Okay, so I've got a 'loan' NC30 for a bit, and it's got a problem that is doing my head in.

It starts fine and runs beautifully, but after about 5 km/warm temp the revs drop to below 1k at idle and I have to give it some throttle when stopped to stop it from stalling. When this happens, it also misses on one cylinder (unknown which) and is understandably groggy at low-mid revs, but clears up higher in the rev range. So I limp home and after all that, it will start perfectly the next time and repeat.

So symptoms indicate that there's something not right with the electrical system as I've ruled out carbs/fuelling/fuel tap/vacuum, etc. and spark plugs. Reg/rect has been replaced recently and the battery is new and reading correct voltages. The intermittent nature of it is baffling me. I'm thinking possibly coils? But surely they'd either work or not at all? I've never had a problem with coils before. Or maybe genny? I'll check these when I can be arsed taking all the bodywork off again

But before I start raiding the race bike for parts, has anyone had a similar problem or got any thoughts before I park the bike in the garage for summer?

Sounds like the genny dying to me..it supplies enough at top revs but a bit of slow running sees the battery start to lose charge...

don't think it's coils, i had a coil dropping on my bike years ago and it would idle fine but when it was asked to do a bit of work, ie higher revs, the bike rode like it had square wheels, constantly missfired, spat flames and had 35bhp, i have a dyno graph to prove it too lol

Thanks fellas. I'll check the genny over and see what's what

Whilest you're checking things, check the starter solenoid (main connection right hand side of the battery) mine had melted causing the exact same problems you're describing.

Strange symptoms, this bike never had all those issues when in my posession.

If the genny checks out fine, check the coils and all of the connectors, leads etc. A cdi caught fire on this bike (!), I checked everything over but it might have caused damage to the coils.

Might be worth checking other basic stuff also: cdi plug connections, etc.

Okay, so I've checked over the whole electrical system and everything was within spec - that's a good thing! That led me to check the 'new' reg/rect which was supplying ample volts/amps at idle, but only 13.2 v @ 5000 rpm and upwards. The reg/rect should have been my first point of call I guess, but it's a Yamaha unit, so I assumed it was fine.

Anyway, I'll swap it for a known working reg/rect in the next couple of days and re-test.

Perhaps a case of the oldest known issue with these bikes trumping me again (and again)

Well, the replacement reg/rec didn't solve the problem.

Even though the generator tested out alright, I decided it was the last port of call. So after a false start which saw a mismatched flywheel and stator (wouldn't fit), a matching pair were finally bolted on.

Passed the test on her first run today in the glorious sunshine. I'll take her out for a longer ride tomorrow to blast the cobwebs out.

Top marks for picking the genny fellas

Sounds to me more like fuelling prob, sticking choke plungers/cable? Float heights? Pilot jets/screws etc? poorly seated carbs?

Phil