Tag Archives: Roger Cramp

June 2014 – a review

The rebuild of the Super squirrel racer is in its final phases.. and so it has to be as it’s entered for the Beezumph at Cadwell park on the 11th /12th July. I need to do a piece on the final assembly and some of the things I’ve experimented with.
I’ll default to the single carburettor that I know works if time really dissapears but I’d really like to try and get the twin carb set up finished and ready as it really might fly with a bit of extra gas coming in.

Ovally bored single carb used on the Super Squirrel since 1970.
Ovally bored single carb used on the Super Squirrel since 1970.
One of the main reasons that Roger evolved from this single down-tube frame to the duplex frame on his bike is the ability to fit a bigger carb. He obviously felt it was holding the engine back. I thought I’d have a look at this further.

I’ve had a twin carb manifold for a few years which was made by Eddie Shermer. It splits either side of the single tube and gives you the advantage of standard two stud carb mounting rather than the unique Scott three bolt pattern. I have been intending to use the two Amal 289 carbs that I have previously used, albeit briefly, with this set-up. Although it seemed to go well at the time there was insufficient opportunity to really test its performance. That was with petrol, not methanol so a direct comparison is not possible. I have had a feeling that the 289’s will be too big though.

Twin carb manifold in position
Twin carb manifold in position

A couple of years ago, when I first set the bike up on methanol I approached various people for advice. Roger Cramp of Velocette racing fame had built and developed two strokes to run on methanol and he kindly gave me the benefit of his experience about carburation. One of the things he said was that with methanol he’d found it very important to make sure that you had sufficient gas-speed over the emulsion tube to ensure that you had adequate atomisation, and he found that he’d reduced carb throat size to achieve better results. I imagine this principle applies to any fuel, but methanol is more reluctant than petrol to diffuse it seems. My single carburettor that sits behind the downtube is quite small and it works perfectly with good clean pick up throughout the range and it’s difficult to imagine that the pick up could be better. I don’t want to lose tractability so I thought I’d look at the relative areas.
Twin carb manifold made by Eddie Shermer
Twin carb manifold made by Eddie Shermer

The inlet port on a single cylinder measures about 61mm x 16.8mm which gives around 10.5cm². I make no apology for change in units as I use what helps me visualise better! I’ve ignored the single bridge in this port, but reason that it will effectively make the port a little smaller.

The single carb I use at the moment is an Amal 289 bored out to about 32mm. This area is 8.04cm²
The 289’s I have are about 28mm bore and this is about 6.15cm². Two of these is 12.3cm²
A 1″ 276 is about 5.06cm². Two of these is 10.12cm². This would seem to be a better match.

It seems to me that I should try to at least have the carburettor inlet area quite closely matched to the actual inlet port area and that all my work on getting more gas in is a little pointless if I don’t increase the carburettor size. I think it will be very interesting to see what two of the 276’s will be like though although it’s going to be a push to get them and do the calculations for needle and jetting modifications before the Beezumph.

The engine is now together and primary chain and ‘magneto’ belt fitted and tensioned.
Securing the engine is a procedure on my Scott as it is fitted with ‘tie-bars’ which replace the lower frame rails. We tension these before the engine bolts are finally done up to pull everything together.
Also requiring a procedure is fitting the primary chain.

