sounds like an air leak...

[Guest]

Hey gang,

Hoping for some help in diagnosing the following problem with my 1980 GS400.
Starts no problem, is all fine until it warms up (5-10 minutes of idle) then the throttle gets stuck at 4000 rpm. All I can do is play with the clutch at the stoplight to keep it lower.
There is some damage to one of the boots between the airbox and the carbs, but I passed a propane torch (unlit) around there while it was running and found no change.
Any ideas where I should go from here? (Obviously I should replace the damaged boot, but without having a parts shop close by, are there any other tests I can try?)
thanks,

J

[t3rmin]

Hey gang,

Hoping for some help in diagnosing the following problem with my 1980 GS400.
Starts no problem, is all fine until it warms up (5-10 minutes of idle) then the throttle gets stuck at 4000 rpm. All I can do is play with the clutch at the stoplight to keep it lower.
There is some damage to one of the boots between the airbox and the carbs, but I passed a propane torch (unlit) around there while it was running and found no change.
Any ideas where I should go from here? (Obviously I should replace the damaged boot, but without having a parts shop close by, are there any other tests I can try?)
thanks,

J

Hmm.... haven't heard of the propane method for checking for intake leaks. Interesting. Sounds much less messy.

[rustybronco]

propane enrichment test, haven't heard of that in years it was used to check the a/f ratio of carbs at idle. but remember propane ignites...

[nabrams]

I was hugely surprised by how much better my friend's bike idled and rode after I sealed up the leaks between his airbox and carbs. So, yes, I'd say that's the most likely culprit. If the plug that corresponds to that damaged boot is white, then that is almost certainly the culprit.

[Jagir]

I was hugely surprised by how much better my friend's bike idled and rode after I sealed up the leaks between his airbox and carbs. So, yes, I'd say that's the most likely culprit. If the plug that corresponds to that damaged boot is white, then that is almost certainly the culprit.

sealed with what?
j

[nabrams]

Ideally, you should seal it with a new, undamaged boot - but, for testing purposes only: Use silicone sealant (let it dry for 24 hours) on the suspected areas and re-test.
By the way: No venom intended here, but this has been discussed a million times on this forum - Have you used the search feature? It will save you tons of time and money.

sealed with what?
j

[chef1366]

If the head on the 400 has o-rings between the engine side of the carb boots you should consider changing those.