How can I tell when I need to redo the top end on my YZ400f?

8 replies to this topic
  • shiftracer

Posted April 23, 2006 - 07:46 AM


I am just checking to see if anyone knows any signs of a needed top end on a 1999 YZ400f?

  • 642MX

Posted April 23, 2006 - 03:19 PM


Welcome to TT. When to replace the top end depends on how often you change your oil, how clean you keep your air filter, and how many hours has the motor ran. If your 99 model has not been touched yet, then its time. New cam chain, piston, rings, pin, valve springs, valve seals, and possibly valves. While your at it, put in the 450 exhaust cam so it kicks easily like the newer bikes.

  • PK

Posted April 23, 2006 - 06:21 PM


As 642 said, it's probably due. I have an hourmeter on my bikes and it helps to keep track when you start building up a lot of hours. I can get expensive if you don't replace the parts prior to failure. You can't do a compression test on these bikes because of the compression release doing it's job, or in your case you can't kick the bike over without the manual compression release in, so base your maintenance on hours on the engine.

  • XriderR00

Posted April 24, 2006 - 02:19 PM


My '99 400f, to my knowledge, hasn't had any work done to the top end yet and the thing starts easily, has good compression, and runs great. It started 2nd kick after sitting all winter (after I turned the fuel on :thumbsup: )

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  • beasleys

Posted April 25, 2006 - 01:33 AM


I was wondering the same thing about a 01 426. What :thumbsup: is the hour mark that this stuff should be performed?

  • MotoGoalie

Posted April 25, 2006 - 05:59 AM


This might sound like blasphemy but, the bike is 7 years old as of this year. Why spend 1000 on a complete rebuild when you can just go ahead and run it into the ground and get a new bike?

Getting 7 years out of an MX engine is only something Yamaha could pull off. :thumbsup:

  • DRH

Posted April 25, 2006 - 01:34 PM


The 400's are a very solid bike. They don't last forever however, but compared to the rest of the bikes they are the best. Your best guess is by the seat of your pants. Does it feel slower? How about oil comsumption? I know we change oil so often its hard to tell if we are burning it or not. How about blow by, see any blue smoke out of the exhaust. Hard starting is another element of wear especially when warm. I rebuilt by 400 last year for the first time. I thought it was about due. When I tore in to it, I was amazed how well everything looked. It did not appear to show any signs of wear what so ever. I planned on doing the valves but they didn't even need to be shimmed. I put a new piston in it anyways because I was already there. I buttoned the thing back up and away she did go...Still haven't had to redo the valves, but I did reshim just last month, Not bad for a nearly 8 year old bike.

  • yz_for_me

Posted April 25, 2006 - 04:13 PM


...but I did reshim just last month, Not bad for a nearly 8 year old bike.

Not bad indeed.

I find it funny on some of the other forums when people say things like, "This bike is so reliable. I've gone like 30 hours without reshimming my valves."

  • ryan c

Posted April 25, 2006 - 10:52 PM


mine is an 01 426 rode often I checked them yesterday for the !st time EVER to be checked and there right in the middle.


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