Help 1999 wr400f breaking up 1/4-3/4 throttle

[Glocklife]

I just bought a street legal 1999 wr400f it has the gray mod, air lid off, full exhaust with a big gun pipe, I dont know what jets are in it. The bike runs greats expect between 1/4-3/4 throttle. So I read and read and found out about the TPS I look at the bike and it's already disconnected so I connect it.... NO difference. So I dont know what to do any advice?

[flyandride]

1/4 to 3/4 would be controlled by the position of the needle.  You should be able to loosen the carb and rotate it so you can get to the top of the carb.  Pull the slider and needle out and raise or lower the clip on the needle.  See which way improves it.  While you're at it you should pull off the float bowl, pull the jets and clean them.   

[FORMA MOTO]

Yes, but HOW does it run in the 'bad' throttle range: gurggly, or kill-button-y, or on-off-y....

Edited March 22, 2014 by TheKoolAidMadeMeSick

[Glocklife]

Ok thank you for the advice on the needle it was on the 3rd notch of the factory needle I moved it down 3 spots to richen it and it got worse so I raised it to the top and it really really well now but still a little hiccup here and there at a sustained throttle between 1/4 and 3/4. I also found out I have a 38 pilot jet and a 160 main jet.

[Glocklife]

Yes, but HOW does it run in the 'bad' throttle range: gurggly, or kill-button-y, or on-off-y....

It was running on-offish like spark! Then no spark!

[FORMA MOTO]

A 38 pilot is too small. 42 or 45

160 main is fine, but small, like for high altitiude or really wet air

The most common problem with older carbs is the needle jet wearing out: it ovalizes, and you end up chasing jetting issues to insanity.

 

You should go to the 400/426/450 FAQ and see what others use for jetting, and replace your needle jet (not jet needle), apump diaphragm, pilot, and adjust your valves.

[Glocklife]

Thank you guys