Hi all, wondered whether anyone could provide some possibilities as to what might be the culprit for the little reluctancy my NC30 seems to be suffering from.

To cut a long story short, the bike hadn't run in over two years,and I was never expecting it to be 100% when I began working on its reawakening, so to speak.

Recently, the bike has received a whole newly clean set of RVF carbs, with new rubbers, jetted to suit, etc. One of the throttle slides was initially a bit sticky, but seemed nicely free after the rebuild. Also, one of the inlet trumpets wasn't seating properly, but was clicked into place after the carbs were reinstalled. Air filter is new, airbox clear, air scoop in place, new fuel lines installed. Tank was cleaned, as was the fuel tap, and the bike has been treated to half a tank of fresh premium unleaded.

Plus were removed and checked, at which time I dribbled a little more oil down each cylinder bore. I then turned the engine over by hand a fair few times, fitted a new battery, and thumbed the atarter. The bike idles very smoothly, with no hunting on idle whatsoever, almost surprisingly so. After which, I changed the oil and filter, and flushed the coolant system. Clutch is free too.

Howvever, this weekend, despite the bike starting on the button with a little choke, it presented a little gremlin. It doesn't rev properly under load, when in gear. At first, I could only pull away, but when the bike approached 4k or so, it stopped. It didn't stall, neither did it stutter, it just died as if it had run out of fuel.

But it started on the button afterwards, and gradually got better, to the extent that it was guadually able to run about to 8/9 thousand when riding. However, it does not feel like it's running freely, almost like it's only running on 3 or something, but without any jerkiness or smoke. There is no stuttering, or any leaks from the engine, it simply doesn't pull when riding.

I wonder whether it just needs a good run to clear itself out, as the bike seems to show no other signs of ill effect. Would anyone care to offer some suggestion as to what might be causing this in the meantime? Dad seems to think it might be sticky piston rings after having sat for so long? Or maybe one of the carb sliders is sticking?

vaccum line from fuel tap ok / diaphram working properly?

sticky float in one of the carbs?

Are all the pipes heating up on idle? As neo may remember I brought one back from 10 years hibernation, cleaned carbs 3x before I got all cylinders working...that and check fuel flow from tank..


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Thanks for the pointers - the floats did seem nice and free when I had the carbs apart (I bought a second set of RVF carbs to do the job, as mine were gummed up a bit from the bike sitting so long).

From memory, the fuel tap has had the diaphram mod, and seemed okay when I began to empty the tank. I'll take another look at the vacuum pipe though... as well as check if all the pipes heat up the same. From how steady the idle is (which puts my daily bikes to shame), I had thought they were all firing okay.

Something's not right though - the odd thing is, it seemed to get better with use - although still didn't feel like it was revving and giving the power it should.

Plugs looked almost new when I was prepping the bike - I assume a dodgy coil would prevent the bike from starting so easily (a little choke is all it seems to need)?

could be electrical, all the fuses (including the main one in the starter relay) ok and not corroded?

either fuel air or spark most likely (sorry that doesnt narrow it down)

hard to say without seeing it, if the tap is modded then just make sure the vaccumn pipe is blocked off else it will cuase an airleak if its just floating aroud

Now there's a thought, did I reconnect the vacuum feed out of a matter of course as if it were a stock fuel tap set up.... Could this be causing the problem?

It's been too many years since I last worked on this bike

Little update:

Started the bike up yesterday and at first it seemed to be firing on three: It started very easily and 'sounded' as if it was running on all four, but the rear left cylinder's exhaust pipe (from where it appears from beneath the right hand rear set) was cool. I dolloped a bit of Redex into the tank along with another couple of litres of new petrol, checked the fuel tap was working as it should, which it was. Choke is also shutting off fully as it should.

I couldn't see where the other end of the vacuum line disappeared to - looked like somewhere beneath the carbs on the rear right cylinder? It is reasonably new and does pull a vacuum - is it easy to get to to check it properly?

Had a look at the plug, earthing it against the frame and noticed it was getting quite a weak spark. Fitted a new plug and started the bike again. Similar symptoms as before, but the pipe began to warm a bit, so decided to take try it out.

The bike was better than it had been the previous week - pulling cleanly, like I'd expect an NC30 would up to 7k or so. But passed that, it seemed to surge a little. The engine would run up to 12/13k or so, but it wasn't pulling cleanly and lacked power.

Got home and noticed the rear pipes (running a twin stack system) were a bit sooty. Although the bike didn't smell rich. Having a look at the plugs again revealed that they were dark, but didn't look as if the bike was running terribly rich. I changed the other plugs anyway, replaced the air filter with some new Pipecross foam and gave the bike a run again.

Now it seems to run fine up to 10k or so, maybe even stronger lower down.

I'm still unsure about whether the work I'm doing to the bike is improving things, or its simply clearing itself out after such a lack of use? Pretty confident its ready for its MOT either way.

Any thoughts? Vacuum line needs a proper check? HT coils? I'd dread to have to pull the carbs apart again....

new airfilter and plugs dont usually do any harm,

if its pulling a good suck from teh vaccum line then id leave it that way for now.

Is it old fuel in there? Not had too much of a problem with old fuel but you never want to rule somethign out.

Id take it for another decent run seeing as it seems to be running on all 4.. put 30 - 50 miles on it and see how its running.. at least after that if its still running bad you know its not liable to clear up from a run and needs further investigation.

likely issues fro not pulling stong at speed / high revs could be fuel flow / could be the rubber flap not being in place.

i take it that it used to run fine?

Thanks for all your help so far Neo

Tempted to get it properly back together again and see how it performs over some decent miles as you suggest.

The bike has an aftermarket top radiator, so not sure if the rubber flap is sitting as it should, but does wrap around the inner frame bar (behind the headstock) okay, and lines up with the snorkel when the airbox is fitted. Does it need go sit at a specific angle?

The tank had old fuel in it - removed as much as I could when I started working on the bike again - I'd taken off the tap at the point to get as much as I could out of the system (hence the use of Redex yesterday just go help clean any possible sediments further).