The Scott uses an ‘outrigger’ final drive sprocket which is secured through slots in it’s casting to the undertray. The undertray is an aluminum casting which bolts into a Scott frame and carries the gearbox and final drive as an assembly. The gearbox itself is secured using two long studs projecting from the bottom of the gearbox and passing through slots in the undertray to allow adjustment of the primary chain. Under acceleration the outrigger tends to get dragged rearwards along it’s slots, thus wearing the ‘high gear bush) in the gearbox putting bending moments on the output shaft and also encouraging the entire gearbox rearwards also. When this happens the primary chain tightens which puts pressure on the main bearings as well as buggering the chain, wearing the drive sprocket and wasting power.
One of the ways to avoid this is to cut out a little piece of metal to very closely sit in the slot of the outrigger to prevent it being dragged rearwards. Roger did this for years. Now we have snail cams fitted to the rear undertray mounting on the drive side to wedge against the back of the outrigger.
The other part of the gearbox bolting procedure is to make sure that after everything is locked in position, we make sure that the backlash in the adjuster for the gearbox position is taken out so that it also is playing a part in making sure that the gearbox is not pulled backwards. I then wire lock this adjuster nut in position. If this is not done, the gearbox will be pulled. The Scott 3 speed gearbox is a rugged device, but simply ‘doing the bolts up’ is not enough. These two procedures make sure that the gearbox stays where it should.

The Super Squirrel racer … where we are.

It’s not been the best couple of weeks for getting on with the Super Squirrel racer’s engine with both my wife and little girl both poorly and work being very busy indeed. A very good friend of mine also passed away, although this did rather contribute toward the quietness and patience required for gas flowing with riffler files.

So what is the plan?

A bit of history… In 2006 I finished re-building the old Super Squirrel racer and into it went a good Scott engine that I’d built with Moss crank. I sold this engine to fund another more radical engine build, and machined up my own head and heavily modified a Scott barrel to suit. I also welded up my own expansion chamber.
The crankcase I used had been a damaged case that we’d had welded, but had some evidence of cracks still remaining beyond the welding.

Anyway, I took it to it’s first BHR meeting at Mallory and it felt really strong for the first two laps, before it died. I didn’t really look at the engine until I’d pushed it back to the van, but when I did I realised that the damage was absolute to the crankcase. It was split in two and completely irreparable.

oh dear.
oh dear.

Upon reflection, the case wasn’t up to the job. I might have had some tiny amount of piston/ head contact too.. I know they were close as I’d had it before during testing and had worked to increase the clearance. The main bearing assembly was experimental and I think that also may have been a weakness. You live and learn and competition sometimes just brings the answers a bit quicker.

I had been working with my dad building the engines for a few years and I think this just happened just as I was going to move on to do a contract working to changeover a cylinder head line to a new head in the casting plant at Nissan in Sunderland. As I was not in a position to build another engine, my dad resolved to machine up one of his crankcase castings to at least provide a sturdy basis for a race engine. Into this he built the internals of the previous engine, and the new engine was badged ‘Phoenix’ in reference to it’s resurrection from the remains of its predecessor.

from the ashes...
from the ashes…

It was a fantastic thing to do for me.. I think because he felt quite sorry as I’d put so much work into the previous engine and also, that he felt that the far stronger crankcase casting was a far better place to start.

So I ran the engine for a couple of years, on petrol, and it kept going, but it wasn’t really competitive. It wasn’t really ‘tuned’, just solid and although I really enjoyed my bike, it wasn’t anything like as good fun to ride as my dad’s Flying Squirrel, which just had a sense of thrilling urgency that mine lacked utterly.

The catalyst for the major improvements that came was when my wife and I received a wonderful wedding present in 2011, in the shape of a new expansion pipe that my dad had made to fit by Gibsons exhausts in the South East.IMG_6831IMG_6828 My wife found this quite amusing. It was somewhat better looking than the one I’d welded up myself.. but did it work?

The first test came early in 2012 when we participated in the Prescott hill climb in aid of the blood bikes. With the same jetting as previously used with my old pipe I accelerated from the line hard and then pulled in the clutch quickly as it seized on the needle as I rolled it off.

We played around with it in the afternoon, changing plugs, altering the timing and jets but it just seemed to be running very hot. The next outing was at Lydden with the BHR club and we put in an extra head gasket to decrease the compression. It was still running a bit hot, but better… at least it finished a race. It really wasn’t quick enough though. I realised that I needed to make a decision.

It may be that the exhaust is not of the optimal shape and there may be a build up of heat because of this and not simply because it’s charging the cylinder so effectively… but we are not running a blank cheque development program (!) and so we needed to try and see if we could get it to work.

