What is it about these gen 1s that they supposedly run so badly if you remove the EXUP? Is it the way the ignition timing is set up, the carburetion, a combination of such things? I mean, there used to be plenty of carb bikes that ran perfectly well without having any kind of valve in the exhaust system. Or is it that they just lose some of the low down oomph without it?
More than one reason I'm asking this. A little bit of history. On my 3rd gen 1, I occasionally used to get the 7k EXUP warning, but didn't notice any effect on performance when this happened. After a short while, the warning would just clear itself.
On the Hedgetrimmer, it turned out it never had an EXUP valve at all
Luke discovered this when he built the new bike for me, and took the foliage-lover off my hands. Now, that bike ran like crap at lower revs when I first got it. Glitchy, especially on small throttle adjustments, and most noticeable when coming off throttle in small increments in the rev range up to about 3k. After Ivanising funnily enough, there was hardly a problem, you could
just notice it. Even Luke failed to spot it after Mike worked his magic.
Now, on the current bike, it's like it's running on 3 cylinders below 3k. The EXUP appears to be working fine, get the little whirring noise when I switch the ignition on, and no 7k warnings (btw, I did notice the extra grunt after the Hedgetrimmer, but I wouldn't call it a huge difference). When this first happened, the bike got a full service including carb balance and was fine for about 5-700 miles or so, then quickly went back to this poor low rev running. At first I got suspicious about the vacuum take-off fitting for the Scottoiler, as it is very loose, but taping it over with plumbers tape has made no difference, and anyway, I've been told since that it's such a tiny amount of difference this would make as to render it very unlikely (true?).
Something else - these are the same carbs that were on the Hedgetrimmer, in their Ivanised state, but as the manifolds were in nigh on perfect condition on this bike, but quite deeply cracked on the Hedgetrimmer, the ported ones were not transferred over (not noticed any adverse affect on performance due to this when all was running well). So these carbs probably have something like 40k miles on them in total.
The bike is booked in for a thorough service, this time also to include a valve clearance check, carb balance, EXUP service (Luke did this thoroughly about 15k miles ago), and I will be disconnecting the Scottoiler and putting a proper cap on the vacuum take-off - I've never found them to be any easier than just using chain lube or oil, even if this has nothing to do with the running problem.
All of this history has been explained to the chaps who will be doing the service, and they also seem to find it a little odd that after a service, things would be fine for a few hundred miles, and then rapidly deteriorate again. Can anybody enlighten me as to what might be going on here?