Jetting causing overheating?? Please help!

My bike is a 98 WR400, YZ timed, grey wire unhooked, FMF powerbomb and Q-pipe and no airbox lid. The problem with my bike is that it overheats on a regular basis. I have tried everything to try and stop the problem including replacing the water pump impeller. Currently I run a 1.6 cap and Engine Ice Coolant to try and keep the problem contained. When I ride with friends also on WR400's or YZ426's they don't boil over when my bike does. I'm thinking it could be a jetting problem. My bike does pop on deceleration and when cruising at 1/8th to 1/4 throttle it stumbles but that immediately goes away if the throttle is opened up. My current jetting specs are:


178 main jet

48 pilot jet

100 pilot air jet

200 main air jet

82 starter jet

1.5 turns out

Any suggestions would be greatly appreciated. Their is nothing worse than an overheating bike and friends who don't want to wait for it to cool down. Thanks, Eric

Maybe an air leak with the hot start, or carburator rubber? Blow by on the head gasket? You might try a leak down test on the cooling system.

Just take your radiator cap with you to the local auto parts store and purchase a 16 lb replacement cap off the rack. My YZ was doing the same thing after idling for a couple of minutes, all fixed now for about 4 bucks. Higher pressure = higher boiling point.

Additionally your main and your starter jet are way too big....But that's not causing your heating problem....The stumbling is most likely caused by the needle being on wrong clip. Put it on the 3rd clip (from the top)....That should clear that up. If your still getting decelleration popping open the Fuel Screw to 2 turns....

Bonzai :)

[ July 17, 2002: Message edited by: YAMAKAZE ]

I've checked for air leaks by spraying the carb area with WD-40 while running and get no signs of leakage. The folks at Stroker also mentioned a leak from the combustion chamber but I don't think that is the situation because my coolant remains clean. I think I am going to try a new radiator cap because mine could be bad. I see that my starter jet is way to big but it always starts fine so I'm not sure I should change it. Any more jetting suggestions? Thanks for the help, Eric

[ July 17, 2002: Message edited by: Blue Boner ]

I had the exact same problem. I have a 2000 WR400 and even short rides I could hear the fluid in the radiator boiling. Sometimes I would come back from a ride and all the radiator fluid would be in the overflow and as it cooled it would suck back into the radiator and cool down. My mechanic said it was running way to rich and he rejetted it for high altitude (Utah) and it hasn't overheated once!! He said when it runs rich there isn't enough fuel running through it to help cool the engine.

Here is another opinion:

You have a rich pilot and main that is compensating for a lean needle setting. At mid throttle settings you are probably pretty lean. The D_R needles come on WR's, but when you change to YZ timing you should consider changing the needle to something like an E_N.

My "WR with YZ timing, Powerbomb SX, WB Hawkins, grey wire and no airbox lid" jetting spec:

ELN #2

42 pilot

~1.25 turns

170 main

100 paj

200 maj



Bonzai :)

Blue, where ya-bee?.. I don't know if your missing out on my rides, or I'm missing out on yours!.. Anyway, the solution is simple....BUY MY WR.... Viola..No more problems..


Folks, I appreciate all your suggestions, but I have to admit now I'm more confused than ever. I am going to assume that my low speed stumble is from a lean condition. If this is correct can someone explain why I would move my clip to a leaner setting? Personally, I don't know anything about needle profiles and what that all means. I am more than willing to switch to different jets and needles to fix my problem if I have to.

Another question I have is, do jetting specs from a 2000 and up carb also work on a 98 or 99 model even though they have different carb bodies?

Trick Daddy, you couldn't be further from the truth. A rich engine runs cooler than a lean engine because it has excess fuel to cool it.

Yamakaze- check your PM's

SoCal- I rode last Saturday out of Lake Arrowhead and was almost struck by lightning then got right back into the heat. Then on Sunday rode out of Big Bear. About buying your bike, I know its nice but it's not Orange and doesn't have a 520 in it.

Thanks again for any suggestions, Eric

[ July 18, 2002: Message edited by: Blue Boner ]

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