YZ400 Rebuild


4 replies to this topic
  • Wopstar

Posted July 13, 2007 - 01:59 PM

#1

I am planning on rebuilding my 98 YZ400 soon and was just wondering if anybody had suggestions on what to look for or any insight to what else I might need to do.

I have only had it for about 7 or 8 months and it runs great except it has overheated the last few times out and it uses a lot of oil. I plan on putting a new piston in it, new clutch, carb rebuild & valve job. Anything else I should do?

I also was thinking about putting a crankcase filter on the breather tube that just vents out, has anyone ever done this?

  • ericyzf4

Posted July 14, 2007 - 05:30 AM

#2

Watch for the cams binding. On the 400's when the valves are adjusted, the cam caps tend to start binding, and the cams don't spin freely. Set the head on a bench (with a towel or pad under it) and reassemble it, cams, caps, etc. just like you would in the bike, torqueing everything. Then check your valve lash. This is much easier than trying to do it while it's in the bike. When my cams were binding, I sent my head to Engine Dynamics in California, they are super friendly and very knowlegeable. They also replaced valves, valve seals, seats, lapped the valves, and set the lash for me for about $300.

http://enginedynamic...m/contact.shtml

The 426 cylinder from 2000 (only 2000) will bolt on your 400 making it a 426. You will have to split the cases and have the crank machined to accept the 426 rod and journal bearing and have to add the 426 piston. (you will need 426 cylinder, rod piston) This is many times more reliable than trying to bore the original cylinder.

http://www.powroll.c...A_YZFWR426F.htm

Replace your timing chain, it is cheap insurance. (about $30.00)

There are many other things that can be done to the 400. The aluminum subframe from a 426 is a direct bolt on and will shave a couple of pounds.
The radiators from an 04 - 05 450 will also bolt on and will add cooling capacity. (you will need the shrouds and fins from an 01 - 02 426)

These are a few of the things I have done to my 99 400 and they all work.

E---

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  • tntmo

Posted July 14, 2007 - 06:46 AM

#3

ericyzf4 pretty much summed it up. It might be cheaper to just buy a complete 426 crank assy than to get yours machined. The TT store has everything you need at really good prices. I'm looking to rebuild my 400 after the next desert season and am probably going with the 440 stroker kit from Powroll. Check your cylinder head, especially around the shim buckets. Lots of them are mushroomed out there and make it hard to remove the shim buckets. Mine is doing that, probably due to excessive torque on the cam caps. It still runs fine but I will need some machine work done. I have an extra head that I will build up to use and hopefully rebuild the original one as a spare.

  • grayracer513

Posted July 14, 2007 - 08:00 AM

#4

Watch for the cams binding. On the 400's when the valves are adjusted, the cam caps tend to start binding, and the cams don't spin freely. Set the head on a bench (with a towel or pad under it) and reassemble it, cams, caps, etc. just like you would in the bike, torqueing everything. Then check your valve lash. This is much easier than trying to do it while it's in the bike.

This method has its advantages, but some pitfalls as well. Most of the time it works out fine, but you should always recheck the clearances once the head is on the engine and fully torqued up. Things can tweak just enough to change something.

Also, in setting the clearance on the bench, it is not necessary to torque the cam caps past about 40 inch pounds or so.

Which brings me to the subject of cams binding. This problem is often caused by the mechanic not correctly torquing up the caps. They must be seated squarely before being tightened, and not drawn down with the bolts, and they must be torqued in the correct sequence, and incrementally, in small steps, as a set, rather than tightening one completely, and then each of the others. I tighten them all to 45 in/lb, then to 60, then finally to 75 in/lb, with the threads lightly oiled. The specified 86 in/lb feels excessive, based only on my professional experience, and I have never had a problem with using 75 in/lb instead.

  • Wopstar

Posted July 15, 2007 - 06:22 PM

#5

Thanks for the advise. I would like to try the 426 conversion but it's not in the budget right now so I will probably just do the basic rebuild and save up for the conversion later. I will definately put a new chain in it.

Anybody have any experience rmoving the breather tube and replacing it with a breather?





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