yz 400 backfire and smoking

4 replies to this topic
  • roostyrm250

Posted November 26, 2010 - 10:13 PM


not sure whats wrong and ive been looking all over this forum for a clue to what needs fixed???

Edited by roostyrm250, November 28, 2010 - 01:49 AM.
to long for a newbie

  • brentn

Posted November 28, 2010 - 03:22 PM


Backfiring can be too lean, or a leak in the exhaust. Either the bolts to hold the header are loose, the gasket is F*ed or you have a crack in the exhaust.

If it's smoking, usually the color of the smoke can tell you what's going on. White usually means that coolant is burning, blue is usually oil that's burning, and black usually means a really rich condition.
You can rub your finger inside of the silencer if it's been smoking for a while and check to see what kind of soot is in there. Dry black soot is normal, but if it's greasy and oily and does not dry when you rub your fingers together it's oil. If it's wet it could be coolant. That's just a small step into trying to find out what's wrong, the best way is to check your coolant and oil capacities after you ride the bike for a good couple of hours. Measure before and after.

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  • Deadend

Posted November 29, 2010 - 09:22 AM


if your valves are out of spec on the bike it likes to backfire a lot as well. Remember though, that these are the first of the high compression 4strokes, and some backfire will be occuring naturally on decel even on a properly jetted bike (my experiences from sealevel to 3k feet )

Edited by Deadend, November 29, 2010 - 09:25 AM.

  • roostyrm250

Posted November 30, 2010 - 08:07 AM


well im going to get some new pilot jets for this bike and see what works. im gonna try stock size first then go up a size if the issue continues. thank you for all you help guys, this is my first 4t and its all a learning process for me at this point.
ill keep yall posted on what happens when i put the new jet in...:thumbsup:

  • tech24

Posted November 30, 2010 - 08:19 AM


looking in the jetting database I see stock - 185 main with slip-on or full system but all have 45 pilot jet which I assume is stock and 1.5 to 2.5 turns on screw. Not sure if that helps any.

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