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.

thanks to Ted Parkin for this one

Lester Young (Tenor Sax) was sitting in the band bus on his way to a gig when a young Charlie Parker “wannabe” came to him and proceeded to play every note possible (and loud) on his alto sax into Lester’s ear. Lester sat there and suffered the racket. When the lad had finished he said to Lester. “So Pres, What did you think of that?” Lester turned and said. “Not bad Lady. But can you tell a story?”

Holiday thoughts..

I’ve been away in France for a few days Holiday with my wife and our little girl and I thought I’d take a couple of motorbike magazines with me that I’ve never bought just to give me something to stick my nose in (as well as Jennings book on two stroke tuning, Tuning for Speed and a a great book about some of the lesser known stories from within the drama of the Tour de France). One of these was ‘Practical Sportsbikes’ which seems to be largely written by one man but has some really interesting bits and pieces in it. Mainly aimed at people who are interested in 70’s, 80’s and 90’s sportsbikes and still actually doing things to them to make them faster or better. One bit was concerned with the re-commissioning of a Suzuki RGV 250, which I’ve always had a soft spot for after having sat on one at the 1989 motorcycle show at the NEC. Anyway, they had decided to fit a programmable ignition unit which gave them the opportunity to pre-program advance curves and also to alter the timing using a plug in remote control. After realising that this wasn’t something that was that far beyond the realms of possibility, I wondered about using this on the Silk Scott racer, since no firm ignition set up has been defined as yet, beyond the use of the flywheel as part of the generator and ignition trigger.

Roger working on generator flywheel.
Roger working on generator flywheel.

We're hoping it's all going to work out...
We’re hoping it’s all going to work out…
I wrote to the manufacturer mentioned in the piece at the beginning of the week outlining my interest and telling him the current situation.
I’ve had an email exchange this week which has been interesting. He admits that the benefits of an advance curve are likely to be greater in a higher revving engine, but he reckons that all engines benefit from it. It also gives speedy possibilities to set up in a dyno session, where the timing can be changed very quickly.
I’ve never had an advance curve on the system I run on the Super Squirrel and I’ve always thought it was fine. Without actually putting it on a dyno, It’s pretty difficult to know though.
He also says that the spark output is really good at low revs which means good starting… much better than a PVL system he said, which didn’t really crank out the voltage until the revs were higher. That would be nice.

The programmable ignition system is ‘zeeltronic’ (apparently popular according to this magazine article) and the website is here:


They do systems that also control exhaust valves at different revs but the one he specified just does a couple of ignition curve programmes.

It certainly means that there wont be the fiddle of trying to make sure that the pickup assembly is adjustable. Apparently you set the pickup to sense the trigger just before the range you are going to be using and then the actual ignition firing points are decided by you in two programmable maps.

Also, since we won’t have any ignition or oil pump related gubbins hanging off the doors, I think that we should also do a set of reed doors for it such as Roger made for his brother’s bike back in the late 1960’s. Recently he’s had more castings made as they were used on the ‘phased transfer’ engine that Bob Collet has designed and built using Scott components as a basis.

Reed valve doors as designed, made and fitted to his brothers bike in the late 1960's
Reed valve doors as designed, made and fitted to his brothers bike in the late 1960’s
The more I think about it, the more I reckon that although reeds can be restrictive to flow in high revving two strokes, with the engine speeds that we are using there could well be a distinct advantage and with the high comp head and a resonant exhaust, it could be significant!