Carburetor Tuning - Accelerator Pump

[Vintage Engine Repairs]

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

 

[altatim]

I just recently tossed a C1Q that I thought was just dogbone tired, I might revisit is as I couldn't find anything wrong other that that is was almost impossible to tune even with new rubbers. Might do a full reset on the jets and see if its better but I only tune by ear so we will see if thats possible lol

 

[Vintage Engine Repairs]

I just recently tossed a C1Q that I thought was just dogbone tired, I might revisit is as I couldn't find anything wrong other that that is was almost impossible to tune even with new rubbers. Might do a full reset on the jets and see if its better but I only tune by ear so we will see if thats possible lol

It’s likely a worn accelerator pump o-ring I have a video on how to diagnose it.

Don’t delete the accelerator pump like I did here, just buy the o-ring. They run best with the pump in place.

 

[sean donato]

Often times the accelerator pumps arnt serviceable, even if you can get them apart there's no parts available for them. Walbro is a big ofender for this with many of their carbs. It's also a bad spot for dirt to collect, often times wearing out the plunger and or bore it sits in. Once thats been done it's either plug it or toss the carb.

 

[ZeroJunk]

Those carbs and a sister with a little different linkage fit most all of those white Stihl weedeaters.
We buy them 25 at a time. If it won't prime and you are sure your fuel line is good might as well chunk it.
They are only $45 . Not worth chasing your tail over too much.

 

[Vintage Engine Repairs]

Out here they’re 140 AUD / $91 usd per carb, but Aus prices are always stupidly high in comparison to the USA. I just put a 5 cent o-ring I keep spare and charge the customer 15 minutes labour.

 

[2StrokeFondler]

Out here they’re 140 AUD / $91 usd per carb, but Aus prices are always stupidly high in comparison to the USA. I just put a 5 cent o-ring I keep spare and charge the customer 15 minutes labour.

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