Vintage Dirt Bike Q & A

My bike seems to run rich I have a 300 main 50 pilot and 2.0 air jet the bike has a mossemberg reed valve and a circle f pipe I dropped the main down to a 290 and it did lean it up but it's not running clean it seems to struggle at low revs then goes ok but doesn't seem to rev out , it also has a pvl ignition can anyone give me their jet settings or any advice . thanks

the needle is a 6h4

Do you know what needle jet you are running??

if you mean the main jet it's a 300

Not the main jet. The needle jet is the brass part the needle goes down into and the main jet screws on the bottom of. For instance, I'm running a 159Q4 needle jet on my 36 Mikuni. I also run a 6f4 needle, 270 main and a 35 pilot. The needle you use will depend largely on what needle jet you use. Every bike is different and With your pipe, I'm guessing you're real close on your main jet. You are going to have to figure out whether you are currently rich or lean on your midrange, though. Once you determine that, try readjusting your needle in the proper direction. If you get to the end of the adjustment and are still rich/lean, you are going to have to change the needle a size.

Go to http://www.sudco.com and download their catalog. It has lists of all the Mikuni parts with their specifications, and some good background on tuning.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF