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My YZ400F runs badly when i ride it for about 20 minutes

I just bought this bike a couple weeks ago and was told it had sat for around 3 years in the guys basement because he kind of just got out of riding he had the engine checked out by the local shop and they could still see the honing rings in the top end but i guess they put a new piston and rings in it just because they had it apart and so it wouldnt have to be done so soon in the future. anyways i got it home and it wouldnt start so we took the carb apart and all the jets were plugged so we cleaned them and put it together. it ran good for one night.. the next day i started it and it ran good for about 45 min. and it started losing alot of power and bogged down alot when i gave it throttle. so we took the carb apart again and a few jets were plugged again. cleaned it up and put it back together. cleaned the tank because it had some yellow stuff at the bottom and put a new air filter and a fuel filter on the gas line on it. but still when it warms up (now after riding prob. 15-20 min) it will run like its running lean and bog down really bad and pop and crackle. the oil looks new and is good, and the coolent looks fine. we took the head cover off and measured the clearence for the valves and the exhaust valves are in spec but the intake vlaves need adjusted. i dont know how it only runs bad only when it warms up though? also we set the air-fuel screw to factory specs. im not sure what it could be. help would be much appreciated! thanks!

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  • Brad Brubaker Oct 27, 2012

    thanks all for the help! i really appreciate it! i just took it to the local yamaha dealer today to have them see whats going on. they said they'll have it running like a top by the first of next week. when i get it back ill let u guys know what was wrong. thanks again!

  • Brad Brubaker Dec 04, 2012

    took it to the shop and they rebuilt the carb and adjusted 4 of the 5 valves haha. ended up being the valves needing adjusted. runs like a ape now! thanks for all your help!



4 Suggested Answers

  • 2 Answers

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Posted on Jan 02, 2017

  • 333 Answers

SOURCE: hi i just got a 1988 gsx600f i put new plugs in it

Was the bike was running OK with the old plugs?
Try these checks:
1:Run the motor for few minutes and switch off.
2:Pull the plug lead off the offending cylinder
3:Fit another(known to be working) plug to the lead(leave existing one in the motor), start the bike and check for spark.
4:If there is spark, pull the original plug out and:
(a)if it is wet change the plug.
(b)If it is dry service the carbs
5:If there is no spark, it may possibly be wiring connections etc and you can get back to me. I doubt it is a coil issue as 1 & 3 will be fed from both sets.

Please get back to me if you require further assistance.

Posted on May 27, 2010

  • 1508 Answers

SOURCE: pw 50,

cover up the drilled holes this can cause problems, why was a new carb fitted? did the shop have it running?
not likely rings .
check carb for flooding and correct jet sizes.
check safety kill\start\run switch for shorts.

Posted on Feb 25, 2011

  • 3567 Answers

SOURCE: Canadian bike 1991 Yamaha fj1200,50,000kms.

Chances are there is a water build up in the carb float bowls. The water comes from condensation in the gas tank. Try draining the carbs. Another possibility is carburetor main jets partially plugged up. You must clean the carbs if plugged jets are found to be the problem. Please rate my answer. Thanks. tombones49_152.gif

Posted on Jun 02, 2011

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