Enjin runs away after start up

[Danie.e]

When I start my 650 it idles quite well for a short while then suddenly it take off and revs up quite considerable. This indicates lean but I cannot find an air leak. Standard air box with filter and all joints sealing well. Spraying carb cleaner over all the recommended joints does not make any difference.

Carbs was cleaned properly and the correct jets used as per the service manual. Floats set at 22.4mm (as close as possible) - there are no holes in the diaphragms.

Any suggestions?

[93Bandit]

This is often caused by leaking O-rings on the intake boots. Remove the carbs then remove the intake boots and inspect the O-rings. If they've never been replaced, they're probably bad and should be replaced. If the intake boots are original, you should probably replace them as well.

[Steve]

How long from start-up to the runaway condition?

Is the "choke" still on?

Has it ever run properly since you have had the bike?

Other than the recent carb cleaning, has anything else changed?

You mention "correct jets used". Does this mean they were changed? What happened to the original jets?

.

[Gorminrider]

steve's questions will help narrow it down for sure but I'd guess a few easy ones based on what you say.

"Choke" (cable not working)

or throttle setting...turn down the throttle setting (the big knob that is between the carbs)... also the cable...sometimes turning the handlebars increases throttle due to bad routing of cable.

[tom203]

Likely the "choke " plungers not closing completely....maybe sticking? or cable issue?

[Danie.e]

Thanks for all the replies. I think I have found the problem. I recently purchased a set of inlet manifolds on the internet and found that the thread for the cap screws on the side strip very easily. I re-tapped and put in larger screws with a decent copper washer for better seal. On installing the "new" inlet manifolds, I noticed that the actual aluminium part formed a very slight arch (nearly not visible) when tightening the to bolts due to the O-ring not being compressed completely flat against the cylinder head. Being ignorant, I assumed that it was a good thing and that it was "really" sealing well.

The problem is quite simple: due to the low quality of Aluminium used, with the slightest heat, the aluminium expands much quicker than the head hence the engine running smooth for a short while and then takes off simply because the seals do not seal properly anymore.

May this lesson also be a warning to all that the "cheap" inlets currently being offered on E-bay, ALi Express, Picklick etc. are all from the same source and profess to cover the 1980 to 1983 range of 550, 650, and 850 models. These items are pure rubbish and should be avoided. Close scrutiny of the pictures on the internet after the fact show that they are all from the same source. All the "850" ones on offer actually have 28mm instead of 32mm inlets which is false advertising.

I added a picture of an "850" inlet as I also bought a set for my 850 which I am going to return as they are the same as the ones I tried on my 650.

[Gorminrider]

That's Interesting and a caution for sure! Good on you! Danie, for finding "it" and reporting back. (but score one for 93bandit who nailed the cause ...even not knowing you had bought new ones.)