High Idle - Curvy.

Hi Guys,

I have a ill running '99 model. The problem is that once started the bike will idle for a few seconds then gradually rise untill it wants to sit at about 5000rpm idling.

All the rubber joints between the inlet/carb and carb/airbox are fine with no splits, and I have checked they are tightened.

Carbs had a good clean last Winter and I've done 4 laps on track since then.

I have noticed that with the airbox off, and this problem occuring, that once turned off for about 10 seconds after that vapour (I presume fuel vapour) rises out of the rear carb, but not the front? Could this be anything to do with it? I would have if they should both do the same?

Cheers for any help.

[andrewsmith]

Stuck choke plungers, would be the RPM rise and possibly a stuck idle adjuster.
The vapour I'll plead insanity at that

So how did the first ride go?

Hi Andy,

Sounds like a plan, I'll have a look at them tomorrow, cheers!

The first ride went well (in terms of the bike doing what it should do and not scaring me). I had to pull in after 4 laps as having finished prepping the bike at 10pm the night before, I missed tightening the lock nuts on the gear linkage rod. It managed to unwind itself then the gear lever became wedged on the bellypan so I couldn't change gear. It had also cut out on a couple of occasions going into corners at this stage, which I think is related to this idle problem. Back in the paddock we had a quick look at it, but at this stage a big shower was coming down so thought we'd leave it to look at properly back home. I had a 1937 Norton 500 to spend the rest of the day on anyway. Would like to get it sorted soon and book another track day, so we can just go round and round and get it setup right.

[andrewsmith]

you running the curvy vac tap and fuel system?

Vapor rising up out of the cylinder means there's valves open on that cylinder. Nothing to worry about.

Originally Posted by Todge

Hi Andy,

I had to pull in after 4 laps as having finished prepping the bike at 10pm the night before, I missed tightening the lock nuts on the gear linkage rod.

This is why race orgs. require lots of safety wire. It's not so much to keep things from coming loose, since they won't if they are properly torqued, it's more to keep things that haven't been tightened from falling off.
Just be careful you don't leave something loose that causes you to crash- (you might take me out when you do!) I really hate it when parts fall off a bike onto the track too, as they can cause somebody else to crash, (possibly me).

[Biker Biggles]

5000rpm is quite high for a choke plunger problem,and your description of the revs drifting up suggests it may be worth looking at the throttle cable.Is it too tight,allowing the throttles to open and not self close?Does turning the bars cause the revs to rise?Otherwise Id be having (yet)another look for an air leak somewhere.

Originally Posted by andrewsmith

you running the curvy vac tap and fuel system?

Yes, thats all standard.

So I have have the choke plungers out, checked them over, lubricated them and put them back in. They were free moving when they came out and had a look at the whole cable / operation and saw no issues there.

I havent had the bik running long enough since to see whwether it still does it as I have new probelms!!!

When I did run it last night, I noticed the front cylinder wasnt running (cold pipe). Tonight, I checked the plug (both are brand new (4 lap old) plugs) and it was sparking ok in the HT lead. What could this be??? Its also been a pig to start (long time on the starter button) before it fires (maybe this is due to only one cylinder firing)???

Any ideas - masssively appreciated. It really is doing my head in. I thought building the bike was the hard part and using an exisiting running engine would be easy!