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I am stymied....vacuum leak.

gustovh

gustovh

Forum Sage
Past Site Supporter
Been fighting this problem on my 80 1100L 16 valve machine. Stage 1 kit and Supertrap exhaust, KN pods. I picked up a vacuum leak and hanging idle on the return leg of a ride with Sandy. I Can't Find It!!!!!! starts and idles good until she warms up then the idle climbs and climbs. Have tried start fluid checking for leak.

Am ready to pull what hair that I have left out.

What's been done.
Carbs have been cleaned 3 times.

fresh carb mounts.
polished head pipe mounting surface.
dipped and sonic cleaned/ rinsed in boiling water/ blown out each carb body (yes I cleaned them)
fresh O rings
new diaphragms
sync screws plugs cleaned polished
pods cleaned and oiled

V
 
Have you synced the carbs? How far open are the pilot screws?
 
Is it a vacuum operated fuel tap ?
Check the vac pipe to the tap and also the tap diaphragm for splits and leaks..
 
If you think you have a vacuum leak: take a propane torch, not lit, and poke it in around where it could be leaking vacuum while the bike is idling. If you find a vacuum leak the idle will go up.
 
Carbs have been synced, screws are out on average 11/2 turns.
Vacuum line to fuel tank is new, tap is not an 80s tap but has a reserve and works properly.
I used starter fluid looking for the leak.

I think that I will plug the screw holes with rubber plugs and see what happens.

I might add that I have been running this set up for quite a while.

V
 
Last edited:
As an experiment, I'd open the pilot screws to 2.5 or 3 turns open. 1.5 is typically a little lean on these old bikes.
 
Sounds like you have been VERY thorough.
A long shot but something s simple as throttle cable routing? Heat from the engine expanding the sheath around the cables?
 
steve murdoch said:
Sounds like you have been VERY thorough.
A long shot but something s simple as throttle cable routing? Heat from the engine expanding the sheath around the cables?
Click to expand...

The throttle cable actually has a little extra slack in it and there is no change to it when idle starts to rise.

Todd seems to me it would show bad seals with the starter fluid.

Well going to take them off again and go through them one more time, gotta be missing something.

V
 
Maybe a long shot here, but what about the idle speed adjustment screw/bolt? The one that pushes the throttle open, located on the carbs? I had the same issue on my 750 after I installed all new rubbers and rebuilt the carbs and sealed the airbox. Every time I'd go for a ride, as it warmed up idle would creep up. At one point it got as high as 4,000rpms. I was pulling my hair out trying to figure out why. Turns out the spring that keeps tension on the idle screw was broken in half, so engine vibrations actually caused the screw to thread in, increasing idle speed. I stole a spring off a set of carbs I had on the shelf and bingo. Fixed. That drove me bonkers for a while.
 
93Bandit said:
Maybe a long shot here, but what about the idle speed adjustment screw/bolt? The one that pushes the throttle open, located on the carbs? I had the same issue on my 750 after I installed all new rubbers and rebuilt the carbs and sealed the airbox. Every time I'd go for a ride, as it warmed up idle would creep up. At one point it got as high as 4,000rpms. I was pulling my hair out trying to figure out why. Turns out the spring that keeps tension on the idle screw was broken in half, so engine vibrations actually caused the screw to thread in, increasing idle speed. I stole a spring off a set of carbs I had on the shelf and bingo. Fixed. That drove me bonkers for a while.
Click to expand...

If only it was that easy. The problem developed before the new mounts and diaphragms.

Here is a little more feedback.
Turned the pilots out to 21/2 turns, ran the bike till warm and adjusted to lean best idle. I could turn #1/3 screws almost all the way in and not much change….this is not right……hmmmm
 
I've never had one on pods that ran without the stock pilots adjusted 2.5-3.5 turns out..... Maybe the extra cold weather having an effect? Any new formulation rules in Gas up there? :)

Have you checked the floats are sealing/opening? Any of the floats leaking (full of gas). I guess a sticky float valve could make you run lean & perhaps seems like a vacuum leak...
 
gustovh said:
If only it was that easy. The problem developed before the new mounts and diaphragms.

Here is a little more feedback.
Turned the pilots out to 21/2 turns, ran the bike till warm and adjusted to lean best idle. I could turn #1/3 screws almost all the way in and not much change….this is not right……hmmmm
Click to expand...

Well bummer. Figured it was a long shot but didn't want the chance of withholding a suggestion from keeping you from fixing it.
 
If you are spraying starting fluid, and getting no change (I always make sure I have the little Red Tube on the Spray can) so I can isolate where I spray, but if you aren't getting any change in you idle, then it is not an intake leak. The response on the smallest of leaks is definitive and fast. I asked about the valves as my GS750ES has cam followers with the locknut and square head screw, and I wasn't paying attention the way I should have, and tightened them to tight, boy did that bike run like cr@p
 
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