More carb questions

1 reply to this topic
  • AFelix

Posted July 11, 2014 - 09:15 PM


Mine is a relatively new to me 99 YZ400F with a keihen carb, everything was great during the 1st several trail rides. My last ride was at +\- 6000 feet and she had trouble idling and staying running.

After reading many carb post on this site, I decided to check my jetting. At the same time I replaced all the carb tubing, and purchased a new easily adjustable fuel screw. I have 170, 65 and 45 jetting, the main is a JD, not sure about the rest, original fuel screw was a red with a knurled end which was originally set at 4.5 turns out, anything less and she would not start. (I have read some comments about a JD red fuel screw).

I pulled the carb, pulled the bowl to check jets, ( had to pull the 45 jet to read size), reassembled everything with new hoses and new fuel screw, started at 2.5 turners out, and she started fine, seemed to run ok ( in my driveway). I tried adjusting the fuel screw using the instructions from this site. Upped the RPM to +\- 1500 rpm and turned the screw in waiting for rpm changes. I turned the screw in all the way with only a small rpm change just before bottom. I then turned the screw out to at least 4.5 with virtually no change. I then ran it all the way back in and then out 2 turns. I have not had an opportunity to really ride yet, but can anyone explain this? BTW, home base is +\- 600 feet.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Thommo_400f

Posted July 14, 2014 - 02:03 AM


Be sure to check if your needle jet is worn. It will cause really weird pilot adjustments and difficult low throttle jetting.

Related Content


Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • - - - - -
  • 0 reviews

Michigan Motocross Tires by 288yz450

Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply

Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.