99 yz 400 off road mods?

Hey all,

I got a good deal on a 99 yz400f, Been riding it for about 3 weeks now and I love it! Huge upgrade from my 82 xr 200:eek: . I have been riding in the trails and thick bush a bit, and it has overheated a couple of times - is this damaging to the bike? It starts spraying antifreeze out the bottom and the Rad cap popped last time. I just leaned it up against a tree for 10 minutes or so and it ran fine afterwards.

any help with the following questions would be appreciated...

1 I put the bike on the trailer and noticed that there is a bit of lateral play where the swingarm attaches, i assume it needs new bushings or bearings, is there a kit for these parts, and is it a simple fix?

2 Can you put a wr transmission in it? The gears are too tall in the dense bush but too short for the long straight railway beds. I saw the thread for the 450s and 426's but not much for the older 400s.

3 Any idea where to find the heavier flywheel? I looked in the TT parts store, but nothing for the 400. What is a good weight for a bike that will be primarily trail ridden, and could I just add weight to the stock flywheel?

If anyone in Canada has any of these parts, a heavier flywheel, larger tank, petcock with reserve, or a kickstand, please leave me a private message

cheers -


i just purchased myself a 99 YZ400F and i found the bike to overheat. what i did to fix the problem was to rebuilt the water pump and the use water wetter (the pink stuff) in the rediator. I also upgraded my radiator cap from a 1.2 psi cap to an after market 1.6 psi cap.

that seemed to fix the problem. I have not have the radiator boil over and come out of the overflow tube.

the water pumpe is a big thing because if the impellor fins are damaged, its not going to move the fluid is the system very well.

Hope this helps!!!!:thumbsup:

Patrick M.

Yes over heating can damage your motor. Sometimes, as stated in another post, the radiator cap may need replacing. Do what was suggested in that psot. Yes, there are kits made for your swing arm. Its not a fun job but it can be done. I put on a flywheel weight and loved the results. You might want to try and get a catalog from Rocky Mountain atvmc. They will have all the parts you are asking about. I went with a 10oz flywheel weight

If you want a reserve petcock order one from a WR400, they are about $15.

For a heavier flywheel, you could get the WR426 flywheel along with a stator. Then you could run lights if you wanted to.

This guy sells reasonably priced sidestands on Ebay.


There is a swingarm kit, can't remember who makes it but if you have a friend with a press it makes it easier to install.

Good luck and enjoy the bike!

Steahly flywheels They got complete flywheels and wheights.

I use a 12 or was it 14 OZ flywheel on my yz 400 converted for technical trail riding.

Perfect if you dont want stalling whith the yz gearbox(it goes at tad bit to fast on first gear) Sprockets 14/51

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