Anytime a carburetor is replaced, it might need some preliminary adjustments. New replacement carburetors are often shipped with the plastic limiter caps not permanently seated. Only after the final adjustments are made to the carburetor should the limiter caps be pushed down over the serrated part of the adjustment needle.
After a new replacement carburetor in installed and before any adjustments are made, start and run the equipment until it is at its full operating temperature (around 4 or 5 minutes). If the equipment will not start, back out the idle screw slowly until you can see a gap between the lever and the screw. Then turn the screw back in slowly until it touches but does not move the lever. Now turn the screw in 1/2 to 3/4 turn more to move the lever (which will open the throttle valve slightly).
Next remove the limiter caps by gently pulling up on them with needle nose pliers. Now close both adjustment needles very slowly clockwise, until each gently bears against its seat.
The idle mixture adjustment is called the low needle or lo needle. It meters the flow of fuel in the in the idle system. Open the low needle one turn.
The main mixture adjustment is called the high needle or hi needle. It meters the fuel drawn through the main jet for full power operation. Open the high needle 1 turn.
For idling, start and run the equipment until full operating temperature is reached. Then turn the low needle slowly to the left and then back to the right while noting the effect on the engine idle speed. At the point, do not adjust the idle speed screw to change the speed, but set the low needle at the highest speed obtainable, then back off on the needle until the RPM just starts to decrease. The needle is now adjusted to rich optimum position.
Now you can adjust the proper idle speed with the idle speed screw. The proper speed is lightly below that which would cause the equipment clutch to engage but fast enough that the engine idle will be stable at any altitude. Adjust the idle speed screw so that the unit idles between 2800 and 3200 RPM.
Press the throttle to wide open position to make sure acceleration is good and without hesitation.
With the throttle held in wide open position, apply load to the equipment. Adjust the high needle to a rich optimum setting. This is achieved by setting the high needle to a maximum RPM under load and back off needle counterclockwise until there is approximately a 200 RPM speed drop.
Both the high and low needle limiter caps can now be pressed into their tamper proof (rich stop) position without changing their settings (without the needles turning).
The 2-cycle engine maximum no-load RPM should be between 11,500 and 12,000 RPM with most chainsaws, trimmers and blowers.
Download the Walbro general service and repair manual for diaphragm carburetors
Download the Zama service and repair manual
