High idle to 4,000 rpm

[Fizzy]

Hi all,
I have an issue with my daughter's UK model GS500 2002 K2.
It's showing all of the typical symptoms of an air intake leak.
When it starts cold on choke it revs fairly high but if you reduce the choke lever it tends to stall out.
Once it warms up the fun begins - the revs rocket to 4k rpm when revved and refuses to drop.
Two ways to force the revs down, either slip the clutch with the brakes on and the revs behave until you rev it or switch off and start the bike on normal idle again.
I have had the carbs off and fitted refurb kits on both taking care to clear it thoroughly and checking the jets. O rings replaced. Diaphragms checked - no rips or leaks. Sprayed the intake boots with spray alcohol as recommended - no sudden jump in revs so I assume there are no air leaks. I didn't separate the carbs so I hope the balancing is still ok.
Throttle cables are well adjusted and in good condition.
I had the cylinder head cover off and adjusted the exhaust shims down a size but it was still a bit tight on the valve, the problem remained.
Just so you know everything, there is a small leak on the exhaust can, but not enough to even notice the noise.

So here's the freaky thing though.

Engine revving at 4k rpm, if I disconnect the left cylinder spark plug the engine runs perfectly. Idle is good, no sudden increases, revs really smartly, not even a lumpy engine like you would expect with one cylinder down.
If I reconnect the HT lead it starts playing up again. It's a real head scratcher.
If I do the same with the right hand spark plug it runs lumpy as you would expect.
I don't know what to make of this. It idles, revs, returns to idle. No issues at all. All on 1 cylinder (right hand).

Before I chuck the bike at the local garage mechanic does anyone have any ideas?
Could it be left exhaust valve shim is miles out?
A more serious fault with the cylinder?

I'd appreciate your thoughts before I start ripping the guts out of the motor.

Thanks all


Edit : Forgot to add, both spark plugs are showing slightly white on tips, so a little on the lean side?

[mr72]

Yeah probably lean, and probably "idle speed" set too high, probably because if you set it lower it doesn't want to run, because it's lean.

I could put an auto-responder on since this is basically 99% of what I post in this forum these days.

All you new folks, your idle speed is set too high. Probably trying to work around some other problem, which may be lean pilot mixture, which may very well be due to a vacuum leak, but you don't know until you fix it.

[Meukowi]

had to readjust my mixture screws today after mainjet/filter change, ran very badly.
1.idle speed screw all the way down
2.choke on and started the engine
3.closed both mixture screws til it almost stalls
4.opening them to the sweet spot where it runs the best.
this worked for me, i guess i've got stock pilot jet, since dj kit didnt include pilot jets.
Anyway, now she runs better than ever, with 134main and needle clip on third slot (middle)

[mr72]

1.idle speed screw all the way down
2.choke on and started the engine
3.closed both mixture screws til it almost stalls
4.opening them to the sweet spot where it runs the best.

Yeah this is the way to do it as long as "runs the best" is "idles the fastest".

Keeping the idle speed screw all the way out (throttle stop closed) is how it's supposed to run.

[Fizzy]

Hi guys,

Some feedback for you

It was the pilots. It's certainly running a lot better but needs some fine adjustments I think. I will get my daughter to run it through the week and check the spark plug condition next weekend.
I backed the pilots out three full turns and then played around with them until it was ticking over nicely, sometimes a bit erratic, but nothing over 1,500 rpm

Thanks for the guidance. You just saved me a load of cash.

Ride safe!

[mr72]

3 turns out is too much, indicates you need bigger pilot jets.

and idle 1500 is way too high.