yz400f whats wrong

9 replies to this topic
  • gomer334

Posted June 12, 2008 - 05:54 PM


so i have a yz400f and i cant figure out whats wrong with it ive changed the piston and rings did that cause its been 2 years since replaced and redid my valves and got a new fmf power core 4 exhaust and for some reson it still sounds like my valves are tapping but its really really loud i dont know what could be wrong any ideas if the timeing was off would it make a loud noise like this ??

  • matt4x4

Posted June 13, 2008 - 04:12 AM


How much torque did you apply to the cam caps - Birdy426 just went through what sounds like the same situation.
Specs are 86 in/lbs, but Grey (who really knows his stuff) keeps his at 75....

  • gomer334

Posted June 13, 2008 - 04:42 PM


a good bit i got out a steth o scope and listened it sounds like its coming from my piston but i know everything there is good it might be my bottom end barrings i think does anyone know how much they cost to get replaced??

  • Birdy426

Posted June 14, 2008 - 01:48 PM


+1 on the cam cap torque...that's the only change I made on the last reassembly, and the clatter went magically away (though I still can't fugure out why). I would try it and see. Pop off the valve cover and timing covers, double check the cam timing, then loosen up the cam cap blts and retorque them (in a cris-cross pattern, of course) to 75 in-lb and see what you get. On my bike, the noise was so loud and reflected off of so much stuff, that the only way I could find it was with a stethescope. It was very clearly coming from the cam chain side of the head.

Another thing to check...on the 400s, the primary drive gear was keyed to the crank. When that bad boy loosens up or wears, things get rattley in a hurry. You might want to replace the key and retorque the nut before you do a major rebuild.

If you do need crank bearings or rod bearings, the cost depends on if you are going to do it yourself or not. Check the TT store for best prices on Yamaha parts. I just did a buddies bottom end, and it was about a benjamin for a rod, bearing, and pin, and about 1 buck and a half to get the crank rebuilt and trued. You may be able to get the presswork done cheaper, but certainly no less than a hundred bucks. If you need to replace the R/H crank half because the primary drive keyway is dorked up, you may want to think about just replacing the entire crank (about 350 at the TT store), becuase it will be about a push, cost wise, with rebuilding what you have. If it's main bearings, plan on about 30 bucks a side. I would replace the balance shaft bushings while you're in there, at about 20 bucks a side. Plan on another hundred bucks in gaskets, seals, oil, assembly lube, etc.

If you do end up replacing the crank or rebuilding the one that you have, you may want to consider using a 426 part. The 426 has 2 advantages over the 400. First, the primary drive gear is splined rather than keyed (but that means you have to buy a new primary drive gear and balance shaft drive gear); and second, the small end bearing is bigger, and therefore stronger (but you then have to use a 426 piston and cylinder, a 426 piston and send your cylinder out to be bored and replated, or a big bore kit from somewhere like Luke's racing that gets you to 444.

Good luck and let us know what you find. Again, try the easy stuff first (cam baring torque and primary drive key).

  • gomer334

Posted June 14, 2008 - 02:07 PM


ok thanks alot ill try everything i can cheap stuff first and is there suppose to be play in the connecting rod on a 4 stroke?? side to side??

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  • Birdy426

Posted June 14, 2008 - 02:42 PM


Yup, there's some play side to side, but should be none radially (up and down). The book tells you how much side to side is OK.

  • gomer334

Posted June 14, 2008 - 02:46 PM


ok thanks

  • gomer334

Posted June 14, 2008 - 06:32 PM


also one other thing i forgot to mention is the bike will not idle unless the choke is on

  • Birdy426

Posted June 14, 2008 - 08:05 PM


That sounds like a carb issue...plugged up pilot jet, maybe

  • gomer334

Posted June 15, 2008 - 10:55 AM


anyone got an fmf power core 4 on the 400f?? would i have to re jet for this pipe some people told me i should

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