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Carb tuning - Accelerator Pump

Vintage Engine Repairs

Pinnacle OPE Member
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Aug 15, 2019
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I just finished tuning a fs55, not a saw, however this carburetor (C1Q) is on a bunch of saws and some have accelerator pumps.

I have run into this issue on a number of machines:

When tuning carbs with accelerator pumps, it’s a good idea to stick to using a tach and the process they recommend in the manual.

The reason being is the pump piston is actuated off the throttle shaft which can also be adjusted by the LA (idle) screw. If this idle screw is too far in, you’re pushing that pump piston in and fixing it there and reducing the overall volume that can be held in the carburetor. This can cause strange symptoms at idle - loading up, leaning out, increasing and decreasing in rpm.

Stihls typical suggestion across the board is to find half way of either drop off at 3300 and richen L by 500.

I noticed in the manual in this case it says to find drop off at around 3k (300 rpm less) and richen to 2800. I believe it’s all about keeping the cam of the throttle shaft OFF the accelerator pump piston.

YMMV, but I thought I’d put this here in case someone is having trouble tuning with their normal method of tuning by ear and feel. It may get you out of a bind :)