Hi All,

Its been a while, but I'm looking for some reassurance that I'm heading in the right direction.

I've had an issue with my NC for a few months now thats occurring more and more frequently.

Essentially the bike will drop on to 3 cylinders under load, then pick back up again. Usually fine at idle, unless its cold out. She runs a TSR 3/4 system, when I first bought it I went 118 / 120 jets, shimmed the needles as Graham's guide etc etc. All ran fine.

The bike had been sat unused for a while due to moving house, and initially this only happened infrequently, but its been gradually more pronounced.

I initially stripped the carbs, cleaned them out, made sure everything was ok there. I then suspected duff fuel, as I refilled after with higher octane and it helped matters. Its now happening consistently when its cold out, and under throttle load until the bike gets warm.

I put a new Vac tube on 18 months or so back - the thick wall stuff from GF - so it shouldnt be that.

I'm suspecting spark plugs, leads or coils. Sparks are going to be my first guess, as I have a new set - even though they're a pain in the ass to change from memory.

As these eat plugs I would start there,but sounds more like a carb issue to me.check all lines from tank(vac/petrol line)..if it's been standing with fuel in carbs they do gum up,and as these carbs have tiny passages inside it doesnt take much for them to block....electrics usually breakdown when they get warm,so if it's running fine/or better when cold,then worse when warm that's a sign of electrics...but start with plugs first...you may take them out and problem will be self evident...rear plugs are easy,front ones need the top rad swung down out the way,but mo need to drain it...good luck and let us know bud...

Fuel flows easily from tank with vacuum applied?

Hey guys,

Rich - that was my thought, but carbs have been stripped down and cleaned earlier this year, and I've had some in fuel carb cleaner run through it - jets, pilots, floats, mini filters all removed and cleaned with carb cleaner. I used to have an NC30 20 years or so ago, so I've been through this kind of thing before :) The current one is a little bit of a garage queen and only comes out over the summer, but usually gets a fresh tank every month or so.

It had been standing, but after the first time I cleaned the carbs a couple of years back, it ran pretty much perfectly and has been getting gradually worse over the few months - Ironically when its been used frequently.

It goes away over the winter, but I always drain the carbs via the drain screw on each, then run the bike with the tap on Off until it dies.


I am hoping its the plugs - I've had the bike for 6 years now, and they havent been done since I bought it, although its also only done about 5000 miles since then too!


Magg - yeah, fuel runs from the tank easily with vac to the pipe - my first one had this issue, so it was one of the first things I checked.

Problem only occurs while engine is cold, but runs well once hot?

Probs is plugs then bud,change and see how you go..they are finicky to plugs these v4s...if you've done that with carbs it should be all good then...

Got round to changing the plugs this weekend, the fronts weren't as much of a pain as I remember!

2 of them had slight browning / discolouration on the porcelain body, and it seems to have pretty much fixed the issue :)

Glad to hear!!!..at least you now know you wont have to pull carbs off!!!