Letter Of The Week: Air Screw Me!

Dirt Bike Test,

I’m running the JD Jet kit on my 2018 KTM 300XC and have a bog at just above throttle opening to 1/8. I have it set up for tech\trail at 2000-7000ft (live in Reno, NV and ride 4500-5500) with the 30 pilot jet. After 1/8 throttle the bike runs perfectly. The air\fuel screw is set at 1.5 but I will be turning it in to 1.0 to richen it.

Where did you set your air fuel screw? Also, if I go below the 1.0 or 0.75 turns out should I increase the pilot jet to a 35?

I appreciate your input.

John from Nevada

John,

Our air screw was played with the fuel screw regularly to tune the throttle response. this is normal for us but usually about 1/4-turn increments. I don’t recall specifically what our standard setting was but we used it as detailed in the JD instructions and as explained between the two different needles in the test. A bog on a two-stroke, especially right at the crack of the throttle, is typically caused by a lean condition. There is a 32.5 pilot and that may be the right choice for you with all things being correct as described, but here is a way to find out without going into the carb just yet.

I think where most are getting confused is that on the two-stroke carb, the screw is an AIR SCREW, not a fuel screw. So opening = leaner, closing = richer. This screw, on the back of the carb, allows air to go past and into the pilot circuit. The more technical name is the pilot air screw. It is basically the opposite of ones found on four-strokes which are fuel screws.

Here is a really good source of basic information:
https://www.fixyourdirtbike.com/fix/how-to/2-stroke-carb-tuning/

Here is a technique we use to tune the pilot circuit and some basic rules:

-If you are less than one turn from full closed or more than two open, you are likely running the wrong pilot jet. If the screw is closed, you need a richer (bigger number) pilot jet. If the screw is too open you need a leaner pilot (smaller number) jet.
-To test this set the bike to a high idle either with the idle adjust or by holding the throttle just barley cracked. Turn the air screw back and forth to find a “middle position” where the high idle “settles”. At this point you are close (provided the screw is between 1-3 turns) and you can set the idle normal and then play with the screw in 1/4-turn increments to get the “feel” you want.
-Everything in the carb is dependent on a lot of the other parts. Including the float level, vent lines, contamination as well as the other jets. Any obstruction/bad setting in one area will have and effect on every other system.
-Then the final test is to put the bike in 3rd gear, let it lug down to almost no RPM and then do a smooth roll-on of the throttle and pay attention to the throttle position and the RPMs where you are trying to tune. In this case it is the very low RPM and small throttle opening. If it isn’t right, make a large change, in this case 1 turn of the air screw in both directions to determine if it is rich or lean. Then adjust based on that.
-If things are not seeming right, look elsewhere. On two-strokes bad reeds, air leaks, electrical issues, bad gas, dented or plugged pipes can cause lots of issues that seem like they are jetting related.

There is a lot of “feel” in all of this and in the end getting it just right means having it feel right to you. The air screw can give the bike a hit off the bottom and also make it very smooth depending on where you set it. So in the end don’t be afraid to play with it.