Hi there

Just put my NC30 back together from a bit of time standing in pieces (took a while to build up the courage to replace the 2nd gea r), While i had the engine out i cleaned up the carbs, which were pretty blocked up.

I took the bike for a ride today and the beast just wont rev cleanly up through the rev range, it just kinda splutters along, it still does build up revs, but very slowly. It runs very cleanly on a very small throttle opening, but on anything higher it starts to splutter. The wierd thing, to me at least, is that when the bike is in neutral it revs very cleanly.

So i'm thinking that it may be necessary to pull the carbs to pieces again and check for any blockages

any other suggestions? anyone had this same problem?

Thanks in advance for all your help

did it work fine beforee?

it has been a while since i rode it before today, but yeah. it worked great as far as i can rememeber (besides the 2nd gear issue)

I have similar problem, but can only tell you what hasn't worked for me! Got new Rick O rectifier and OEM air filter in hope of improvement, but to no avail. Now wondering if head gasket has gone. Bike revs fairly freely in neutral, but struggles under load. Does anyone know what failed head gasket sound like? Bike definitely sounds odd above 8,000 under load and won't get past 10,000 in top.

A blown head gasket probably won't make a difference to the sound. If it's anything like with cars, you'll notice a lot more condensation when starting from cold, possibly a reluctance to idle properly when cold, and maybe a slight lack of power and poor running. Easiest way to check is to spin the motor over with the plugs out and see if it spots water out the plug holes. However, as I understand it, HGF is incredibly rare on an NC. Sounds more like there's still some old crap in the carbs and they need more cleaning, or possibly they're not set up quite right. I'm not quite sure how it would act if the carbs weren't seated properly, or if you got the emulsion tubes muddled up though....

Thanks for the reply. Haven't ever dismantled the carbs, and bike was running ok earlier this year so doubt if emulsion tubes have muddled up. It would seem a carb overhaul is required to rule out aspiration problems. Not sure if any of this has helped IanNC30 though! Not sure if I have time or facilities to undertake carb overhaul - any workshop recommendations in West London area anyone?

IanNC30 wrote:I took the bike for a ride today and the beast just wont rev cleanly up through the rev range, it just kinda splutters along, it still does build up revs, but very slowly. It runs very cleanly on a very small throttle opening, but on anything higher it starts to splutter. The wierd thing, to me at least, is that when the bike is in neutral it revs very cleanly.

It sounds like the bike is running lean to me. It could be related to induction, these bikes are super sensitive to changes in this area. Have you got the rubber flap installed that sits in front of the airbox lower (part 15 in pic below)? Removing this gives similar symptoms to what you describe. Is the airbox all standard (including lower front tray)? Do you remember what sized main jets are in your carbs?



It also could be an electrical issue, possibly related to bad earths. Have you got both earth wires attached properly? 1 runs from the front coil negative terminal to the engine and there's another wire from the battery negative to the left of the frame if I recall correctly. It's common for peeps to add extra earth wires to help the system out.

I would also check that the cdi in the tail is plugged into both plugs securely - this has caught peeps out before and can give symptoms similar to what you describe.

Simple system voltage checks would also be a good precaution to rule out any issues with battery, reg/rec or generator. These checks only take a few minutes and all you need is a cheap multimeter.

super-red-jim wrote:Not sure if I have time or facilities to undertake carb overhaul - any workshop recommendations in West London area anyone?

Your bike also sounds as though it's running lean, gives the symptoms you describe.

You'll need to open the carbs up to see what's going on for yourself. Until you know what main jets are in them in them you'll only be guessing what the problem may be. It's really a simple process, there's a step-by-step guide in the document library (also shows a simple check to see that the carbs are mounted correctly): http://www.400greybike.com/docs.htm

You only need basic tools for this job. Always a good idea to replace the float bowl gaskets (Rick Oliver sells repros for about 6 quid a set) and you may possibly need to replace the inlet rubbers depending on their condition. I'd hate to think how much a workshop would charge to do this.

Cheers, all good advice. Not sure about rubber flap - will investigate that next and then undertake check for electrical issues including cdi, but just fitted new reg/rec so it's not that. I know it needs re-jetting (it's on stock 110s), but I have been postponing this as I have tried and failed before to get to the front carb inlet rubbers! Never ceases to amaze me how something works one day, then doesn't the next. But then I'll probably be saying that as I make my way to the pearly gates!

Possibly air-leak somewhere, airbox / carb rubbers not on right.

are the clocks / restriction circuit connected ok else it will missfire when revving over 5500rpm

Problem resolved. Without any time on my hands I found a bike shop in Slough with an owner who used to race VFR400s back in the day. Paid him to fit some new jets I'd purchased from Rick O to replace the stock items which I believe cause bike to run lean. In taking the carb apart he found someone had been there before and fitted the right size jets (so £10 wasted there!) but did find cracked spark plugs and blocked pilot jets. I also bought new battery. So its cost me a few quid in parts and a bit more in labour, but it's like a new bike now. Looking at the top of this thread, my advice to IanNC30 would be to check pilot jets are not blocked.