My NC21 has developed a "fault" where it will not run smoothly at steady/neutral throttle when cruising, mainly noticable coming in at about 7500 revs when the engine is warmed up. If you wind the throttle it usually pulls away again until you go back to a constant speed/rev.

I've had it tuned and tried alternative jetting but is now back to original. Bike shop reckoned it could be a compression issue (since it happens when warmed up), I found another motor with better compression and chucked it in but is no different!!! I had to use the same carbs because the others were goosed.

I have checked coils and all are within spec, plugs are new too, haven't checked HT leads yet but I think they did at the bike shop.

Does anybody have any ideas?? I'm unsure whether its fuel or fire. It kind of feels like a fuel starvation thing on one or two cylinders but could be a spark problem, however I'd expect that to remain when accelerating too.

Help!

KiwiNC21 wrote: I have checked coils and all are within spec, plugs are new too, haven't checked HT leads yet but I think they did at the bike shop.

Have you tried a simple voltage check at idle and some revs?

Could also be fuel tap or vaccum line related.

I was thinking maybe the fuel tap vac-valve line. I don't think that the hose is a great fit at the inlet, might try another one. Rubber hose might be expanding when engine warmed up and leaking around the nipple? I don't think it has a clamp on it.

I don't think they have a clamp as standard, but yeah if it can get loose, is split or collapsing under vacuum then it will bugger the fuel flow up.

If there's any doubt about its condition then replace it reckon, they can cause havoc otherwise. You can use a couple of cable ties to secure it at each end.

Hmmm...

Vaccumn line is a good starting point.. they dont' have clips on originally but are a tight fit, a replacmenet line would be good.

Idling / Part Throttle..

Sounds to me like Carb Balancing / Float heights / Pilot Screws / Pilot Jets.

Have you checked all of these...

Carbs go out of balance and need checking occasionally.. this will give you a smoother tick-over.

Float Heights / Pilots

These will affect part throttle crusing.. if one of your jets is blocked.. debris from the tank? It could cause issue.