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400ex rebuild problem

1931 Views 5 Replies 4 Participants Last post by  mizzymatt
I have a 400ex i bought and it smoked like a chimney it was stock bore sent it to a shop ordered a 11to1 416 kit and got the cylinder bored slapped it back together with a stage 1 hot cam runs great has tons of power but after about 15 minutes of riding it slightly smokes oil out of the exhaust could this be the valve seals, oh i also lapped the valves.
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Yeah, leaky valve seals can add some oil to combustion & smoke a bit. Generally speaking, bad valve seals will let oil run down the valve guides when the motor is shut off & smoke a bit on startup. While the motor is running, leaky seals will leak the worse when you chop the throttle on deceleration. When you get back on the throttle it may smoke a lot briefly then clear up until you chop the throttle again.

Look for evidence on the spark plug. Oil will coat the plug and burn off, leaving shiny black & irregular looking deposits on the plug (not sooty, that is unburned fuel) and be most noticeable on the end of the threaded plug body where plug temperatures are cooler.

There can be some residual oil in the exhaust burning off when it gets hot too, since it smoked before you freshened the cylinder and may have some old oil in there yet.

A motor may also start smoking shortly after a topend refreshing if the cylinder is not thoroughly scrubbed clean of all machining/honing grit before assembly. Believe it or not, this is the number one cause of all oil burning issues after rebuild!

Diagnosis is up to you... hope this helps.

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