Keihin 28mm carburetor jet


New Member
May 29, 2024
Hello everyone , hope to get some answers here , I have a 1988 blaster , dg pipe & silencer , vforce4 reeds , air box open , kn filter and a 28mm genuine keihin carb with 45 and 140 jets ... live on south texas with 121ft elevation & 90 farenheit .. currently run very bad , not have experience with carburators , ** any ideas where to start on jets ?
Welcome aboard.

So you are pretty much at sea level. Jetting changes for elevation usually aren't needed until you get a couple thousand feet in elevation or higher.

You've given all good pertinent information about your quad, but I can't just throw jetting numbers out there as every situation is not the same.

What I can do is give you some guidelines and suggestions though.

You say running bad, well, you can have an engine running bad for a plethora of different reasons. Your jetting numbers sound pretty good for a baseline initial setting. But what we need to do before swapping jets is make sure other systems are in proper working order. Replace the sparkplug, always start with a fresh plug when delving into jetting. With a 2-stroke, a wet-fouled plug will often allow the engine to run lower RPM/engine load but will fall on its face when you attempt to grab a fistfull of throttle.

Make sure the carb is squeaky clean. If you remove the carb and then remove the bowl, anything other than squeaky clean will be cause to suspect the carb needs to be cleaned as well as the jets themselves. The fuel circuits in the carb body that the jets supply fuel to can be varnished inside and give you problems, particularly the pilot circuit as it is very small and doesn't take much fouling to cause issues. While the carb is apart, check float adjustment and adjust as necessary to Keihin factory spec.

If all of this checks out check for engine air leaks. Crank seals, base gasket, and reedvalve gaskets are the most common causes of air leaks. Case center gasket can also but is less common.

If the engine is smoking excessively and sounds like it's drowning in fuel, this is a sign of a rich condition. If it is a smooth bog/loss in power and throttle response, this indicates a lean condition. If your symptoms sound like the former, you probably do not have an air leak. If it sounds like the latter, you may have an air leak. But I would not jump into suspecting an air leak until trying some jetting changes and seeing if it responds accordingly.

Once you get to the point of changing jetting, start with the main jet and go a couple sizes richer (if you have determined it's running lean). Always work richer first. If the problem worsens, then you know you can work your way leaner.

Once the main is dialed in and throttle range from appx 1/4-fulk throttle seems solid, then you can dial in the pilot as needed.

A tip for pilot jet tuning... Your idle air screw should be appx 1.5 turns out. This screw regulates idle air. So turning it out adds more air, this leaning the idle mixture. Conversely, turning it in reduced air, enriching idle mixture. Using the target of 1.5 turns as a guide, you can determine if you are rich, lean, or good to go on pilot jet size.
Example: if you find the most stable idle at 2.5 turns out on the air screw, you are a size or two too rich on the pilot.

I source all my genuine Keihin jets from

Good luck and let us know how you make out.