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Engine Carburetor - Low Speed Mixture - How to Set for reliable idle and transition to higher RPM?

Question: How do I set the low speed mixture of my carburetor?

Answer: Generally you should not adjust the factory settings until the engine has been run for 4-6 tanks of fuel at least AND you should only adjust the factory settings if the engine does not idle well or does not transition to higher RPR well.

All engines idle poorly and transition poorly on the first tank of fuel, a bit better on the 2nd, 3rd and 4th tanks and progress to normal operational reliability by about the 6th tank of fuel.

If you find that after this breakin period, your engine dies at idle or dies when trying to increase the RPM from idle, you may need to adjust the idle mixture. Before attempting to adjust the idle mixture, make sure that the problem is not with your glow plug, fuel tank or tubing etc. Then gently turn the idle screw all the way in (clockwise) while counting how many turns or fractions of turns the factory setting is from fully closed. This turn count will enable you to go back to the factory setting at any time. Do so now... turn the idle screw back out (counter clockwise) to the factory setting.

The low speed mixture screw works the same way as the main needle valve... turning it IN (clockwise rotation) makes the engine run leaner (less fuel,  lower ratio of fuel to air) and turning it OUT (counter clockwise rotation) makes the engine run richer (more fuel, higher ratio of fuel to air).

To make adjustments to the low speed mixture, we recommend the following procedure. Review the instruction manual first. All of it! Then read it again for good measure.

Now put your engine on an engine test stand and set the center line of the test stand fuel tank about 1/4" below the needle valve. Check your tubing for leaks and secure attachment. Fill the tank with good quality fuel as described in the instruction manual. 10% nitro maximum. 18% oil minimum with Oil being a mix of synthetic and castor.

Start the engine, reduce the speed to idle. Let it idle (even if rough running) for about 30 seconds. Then suddenly open the throttle fully for maxium RPM and listen carefully to your engine. If your engine gurgles and sputters as you open the throttle with lots of grayish smoke and oil coming out of the muffler, then your idle mixture is too rich. Lean out the setting by turning the mixture adjustment screen IN (clockwish) by 1/8 to 1/4 of a turn. Then try again and see if the engine transitions to high RPM any better. If it is better you are going in the right direction. If it is worse (even more oil and smoke). If at some point your engine goes from idle directly to a high RPM and then dies without notice thereafter, your low speed mixture is too lean... back the idle mixture screw out (counter clockwise) about 1/8 of a turn and try again. The ideal settting is one where the engine does not sputter and gurgle when trying to rev up but also does not stall after reving up!

Once you have this working well on an engine test stand, move the engine to your model and check out the low speed mixture again. You may need a further slight adjustment depending on the height of the models fuel tank relative to the needle valve.

Article ID: 3985