Over revvvvvvv... like full throttle at idle

Sharkins123

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Hey all,

First post... after reading a million threads first. My boy bought an 82 XS400 (I think). And as true bike neophytes, we've screwed 10 things up on it and fixed half of them!

Here's my latest, weird issue maybe you good folks can help with. When we start the bike it kicks into over rev as though the throttle is fully open. Throttle/cables, etc are all mechanically sound...

Thanks for any insights you may have. This machine will run one way or another... even if it costs thousands! It's an illogical matter of pride at this point.
 
Did you check the idle set screw between the carbs? Enrichener/choke stuck?

If you removed the carbs and took them off the bracket, they would definitely need to be synced up again. The master carb could be adjusted way too high.

Vacuum leaks at the boots will make idle high. Replacing the boots is practically required when you start to dig into these machines. If they have any cracks at al, replace them.
 
Does that bike have a vacuum petcock? That's what happened when mine failed. Poured gas straight in.
 
Carb would stop petcock from doing that if float correctly set.
I have no idea what I am doing 89% of the time, not a mechanic. I do know I was going about 25mphs and almost ended up in the back of a car cause the engine went wide open. When I took off the vacuum line between the petcock and carb holder boot, gas was pouring out. Capped that on both sides and it runs as advertised. So now I just drive in the prime position. Probably a better way to do it.
 
Yeah, if the diaphragm in the petcock degrades you can get fuel out the vacuum line, bypassing the carbs entirely.
 
I have no idea what I am doing 89% of the time, not a mechanic. I do know I was going about 25mphs and almost ended up in the back of a car cause the engine went wide open. When I took off the vacuum line between the petcock and carb holder boot, gas was pouring out. Capped that on both sides and it runs as advertised. So now I just drive in the prime position. Probably a better way to do it.

Wow that is pretty scary. Considering the age of these motorcycles I might just drive in the prime position without the petcock vac line also. Something to consider.
 
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