Question about 1982 Yamaha XS 400

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About after a min of the bike running gas starts flowing into the air box, its alos getting into th oil

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Your bike has a vacuum operated fuel petcock....so there is a vacuum line from intake that acts on a diaphram in the petcock to open the fuel supply up upon startup. I suspect the diaphram is leaking so when it lifts the diaphram to admit fuel to carbs, the vacuum is drawing raw fuel down the vacuum line flooding the intake.
Have a container handy, remove the vacuum line from petcock...check for fuel leakage....now manually open the petcock by setting it to the PRI for "prime" position...you will likely see fuel leaking out vacuum port.
Kits to rebuild the petcock are readily available.

Posted on Mar 22, 2011

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First check to see if the oil level in your crankcase is too high. If you have a vacuum petcock malfunction (hole in the diaphragm) or if the fuel petcock is left in the prime position and a float needle sticks, then you get you get gas running into the cylinders, past the rings, and into the oil.
When you start your motorcycle the oil/gas mixture has no place to go, the crankcase is too full, and the mixture exits through the crankcase breather into the air box where it will ruin a paper air filter. Always turn your fuel petcocks off when not driving. If the petcock diaphragms are bad rebuild the petcock with K&L rebuild kits or replace the fuel petcocks with new ones. If the float needles in the carburetors are bad or sticking, take the carburetors apart, clean them, and replace any worn or damaged parts You will have to change your gasoline saturated oil for it will no longer properly lubricate your motor.
Just a note: in 1983 brand new Honda's would fill there engines in gas. All the components were clean and new. This can happen to any machine in any condition at any time. When not riding always turn off the fuel vales never leave them in the prime position.

Posted on Mar 22, 2011

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Posted on Jan 02, 2017

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