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.

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!