Won't idle and backfires 98 yz400f

I have already replaced the pilot jet (45) and the pilot screw. The pilot screw I have set at 1 3/8 like the OM states. I don't have any air leaks at the boots and the hot start works correctly. The bike will start on first kick with choke on but will die instantly without choke.

Could the possible causes be?

Tps sensor adjustment

Valve clearance


More pilot screw adjusting

Forgot to mention the fuel accelerator pump diaphragm looks worn. Could this be causing my issues?

New list, toss the old one.


  • Pilot jet plugged or obstructed
  • Vacuum release plate on the slide damaged or assembled wrong
  • Internal carb body leak

Did you make sure o-ring was on fuel screw?


Is the carb slide valve the right way up?


Pilot jet passage could be blocked..squirt wd40 or brake cleaner up the hole.


Pilot air jet could be blocked.

Thanks guys for your time!

I will tear down the carb once again. I know the pilot jet is good (brand new) and I made sure the (new) pilot screw had its o-ring and also made sure to fish the old o-ring out. I will need to check slider vac seal? I know it's installed correctly because I didn't remove it. Bought a new air compressor so I will blow all air passages out.

I will update what I find.


Looks difficult to remove the slider! But, here is a quick picture of the vac seal....bad?

ImageUploadedByThumper Talk1380583444.250474.jpg

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