YZ400F carburetor help

7 replies to this topic
  • jeremiah.stewart

Posted June 30, 2012 - 01:21 AM


Ran good all summer, then randomly started bogging when i gave it throttle and ran as if lean, and backfired alot. so i cleaned all the jets, and the acc. pump worked properly. still ran horrible. enrichened the fuel screw about 2 turns, no help. so i looked up the issue on this forum. i then checked the valve clearances, all were to tight, re shimmed, no help. then took offthe mufflers end cap, there was no packing at al, repacked it.. nothing. then took the carb apart again, saw that the acc. pump diaghram was torn, replaced it. the bike was now fixed, ran way better than ever. then a couple days later, running lean again, i enrichened the fuel screw, it fixed it for a couple minutes, enrichened more, ran great again for a couple minutes, but kept going lean. so i thouroghly cleaned the carb again, nothing. so i put the needle to a more rich setting (two notches down on the needle). no help. i did check the exhaust for leaks and the intake boot, and the hot start crap, no leaks.
im completely out of ideas, other than jetting.

any help would be greatly appreciated.

  • RasmusDK

Posted June 30, 2012 - 03:45 PM


There is an o ring that seals around the fuel screw check if thats ok, just take it out and you can see it, it can be stuck in the carb house tho..

  • jeremiah.stewart

Posted July 05, 2012 - 08:43 PM


The O ring on the fuel screw is good, the holes are clear, i rechecked all of the jets, one had a little bit of dirt, cleaned it out. still runs bad and back fires. before i did this though, i started it with choke, turned off the choke, and started riding before it warmed up, and it rode great, took it for a couple minute ride, and it ran superb. went home to get boots and gloves, went outside started it the same way, but ran horrible again. its hard to figure it out, because of how random the issue is..

  • Jsladwig

Posted November 18, 2012 - 10:50 AM


Jeremiah, Ive been having some issues with my yz400f running rough and not idling. I ran through all the same troubleshooting as you and eventually made it to the TPS. I tested it with a Digital Multimeter and found no variance in the resistance i.e. BAD TPS!!! I am tracking a replacement down today and will let you know if it fixes everything. Although our symptoms are not the same maybe the solution is and it's a quick thing to check. Good Luck!

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  • Jsladwig

Posted November 29, 2012 - 10:10 AM


New TPS came in last weekend and went on.... Idling issue fixed! :applause:

  • beercan

Posted December 01, 2012 - 11:22 PM


i also have a 400 and have had problems with that model cr flat carb. one thing to check is if you have a stock exhaust check your gaskets the one inbetween your header and muffler can wear and not really show it its 18 bucks for a new one this can solve a lot of problems i have done this and bike ran great it wasnt the carb. but i have allso had to replace the clamps on the carb they can stretch or bend and you wont be able to get a good seal there a few bucks a piece. now i did rejet my carb i went a size bigger than stock on all of them this helped a lot allso oh my main problem was way to lean so rejeting helped. if you can start your bike and do it with the air intake boot off hold your finger over your air jets if it starts running better than your jetted to lean if it does nothing then your probably having a issue i havent had yet and ill agree with Jsladwig. hope this helped iv missed a lot of good riding days do to these carbs there a little finiky

  • jeremiah.stewart

Posted December 06, 2012 - 11:46 PM


well thank you guys, but i did figure out the issue a while back, i just forgot about this forum. I tested the TPS, it checked out good, although it did fix the bikes issue when i unplugged it, but only for a while. So i figured it could only be one last thing, so i bought the cheapest CDI on ebay. it worked perfect, i was so relieved. too bad i had to sell it for college right after that though..

  • vic2340

Posted December 07, 2012 - 11:07 PM


sounds to me like it could also be valves in need of shiming or an exhaust leak

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