It’s interesting that the needle position change seems to have had an effect within an area that you’d expect to be almost beyond its influence, as I understand it. I need to check the needle jet and needle at that position to see whether it’s restricting the main jet. You’d think that it could be, looking at the graph.
As for the beginning and my lost 400revs, I’m not exactly sure. Part of me thinks that it might be weak because the fuel is too low in the needle jet as I’ve set it to not drip out the pilot air screw assembly. As the carb is at an angle, this obviously reduces the level in the needle jet. This theory is slightly supported by the fact that the very first section (around 2750 revs) was a little stronger with the 300 main jet and needle at the highest position.
However, the ‘technical guy’ from the Amal stand at Stafford said that the float level was set at 0.9″ from the top face of the float banjo. Extrapolating that into the carb means that this works out to be within the thread for the needle jet in the jet block. That’s not very far up the needle jet anyway.
Later carbs made for two strokes had the rear part of the emulsion tube cut away as this created a situation where more of a low pressure area was created to lift the fuel from the jet. Apparently (Amal guy) this was done when the higher performance two strokes came along. It may be that the response time was shortened and at higher revs this became important… I just don’t know. The other ‘known unknown’ is that I think the bellmouths aren’t made with the requirements of gas flow as the primary design factor. I suspected that they wouldn’t be perfect but the way they are made leaves quite a significant ridge around the inside against the edge of the carb inlet. Not ideal for laminar flow as I understand. It may be that this, along with the less than ideal brief radius at the end of the bellmouth is responsible for a meaningful restiction to flow. I didn’t take into account in any mathematical way the extra venturi wall area when I originally planned my conversion to the twin 1″ carbs, but the overall increase in aperture is around 10 to 15%. If I’ve got bad turbulence into the carb then that might cause problems..also since the emulsion tube is right at the bottom of the venturi, it could be that disruption to the flow entering the carburettor is effecting the flow over the emulsion tube.
I think I’m going to try to temporarily effect a radiused entry using silicone sealant and see how it works. The radiused bellmouth I’m going to try and do using epoxy resin, just to test. I’m probably not going to know, but I will have the bike back on the dyno at some point.
Longer term, I’m probably going to file both carbs out to 1 1/16″ (around the top and sides) to increase flow, but that’s not for now… oh and make some decent bellmouths.

Stafford Show visit – April 2015

I hadn’t resolved to go to Stafford until the day before, but I knew that Roger was going and that it was also a good chance to catch up with people who I’ve known through my life around Scotts, but who I have seen rarely in the last few years, since I stopped building engines for a living with Roger. I also had a chance to talk to the technical guy on the Amal carburettor stand about the level the fuel should be within the emulsion tube of a type 76.
I’m constantly thinking of how to improve the power delivery right at the bottom end.
The Scott stand looked very good and had a back drop of screens with interesting photographs, including several of Scott’s in competition over the years. Sheelagh Neal had her father Ossie’s famous Scott racing outfit on the stand and there was a Reynolds special, Harry Langman’s TT racing outfit and a Sand racing Scott.

I also had the opportunity to have a look around the Bonham auction. They had a couple of quite iconic Scotts as well as a Silk. One of the Scotts was apparently a 1926 TT entry and was the first known use of the duplex frame. The other Scott is well known to me as it was the actual machine that I always thought to be the most beautiful Scott when I was very young. It is a Sprint Special and belonged to Dennis Howard, and then Glyn Chambers. It is pictured in the Jeff Clew book, ‘The Yowling Two Stroke’. A special bike I think.

My dad also had a small display of engine parts to enable people to see that there was new spares support for the marque. I think it also shows that the bikes are living and that people don’t have to fear using them.

It was a good show. A few pictures below:

Test run observations

I took the Super Squirrel to my friends farm today to run it down the track, as I didn’t really want to take it to the dyno without knowing it even works. I fitted the pair of gauze covered bellmouths as the road is quite stoney and I really didn’t want any unwanted inhalations. Radiator filled, fuel filled and away it went immediately.
It sounded clean but the first thing I noticed was that it was settling to a high tickover, rather than the stop that we need it to have. I need to check the slides and pilot settings.
It measured between 270°C and 300°C on the exhaust temperature sensors, which doesn’t mean anything much at this point, but might mean more as we move toward a better set up tomorrow.
Pulling away up the hill it was getting bogged down, so I imagine that it’s rich on the needles, which are standard 276 needles raised to the top as a starting point. I don’t think I got to the mains, but we’ll check tomorrow.
So tonight I’ll tweak the bits I can and the rest will have to be done tomorrow.
It’s really a tall order to try and get the whole set-up done in a few hours but we’ll see how far we get.

Scott racer developments

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.

Vee twin manifold
Vee twin manifold
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,IMG_5138 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.

The virtual oily corkboard of a vintage motorcycle racing family