I figured I had three obvious choices. The first was to put my pipe back on. I did not want to do that .. It seemed such a retrograde step. The second was to work on getting the heat away. I’ve got a speedway radiator in the bike so a bigger one may well be much more effective. Also my dads bike has an aluminium cylinder barrel which also transfers the heat away from the exhasut port and cylinder head much more quickly than my iron block. Great. A new radiator would be about £1000 and a cylinder several hundred.

The third way was not popular with my dad.’Dope’ I said, that’s what I’m going to do, ‘run it on dope’.
The positives of methanol are that it really cools an engine and allows a far higher compression ration to be run than with regular petrol. Methanol also burns more slowly and that can make for a smoother and more progressive power delivery. On the negative side, it’s comparatively harder to get hold of, more dangerous to deal with and you need a much larger amount to run on. I’ve also experienced lubrication issues since I’ve used it, but it may be that some careful development may improve that. It also doesn’t give much warning in terms of plug colour if you are running lean. It tends to let you know by melting a hole in a piston apparently.

So it was that I invested in a barrel of your finest methanol and talked to a few people who had experience using it. By far the most useful contact was Roger Cramp, who used to race the highly developed Velocette that his son Ian now campaigns with the BHR. He had been involved in the building and development of an ariel leader that he had run on methanol as well as a Greeves. He confirmed the research I’d done about the necessary changes to ignition timing but also said to be aware that methanol was singularly averse to atomisation (at least when bucketing it in) and that high intake gas speed really helped. This encouraged me to stick with my smallish single carb to at least try out and see if it worked.

I decided on a huge 980 main jet and then measured the needle jet with a taper pin. I then put the carb together and drew lines on the needle at 1/4, 3/4 openings at the top of the needle jet and then worked out the dimensions of the needle at those points against the aperture open for air inlet. I ended up with a pretty severe taper on the needle, but it seemed to make sense.

I advanced the ignition by about 7°, closed the plugs right up and pushed. It fired up and it ran, albeit a little roughly, before I put it in the Van to took it to the last BHR meeting of 2012.

It was brilliant. The jetting seemed to work fine and there were no holes in the power delivery according to throttle position. The bike pulled and was so much fun. It felt totally different to my dads bike, but the torque and flexibility of mine suddenly made it feel like a completely different bike. It absolutely hammered the clutch though (as it does tend to with three gears) and I was up to 1am on the Saturday night stripping, releasing, filing plates and and rebuilding it.

2013 came and one of the first jobs I did was to rebuild the clutch with new GFS plates, laser cut. They were perfect really because I had to dremel each one to fit with abut 0.010″ clearance. The less clearance, the less hammering… My dad also had some pressure plate he’d had made out of solid, which didn’t flex like the original ones. IMG_3703These didn’t have the adjustable clutch actuation pins I normally used, which are a pain to set up. I made pins up instead from some silver steel and got them within 0.001″ of each other using a cordless drill as a chuck, a file, a dremel and some emery cloth.

We took it to a couple of track days, and then the last Cadwell park in 2013 and (with new 21″ racing tyres robbed off my dad’s poorly Flying Squirrel) managed a couple of second places and even a fastest lap. It was flying, although not in the league of Mike Farrel on his Rudge, who was really out on his own and un-catchable for us at least. You can lose a lot on the start as it’s difficult to get off the line with three gears when you’ve geared top for a long straight. Except for a CS1 Norton belonging to the famous Lewis family who’ve been campaigning Triumphs and the Norton for many years. I think everyone else runs four gears. It does make a difference.

I ended the weekend having blown three composite head gaskets and with the feeling that there was a bit too much piston slap noise, but apart from that it was the best racing weekend I’ve ever had at Cadwell. I also knew I needed to strip the engine and that I’d do some gas flowing whilst it was apart.

And that’s where we are!