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!

One thought on “Holiday thoughts..”

  1. When I made the reed valve conversion for Brother Trevor’s bike in about 1967, I realised that the piston skirts were no longer acting as valves to control the inlet, so they could be cut back.
    At one mischievous moment, I contemplated fitting a standard carb in the central position, albeit on a blanking plate, just to confuse folks. A three carb Scott? and watch the head scratching. But then I start to think, why not? The idea is to get the maximum inlet charge into the engine. The standard piston skirt controlled inlet has a fixed opening time and duration and yet, the optimum engine requirements as regards timing, will vary with the engine speed and the lag in the incoming charge proportional to the engine speed. And so the thinking goes that as a reed valve is a “Demand feeding” device, then it would be interesting to fit a couple of reed valves on the doors of the Silk Scott in addition to the central carb, as these would react to any extra demand that presented itself which lay inside or outside the timing window of the conventional piston ported inlet. The original reed valves were taken from a Mercury 35 twin, but I would try to increase the throat of the housings if I were to make new patterns today. They gave great torque, but ran out of puff above about 3500 rpm, so I just put the biggest final drive sprocket I could fit on it and it was fine. Of course I could not race it in 1967 as it was obviously out of period, but “Time marches on” and I can prove I did it in 1967, so it would be eligible for up to 1972 BHR race class. So there you are Richard, a 36mm Moss TT type carb exists for this engine and two Mk1 concentrics on reed valve assemblies on the doors would make a very interesting experiment. OK back to rebuilding my race engine. I am only 73 and have a lot of living to cram in before I fall off my perch!

Leave a Reply

Your email address will not be published. Required fields are marked *