99 YZ400F oil leak at exhaust head

5 replies to this topic
  • donnyboy32

Posted May 23, 2011 - 11:14 AM


Hello everyone. I am new to this forum and fairly new to dirt bike mechaincs. I bought a 99 YZ400F last year and it was running terribly. A valve adjustment and rebuild of the carb made it run like new but I have a new problem.

There is oil leaking (about one drip every 10 seconds while running) from the exhaust head where it goes in to the engine. My question is, should this even be happening at all? Is it simply a leaking exhaust gasket? I replaced the gasket last year as you have to take the exhaust off to change the oil filter. Like I said, I am a novice so it could be simple but I have no idea. Thank you for any response


  • silentv311

Posted May 23, 2011 - 05:24 PM


If the bike has never had a top-end job, or at least for some time, could be oil blowing by the rings because of normal ware. Also, be sure the oil level is not over filled.

  • donnyboy32

Posted May 23, 2011 - 05:40 PM


was worried it may be blowing by piston rings. Just started doing this and no the oil level is not too high. Gets too low in fact because it is leaking and I have to add some. Probably has not had a top end job as the last owner had not done any maintenance. Thanks for the reply. Just to make sure, oil should not even be reaching the exhaust head and gasket correct? Sry if this is a dumb question but I have learned everything through the clymer book. As good as it is for repairing stuff it does not help much with understanding or diagnosing problems. This is not simply a leaking exhaust gasket? Thanks again

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • baxterj787

Posted May 23, 2011 - 06:27 PM


Replace the decompressor arm seal. Or find / replace the o-rings around the decomp plug if the bike has an autodecomp system and the PO has removed the manual decomp assembly.

Look at the head about 1.5 - 2.0" above the exhaust port and you will see the leak. You will have to remove the valve cover to fix this. Look at the FAQ area under the heading of: Oil Leak at Right Front of Engine.

  • DeeCee

Posted May 24, 2011 - 12:59 AM


You need to determine exactly where the leak is coming from and advise as this will give some clue about the leak.

Possibilities are the rocker cover gasket, the oil feed into the head, and the decompression fitting.

  • donnyboy32

Posted May 24, 2011 - 01:13 PM


Hey guys, thanks for the replies so far.

Just to clarify. The oil is definitely leaking out of the exhaust head and not out of the decompression port or the head gasket. It is not dripping on to the exhaust, it is coming from the exhaust head. I plan to take a short a video or some pics and post it here so hopefully I can get an answer. Would rather not start a top end rebuild right now but if I have to I will.

Like I said earlier, the bike runs great. Are there any other symptoms that would be present if it was a bad piston ring? Smoking exhaust, loss of compression, or anything like that. Literally besides the oil leak the bike runs great but its got me worried. Thanks for everyone's help.

Related Content


yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies

yz400f throttle bog by yz400fmaniac

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • * * * * *
  • Hot  28 replies

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

Michigan Motocross Tires by 288yz450

Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply

Help with 2000 yz426 by weekend woods racer

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  36 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.