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Written by Vic R   

The Walbro carb has a low speed and a high speed needle (although the names are kind of a misnomer as both needles control the mixture at wide open throttle) The low speed needle is the one closest to the motor and typically has a long rubber extension T on it for tuning. Its basically (very) used for tuning throttle response off the corners. The high speed needle is the one closest to your airbox. Its used to deliver most of the fuel during full throttle operation.

Turning the needles clockwise makes the needle go deeper into its seat, leaning out the mixture. Turning the needles counterclockwise will richen the mixture. In Yamaha classes, we used to set a baseline with the low speed needle around 1 3/4 turns from being seated and start with the high speed needle around 1/2 turn from being seated and tune from there.

Keep in mind these are very generalized settings and explanations. Many things affect fuel flow including pop off pressure, atmospheric pressure, the cleanliness of the carb screen, the condition of the pump diaphragm etc etc. I'd suggest you get some baseline advice from the your engine builder or maybe some of the front runners in your class.

The trick to adjusting carbs is to find the leanest setting that lets the engine pull all the way around the track in the first few laps of the race. When looking for this setting always start rich and go leaner. Try to dial it in before the engine gets too hot and, check it at the start of the next session. From there develope a plan as to when you need to open the low needle to control engine heat. When I see the 1/2 way flag I check the gages.

If you find the engine loads up with fuel in the tightest corners, turn the low needle leaner (1/8 turn at a time) untill it comes off the corner fairly clean. The leanest I would go on the low would be 1-1/2 turns open (Yamaha).

Next listen how it sounds at the end of the longest straight. If it burbles (4strokes) then lean the high down a bit (1/16th turn at a time) untill it runs clean at max rpm. The leanest I would go on the high would be a 1/8th of a turn open. I try not to adjust the high during a race because, small adjustments are hard to make and have big results on the fastest part of the track. For this reason I try to leave the high fairly rich to give myself some head room.

The high and low work together and, do have an effect on each other. If you lean the low then lean the high, you might now be too lean on the low.

If the low is way too lean it will feel dead coming off of turns. Also, you might feel a dead spot about half way down the straight. If you open the low but still feel weak in the midrange, open the high a little.

The exact settings and adjustments you make will depend on the design of your track and how the fuel loads and unloads in your crankcase.