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Tracing Down a Vacuum Leak

crazyramen

crazyramen

Forum Apprentice
I have a hanging idle on my '79 750 with VM26s. I've read a few threads on it and it seems like it's usually caused by a vacuum leak, but I can't find it for the life of me. It'll idle at ~1500-1800 until I blip the throttle and it drops down to 1000.

I check/adjust the valve clearance each spring (less than 2000 miles on in since then) and I cleaned/tuned the carb this spring also. I've tried the propane trick to track down the leak to no avail. If I put it in prime and disconnect the vacuum tube from the petcock the revs will drop very low and it dies, then if I plug it with my finger it goes back up to higher than normal.

Modifications:
Pod filters with air correction jets,
larger main jets (pilot jets and needle position stock),
4 into 1 exhaust,
new (cheap aftermarket) petcock,
new intake boots with integrated o-rings,
rebuild choke plungers

I'm kinda out of ideas at this point and would appreciate any advice
 
A poor vacuum sync can cause a hanging idle.

Those aftermarket intake boots raise a red flag to me. The pods could also contribute to a lean mixture.
 
Nessism said:
A poor vacuum sync can cause a hanging idle.

Those aftermarket intake boots raise a red flag to me. The pods could also contribute to a lean mixture.
Click to expand...
Ya know now that you mention it I definitely forgot to vacuum sync it after I replaced the choke rubber! That'll be my first check. That actually checks out pretty well. I can't quantify it but it feels like it's lean in just one cylinder.

Yeah I was pretty disappointed when I noticed that the intake boots I ordered from caltric had the o rings molded in.
 
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