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.

View attachment 1163268

This man labels.

 

[epo]

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.

In the case of, for example, MS 200(T), which are rather equipped with (C1Q) carbs with accelerator pumps, the service manual says 3600 -> 2800 rpm (800 rpm difference). Am I missing something here?

 

[Vintage Engine Repairs]

In the case of, for example, MS 200(T), which are rather equipped with (C1Q) carbs with accelerator pumps, the service manual says 3600 -> 2800 rpm (800 rpm difference). Am I missing something here?

Interesting! I’ve never had one before. I had a quick look at the IPL, they have a different diameter & length accelerator piston and o-ring. The idle and progressive idle drillings are also in a different location so I’m not surprised that there is a different setting and how much it holds the piston open and how much fuel it pushes will all be different. I’d stick with what I said before with using manual specs if you run into problems though. The specs maybe totally unrelated, but if someone is chasing their tail, it may help them as it did me.

 

[epo]

Interesting! I’ve never had one before. I had a quick look at the IPL, they have a different diameter & length accelerator piston and o-ring. The idle and progressive idle drillings are also in a different location so I’m not surprised that there is a different setting and how much it holds the piston open and how much fuel it pushes will all be different. I’d stick with what I said before with using manual specs if you run into problems though. The specs maybe totally unrelated, but if someone is chasing their tail, it may help them as it did me.

Thanks!
Leaving aside whether it's about keeping the cam of the throttle shaft off the accelerator pump piston, I find it interesting that, in this case at least, for carburetors with acceleration systems they recommend such a richer A/F mixture for the idle.
I thought the idea behind such systems was to keep the idle speed near the optimum point (max. rpm) for better idle and possibly less fuel wasting.

 

[Vintage Engine Repairs]

Thanks!
Leaving aside whether it's about keeping the cam of the throttle shaft off the accelerator pump piston, I find it interesting that, in this case at least, for carburetors with acceleration systems they recommend such a richer A/F mixture for the idle.
I thought the idea behind such systems was to keep the idle speed near the optimum point (max. rpm) for better idle and possibly less fuel wasting.

Yeah I couldn’t agree more, it’s really strange, but your spot on, just as they state in one of their manuals, the pump is to be able to set the L leaner while maintaining good off idle acceleration so it seems really counter counterintuitive to set the L so rich.

You don’t happen to have access to the earlier version of this manual? It’s got the whole list of tuning procedures for their different machines, unfortunately this only features the later 201t. I’d be curious if there is any difference to what it states between the workshop manual and the earlier of this one. I have found different manuals occasionally give slightly different specs and I’m curious! Sometimes different carbs have different rpm guidelines too.

 

[epo]

Yeah I couldn’t agree more, it’s really strange, but your spot on, just as they state in one of their manuals, the pump is to be able to set the L leaner while maintaining good off idle acceleration so it seems really counter counterintuitive to set the L so rich.

You don’t happen to have access to the earlier version of this manual? It’s got the whole list of tuning procedures for their different machines, unfortunately this only features the later 201t. I’d be curious if there is any difference to what it states between the workshop manual and the earlier of this one. I have found different manuals occasionally give slightly different specs and I’m curious! Sometimes different carbs have different rpm guidelines too.




View attachment 1199753

This MS 200(T) manual is version 2012-09 (© 2012).



I checked other service manuals for fairly modern saws, which are rather always equipped with an accelerator pump, and in those cases it is 3300 -> 2800.

I tried to find a fairly new model of saw without an accelerator pump and see in the manual if there is any difference. I guess MS 250 could be an example here, because if I am not mistaken the version with a Walbro carburetor did not have an accelerator pump. Of course, the service manual (2003-04, © 2012) only says 3300 -> 2800.

For comparison, a competitor's manual for a (simple) chainsaw with a carburetor with an accelerator pump:

1. Find the highest idling speed by slowly turning the L-screw clockwise and counterclockwise.
2. Turn the T-screw until the idling speed is 3 000 rpm.
3. Repeat the procedure in 1 and 2 once again.
4. Turn the L-screw counterclockwise 0,5 of a turn.
5. Adjust the T-screw until the idling speed is 2 700 rpm.
EDIT:
According to a fairly modern EMAK service manual for a chainsaw with a carburetor without an accelerator pump, the values are 3700-4200 -> 2800-3100.
Based on this I would say that probably for chainsaws with carburetors with accelerator pump, to enrich the air-fuel mixture sufficiently the typical value is to reduce the rpm by 500. while for carburetors without pump by 1000 rpm.

There is only one problem with EMAK's instructions, that according to them it is enough to warm up the engine for one minute at idle. I'm not sure if this is sufficient.

Taking all this into account, however, I guess the easiest and the most universal method of setting the idle speed will be the method without a tachometer. In that case, for me it is only sometimes problematic to find a high enough speed so that the engine does not happen to stall later.

If I remember correctly, according to the standards, the clutch engagement speed should be higher than the idle speed by at least 25%.