Yes Frank, split FCR41s and that was only a little of the hassles
hwell:
Iain presented me with a partly stripped 900SS and a frame with an engine attached and a load of boxes. The idea was to remove all the running gear from the part stripped bike and fit it to the frame and engine. That was the easy part. Everything after that seemed to be 2 steps forward, one step back.
I first saw parts of this project bike about 7 years ago when Iain brought a pair of bare heads to me to be fitted with ST2 cams and shimmed to suit. The heads had been fitted with twin sparks and gas-flowed by CJS Tuning. Then earlier this year Iain contacted me to ask if I'd be interested in putting his bike together. I was fine with this but due to holidays and other commitments it would have to wait until later in the year. Iain was fine with that so 3 weeks ago he brought me all the bits.
After the initial build to get the basic bike together I then had to fit the Keihin FCR carbs, Dynacoils, a Motogadget speedo, do the mods to the wiring to fit these and various other bits and pieces that needed doing.
The carbs were initially a nightmare to fit mainly because one of them had the wrong throttle cable fitting plate. Depending on which cylinder it was mounted on the cables had either to be routed through the battery box or through the high mounted oil cooler. However Iain managed to turn up the correct plate and this made life much easier.
Meanwhile I had been sorting out the electrics and after turning the engine on the starter a few times noticed that the horizontal belt was working it's way off the pulleys
WTF.
I went home and had a think about this and realised that here were no flanges on the layshaft pulleys and since the 2V engine has no flanges on the cam pulleys nor the tensioners there was nothing to keep the belts centred on the pulleys. The layshaft pulleys were from a 4V engine which of course has flanges on the tensioners that keep the belts centred. Fortunately the donor bike had a proper set of 2V pulleys so these were robbed from it and fitted on the project bike. Good job I spotted this before trying to fire it up
After the belts were back on I did try to fire it up. It started up fairly easily and would idle, albeit a bit roughly, but as soon as any attempt it popped and banged, spat back through the carbs and cut out. It was if there was no fuel getting through at the 1/4 - 1/2 throttle stage.
Next morning I pulled the carbs apart expecting to find clogged jets but no they were all clean inside although the jets were slightly different in each carb. I just couldn't figure out what was going on so got on with some other work in the meantime. That night I had another Eureka moment. What if the pickup wires were reversed?
Now when I wired them up I'd noticed that the pickup wires were much longer than normal so someone must have extended them, neither did they have connector housings on the ends of them. I never thought anything of it and fitted them to ignition modules taking care to match the colours for each cylinder to that in the wiring diagram. I looked out pictures I had taken of the pickup connections to the ignition modules on the 900SS that I used to have. They were exactly the same as how I had wired them!
So what the hell, I decided to swap the wires round anyway, went and did it, fired up the bike and it ran perfectly
For those interested in the technical details; swapping the pickup wires round reverses the phase of the output pulse. The ignition module triggers off the negative going edge of the pulse, so reversing the phase means that it now triggers off the leading edge of the pulse instead of the trailing edge, advancing the ignition, apparently by around 8º. No wonder the engine wouldn't take any throttle. At idle it was already running too much advance and increasing the revs advanced the ignition so much that it wouldn't run at all. Whoever extended the pickup wires must have swapped round the colours but I couldn't see this beneath all the heatshrink that had been added.
At the end of the day Iain now has a bike that runs nicely and needs a bit of tidying and wee jobs to get it on the road. It has taken a long time but should be worth it. The spec includes 748 Öhlins forks and Öhlins rear shock, CJS gas flowed twin spark heads, ST2 cams, 944 barrels and pistons, Keihin FCR41 flat slide split carbs and Dynacoil twin output coils. This bike should certainly be a go-er
