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yz400fsmasher

98 400f smoking on start up

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alright my good ol' 98 400f is burning oil and blowing smoke out the exhaust. mostly on a cold start, not so much when its warmed up. could it be bad valve guides or piston rings?

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smoke on start up is usually valve guides but if it has not been rebuilt in a while then it could be a mixture of both rings and valve guides.

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hey buddy.now i dont know how true this is but i have been told it is a possibilty.if you have done a oil change and have over filled it ,it could cause valve seals to swell and oil will burn.sorry if this doesent help much but maybe gray could tell us wether its true or false and if its true explain it better then wat i have because i carnt remember all of what i heard but it was to do with overfilling oil and valve seals swelling.

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if you have done a oil change and have over filled it ,it could cause valve seals to swell and oil will burn.
That falls under the general category of Bovine Scatology (BS for short).

Overfilling beyond the absolute capacity of the oil tank causes there too be too much oil in the crankcase, which causes the crank to dip into it and throw it on the cylinder wall, overwhelming the oil rings. In a '98~'05 model, it also means that the excess is going to the tank is being dumped on top of the head via the pressure balance vent, causing oil out the breather and taxing the valve guide seals.

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when you say the absolute capacity do you mean filled all the way to the top? i did just change the oil but put the 1.80 u.s. ounces the manual said to put in.

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My bike (04) blows black smoke upon start up just a little bit though. I had a leak down test done on it and it is blowing past the rings. Good luck

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Absolute capacity means all the tank will hold. In a YZ400, you reach this point at somewhere around two quarts added after an oil change.

Also, your manual does NOT tell you to add 1.8 Qt after an oil change. 1.8 is the total amount of oil in the system. You can expect to add that much to an engine that is being filled for the first time after being built or rebuilt. The amount you put in after an oil change is 1.7 Qt if you change the oil filter , and 1.6 Qt if you don't (page 2.1)

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My bike (04) blows black smoke upon start up just a little bit though. I had a leak down test done on it and it is blowing past the rings. Good luck

Black smoke indicates and overly rich condition? Blipping the throttle with the choke on?

Blueish smoke would be oil burning.

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Black smoke indicates and overly rich condition? Blipping the throttle with the choke on?

Blueish smoke would be oil burning.

Top end is shot so I keep having to richen up the bike as the compression goes down the motor will pull in less fuel causing the bike to lean out.

It only smokes at start up after she warms up no more smokin and no blippin with choke on either.

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