WR400 goes wide open throttle upon startup.

Was burning a load of oil so I ripped her apart Made sure the rings looked good, which they did around a brand new piston. The valves on the other hand were nasty. Exhaust valve seals were shot. Replaced all the valve seals and gave everything a once over. All looked well so back together it went. Bike fired right up and idled. On the test ride I noticed a lot of popping on deceleration. Made sure the exhaust and intake work place with no leaks and took it back out to test it again. Still pOpping a little bit on deceleration but not as bad as it waS. Only problem is now the bike is holding a very high idle out of nowhere, and that's after riding around for a half hour or more with no issue at all. A soon as you kick the bike over it rEvs for red line

Adjust the idle with the black idle knob to the left. It is found near the carb on the left side.

Bike does not respond to the idle adjustment.

Should! Are you turning it to the left? It should die if you turn too much.

Did you work on the carb, remove cables, or slide

Had the carb off the bike while doing work to valves.no adjustments were made to the carb other than opening fuel screw 1/4 turn to try an eliminate decel pop. Bike ran fine for half hour or more. To the best of my recollection I hammered on it through 3 4 5 and let off.while down shifting for a light I realized the idle was way up. Burped the throttle a few times to try and free it up then killed it. I tapped the carb with my knife a few times , made sure the throttle was fully returning to closed position , kicked it and it rev for red line. So I slipped it into gear and had cruise control the whole way home.

Just out of curiosity, what is the factory fuel screw setting/turns?



If you start with a high idle, then set your fuel screw, you will have a problem

Must start with the lowest possible idle first, then turn the fuel screw for max idle.

If it goes to high, start over with a lower idle

Does that bike have a hot start lever?  I had mine get in a bind and it stuck open.  High idle, popping on decal, no amount of idle adjustment did anything.


I don't think the older 400's had hot start levers, but thought it was worth mentioning.



Also, check for vacuum leaks in the hoses.

Nope. This one only has choke.

Is the fuel screw original or aftermarket? I worked on a bike and we put in a cheap aluminum and the bike ran like crap. Something was off, could have been spring, washer, etc.

If it is original, check the parts. If it's aftermarket pull and put in original to see if there is a difference.

Full screw is stock.

Sounds like you might have an intake air leak. Check both rubber boots on either side of carb, but that usually  does not cause red line....usually more like high and/or inconsistent idle.


Check your throttle cable routing, this could be it especially if it worked for awhile then went haywire. 


 Also I have seen where a broken or cut o-ring on the fuel screw can cause all sorts of idle issues, but there again NOT to red line.


 Popping on decel can also be caused by a bad seal on the exhaust donut.

I'd be inspecting cables, and slide position at closed throttle to see if you have things together correctly. Sounds to me like the slide is hung up or under cable tension far from "idle" position.

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