high idle issues

[graciesdadpaul]

Hi everyone.

ive posted on here a few times now about the running issues I've got with my Gt750J. I had the crank rebuilt by Chris Appleby engineering in the south of England and its had a rebore, new pistons etc. I've had various issues with the carburation and ive done a lot of research and believe that the carbs are clean and set up correctly with the correct jets and the needles in the positions as per the manual.

Prior to this I had a misfire on the right hand cylinder which I traced to a blockage in the pilot system and now that seems to be running ok.

its now started to run on a high idle once it warms up a bit (around 2000rpm) which is being caused by the left cylinder as it immediately goes back down to 1000 if you pull the speak plug cap off that cylinder. Ive checked the throttle cable adjustment and the cold start valve is closing fully so its not as far as i can tell getting too much fuel. Tweaking the throttle stop screw makes absolutely no difference so i don't think its that.
Im at a loss as to what it could be and I'd be very grateful if anyone has any ideas.

Ive also been trying to find a garage locally to me that will look at it but none of them are interested and seem scared by it. Has anyone got any recommendations for a garage somewhere around the Buckinghamshire area or north of London that know these bikes.

I've spent a small fortune on this bike and I just want to get out and ride it

thanks chaps.

[Gary66]

Split rubber from carb to airbox maybe?

[teazer]

Do a leakdown test to determine where the air leak is. It may be as simple as a split or non sealing rubber boot but could also be a leak at the base gasket. Use soapy water to see where it's blowing bubbles.

[graciesdadpaul]

thanks for the responses. It got new rubbers so im certain its not that, im not sure how to do a leak down test but I will look at the base gasket suggestion. My other thought is it could be something to do with the choke on that carb but i'm not sure how i would test that.
I'm worried it could be to do with the crank seals but that seems unlikely as it was newly rebuild and its only been started a few times since and not ridden as I cant get the thing to run correctly.

[teazer]

Look online in youtube for ways to make your own tester and how to use it. Do not use a 4 stroke leakdown tester and do not use more than 6-10psi.

Yamaha RD twins have a lab seal so both cylinders have to be sealed, but Suzuki do not, so you can test one cylinder at a time.

I use a MotionPro test kit, but there are much cheaper options available.

Here's one video with a home made low cost test set up: https://www.youtube.com/watch?v=eGzJz302XiI&t=29s

[barney01]

Just a few ideas.
You say new carb rubbers. Just confirming you have the correct carb to cylinder rubbers. These are specific to the J with a notch at the top.
The union clip that secures the carb to the rubber is also J specific and is wider and more robust than later models.
If the choke valve is not sealing or is stuck open in the bore the result would be the opposite to your symptom i.e. running normal on choke enrichment but when warm would cause that cylinder to stumble due to overfuelling.
Air getting in is definitely the likely cause.
Good luck

[graciesdadpaul]

So to update on progress, ive looked at all the connections between the carbs, head and airbox and these are all fine. I've since discovered that the engine wants to run a 2,000 rpm when it warms up and weirdly if you disconnect any of the spark plugs so it can only run on two cylinders it goes back down to a more normal tickover .
I can't think of anything that could cause that so i'm at a bit of a loss now.

[Wolfgangh]

It is normal that idle revs go down when you remove spark from one cylinder. Have you tried spraying brake cleaner on all rubber boots and their seats on the cylinder and carb?

[asco58]

Have you checked the vacuum pipe from the fuel tap to the carb? If it's leaking, split or disconnected it can cause a high RPM Idle.