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yamahauler44

YZ400 smoking when it first starts until it warm?

11 posts in this topic

i am noticing that when i start my 1999 YZ400F it behaves fine and is easy to start has plenty of compression, has plenty of power, ect. but if i rev the motor before its warm it smokes a little, not a huge plume or anything just a little, if i dont rev the motor untill it warms up there is no noticable smoke. have any of you had this before? is it normal or not? give me the run down. thanks in advance.

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In the automotive world, this is caused by leaking valve seals. I wouldn't think the dirtbike world would be too different...

Overnight a little oil seeps by the valve seals and drains into the cylinder. When you crank it up that oil burns off, then there's no smoke.

If it bothers you, change the valve seals.

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I would say it's normal wear, but not necesarrily normal.

I wouldn't really worry about it though until it starts burning a lot of oil.

If I'm wrong about this, the guys will reply here shortly with the correct responses.

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I dont know. It dosent sound bad but my 400 dosent do it. If it gets worse you should check your seals. Oil can get caught inside the exhaust. Take that off and if there is oil id go into the motor. Thats what I would do. Please correct me if im wrong fellas!

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My 426 smoked on start when it had not been run for a few days to a week. I replaced the oil seals and the piston. The rings were fine but the valve stems had a build up of sludge from the leaking oil.

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thanks guys. i was wondering if it could be because of some dust or grit getting by the air filter? what do you think?

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I would say valve seals as well. As OvrrDrive said that the cause in the Automotive world, it would be the same principle on these 4-Strokes.

Keep an eye on your oil level from time to time.

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My 400 used to blow out a bit of whiteish greyish smoke on startup, especially after oil changes. Call it weird, but it stopped when I switched over to Shell Rotella T.

It only takes a little oil to cause alot of smoke. Normal.

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