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V968231

1998 WR 400 flat spot and backfire when shut off

4 posts in this topic

I have been having carb problems with my 98 WR 400:

Initially the bike was cutting-out so I cleaned the carbs,

then I took it to a dealer and he found that the slide plate had broken. The dealer replaced the slide plate but now I have a flat spot at the start of the throttle and the bike backfires when I shut it off.

From reading other posts I guess that this is down to the air/fuel mixture but I am unsure how to adjust this (so was the dealer?). Could any of you guys or gals give me an idiots guide. I am not sure which screw is the air/fuel mix either.

Any help greatly appreciated. :thumbsup:

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You need to reduce the squirt on the carb accelorator pump. Chances are that your pump was not working and now the dealer cleaned it up and its working again, and the squirt is too long. Basically when you open the throtle, a small arm actuates a diaphragm pump which squirts fuel right into the throat of the intake. When the squirt is too long you get a bog as you are describing - too much fuel. You need to reduce the 'arm' travel so only travels through a small portion of its travel and thus reducing the squirt duration. Look for 'Taffy Mod' and it will give you the lowdown.

Under the float bowl (Bottom part of the carb) you will find a small 'hole'. Inside it is a screw. It should be set approx 1.5 turns from closed. Screw it in gently until it seats then back it out 1.5 turns. Further out = more fuel. If you get beyond 2.5-3 turns, you need to probably change your PJ (pilot jet) for a bigger one - this could help reduce the backfire = lean = not enough fuel.

To adjust you will need a very short (1-1.5 inch max) flat bladed screwdrive. Try epoxying a nail to an electrical screwdriver bit and you now have a tool which is small enough and with enough leverage to turn it. Alternatively, loosen you carb and rotate the bottom out so you can get a normal length screwdriver in there.

If your dealer can't help, maybe you shouldnt be taking it to him ?? If you want more help, PM me.

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Thanks for all of your help so far :devil:

On thing I can't get my head around is the fact that my bike was backfiring, which suggests a lean mix. I checked the fuel/air screw and it was 3.5 turns out which would suggest lots of fuel considering stock is 1.5 turns out :thumbsup:

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I think thats because if you have a too small pilot anyway, opening up the fuel screw won't make any difference because the pilot jet still controls/limits the fuel flow.

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