Fire in vac line

[cwdavis]

Just finished doing a carb rebuild on my 81 gs650. and am very happy with the way its running. idols strong and the throttle responds well. but when watching it idol i noticed in the second carb where my vacume line attaches to the petcock the line is a clear one and a bright light would light it up, or a fire run through the line. its random and when it dose it the engine seems to miss but quickly recovers and continues to idol fine. what dose this mean and what do i need to do to correct it? thanks

CWD

[tkent02]

Your intake valves might be too tight, or the idle may be too lean. Either can cause a backfire through the carbs. Could be a burnt intake valve too, maybe something else.
But this can only ignite the air in the vacuum line if there is fuel in it, which there shouldn't be. The petcock diaphragm must be leaking fuel down into the line.

[Steve]

I agree with the small leak in the petcock diaphragm, allowing gas into the tube, which occasionally gets ignited.

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[1_v8_merc]

The 750 used to run so rich, I could use the flames coming out of the tail-pipe for a reverse light in the PM!
(which I actually did briefly, before settings the carbs correct.)

[Killer2600]

I wouldn't worry about your petcock leaking unless you see the gas running down the vacuum line. Backfires don't always chase fuel, ever seen an old carbureted car backfire out the carb? Fire will chase oxygen and fuel alike and doesn't like being confined.