smoke on start-up and finicky carb


5 replies to this topic
  • autogami

Posted December 18, 2007 - 07:31 PM

#1

I have a 99 yz 400 F. It smokes on start up and the carb seem finicky. I have just done a valve job and rebuilt the carb. I live at sea level and dropped the needle clip on groove to allow more fuel due to cold temps. Carb has always been finicky. I want to sell the bike, but need to know if the problems I see are common or not. I have done everything I could think of to elliminate acceleration lag. HELP.

  • 642MX

Posted December 18, 2007 - 08:20 PM

#2

I have a 99 yz 400 F. It smokes on start up and the carb seem finicky. I have just done a valve job and rebuilt the carb. I live at sea level and dropped the needle clip on groove to allow more fuel due to cold temps. Carb has always been finicky. I want to sell the bike, but need to know if the problems I see are common or not. I have done everything I could think of to elliminate acceleration lag. HELP. By the way...anybody want a 99 yz 400 for a couple grand....new plastic, bar ends, and usfs silencer.


Becareful advertising in the forums..... :banghead:

There is a classified section that is free to list your bike.


To answer your question, smoking on start up is a sign of worn valve seals. When you shut the bike off, the oil leaks by the seal and drips on top of the piston. When you start it, its burning the oil off the piston and then the smoking stops as soon as the piston cooks the oil off of it.

If you just had a valve job with new seals, its possible that who ever installed the valves may have torn the seal.

The YZF carbs are pretty easy to tune, providing that they are spotless inside and have no air leaks. You may want to check that.

  • grayracer513

Posted December 18, 2007 - 09:02 PM

#3

What color is the smoke?

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  • USED YZ426F

Posted December 19, 2007 - 08:56 AM

#4

I have a 99 yz 400 F. It smokes on start up and the carb seem finicky.


Like Gray asked, what color is the smoke?

Blue is normally due to oil burning in the combustion chamber. You would need to determine where the oil is coming from and address that (worn rings or damaged/worn valve seals are two likely areas).

Black is normally due to an excessively rich fuel air mixture and may explain why you think you have a finicky carb.

  • grayracer513

Posted December 19, 2007 - 09:35 AM

#5

Blue is normally due to oil burning in the combustion chamber. You would need to determine where the oil is coming from and address that (worn rings or damaged/worn valve seals are two likely areas).

If the smoke is "blue" (much more of a white, IMO) you next have to determine if it's smoke (stays visible as it dissipates and drifts away) or steam (disappears as it drifts away). Oil indicates the stuff mentioned above, and especially the valve seals, if only on start up, whereas steam implicates the head gasket. The level of one or the other fluid should be lowering as it's used.

  • autogami

Posted December 19, 2007 - 12:20 PM

#6

thanks for all of the replies. The smoke smells of oil, so I suppose that it is a leaky seal. Once again...another satisfied customer.

thanks all





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