Won't run right. Advice needed.

I have agreed to help a friend with a '99 YZ426 that wont idle, backfires and runs terrible. Someone tries clean gas and carb-no go. I took over and found unscrewed chike plunger and moisture(clean gas and carb again). Whats the deal with throttle position sensor? does it have to be set accurately?how?

Are there refined jetting specs to make it easier to start? This is a bear. Where is there more meat and potato information for my project?

Thanks, John

I have agreed to help a friend with a '99 YZ426 that wont idle, backfires and runs terrible. Someone tries clean gas and carb-no go. I took over and found unscrewed chike plunger and moisture(clean gas and carb again). Whats the deal with throttle position sensor? does it have to be set accurately?how?

Are there refined jetting specs to make it easier to start? This is a bear. Where is there more meat and potato information for my project?

Thanks, John

It sounds like someone disassembled the carb and got the little plate on the slide in upside down. The bike will run exactly how you stated. Hard to start and usually requires fair amount of thottle to start, runes like crap missing and sputtering, back firing and will not idle at all, even with the idle turned all the way up...

Download a manual:

http://www.yamaha-motor-europe.com/community/service/manuals.jsp

or

http://www.yamahaownershandbook.com.au/?r=0

Get the year right; there is no '99 YZ426. 426 was 2000-2001, '99 was a 400.

If you had the TPS off or moved it, return it as closely as you can to it's original position by looking at the witness marks left by the mounting screws. The adjustment procedure is in the book, and if you doubt the function of the thing, disconnect it altogether temporarily. The TPS should not affect the idle in any way.

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