Stihl 4-Mix - FS110 Carboned Up?

[SteveSr]

Well, @SteveSr, I tried adjusting the start/throttle assembly and the screw adjustment at the plastic loop above carb and it wouldn’t idle down till I put things back a bit the way they were.

The cable must be adjusted to allow the full range of throttle movement. At WOT the carb throttle must be fully open. At idle the throttle must rest on the screw stop on the carb and the cable should be slack.

Well, @SteveSr, After reviewing the manual Harley put up I took the carb off again and tried to remove the accelerator pump and it wasn’t there, go figure.

The accelerator pump is located behind the brass plug on the throttle shaft bore near the throttle arm. If you have a pump problem (unlikely) you need a new carb.

Well, @SteveSr, I tried the H at 3.5 turns and it stumbled bad. Put it back to 2 turns and it runs at 9600 rpm at not quite full throttle but if I squeeze the trigger further to stop it stumbles.

About 3.5 is where I found this one set. If it is that far off it would answer a lot of questions about what happened to this one. However, opening the throttle to WOT and having it bog/stall seems to indicate not enough gas.

It sounds like you, at least, have carb issues like stiff diaphragms or perhaps incorrectly assembled by p. o. Replacement price will depend on which Stihl distributor territory you are in.

 

[billyboy]

The cable must be adjusted to allow the full range of throttle movement. At WOT the carb throttle must be fully open. At idle the throttle must rest on the screw stop on the carb and the cable should be slack.


The accelerator pump is located behind the brass plug on the throttle shaft bore near the throttle arm. If you have a pump problem (unlikely) you need a new carb.


About 3.5 is where I found this one set. If it is that far off it would answer a lot of questions about what happened to this one. However, opening the throttle to WOT and having it bog/stall seems to indicate not enough gas.

It sounds like you, at least, have carb issues like stiff diaphragms or perhaps incorrectly assembled by p. o. Replacement price will depend on which Stihl distributor territory you are in.

I looked at the manual about the pump and saw there was a filter in the pump and Wasn’t aware of it when I cleaned the carb so I was on my way to dismantle it and it wasn’t as pictured in the diagram. I’m not comfortable pulling out plugs like that and they didn’t even have the picture right. Anyway, I readjusted the throttle cable and pulled the H needle out and shot some cleaner thru it. Put H at 3.5 and it Jumped up to 10200 rpm and it seems, sounds like it is running like it should. This dam thing is way too complicated for me as a string trimmer. I needed something more powerful than my fs45 that broke the timing key but this fs90 is about ridiculous. Is there a regular 2 stroke that has some power to it. Thanks for the help.

 

[SteveSr]

I looked at the manual about the pump and saw there was a filter in the pump and Wasn’t aware of it when I cleaned the carb so I was on my way to dismantle it and it wasn’t as pictured in the diagram.

The fuel pump is a part of the carb opposite the metering side. There is a 1/4" diameter fine screen on the pump side of the carb which can get crud in it if the fuel filter fails or gets a hole in it. This screen can usually be removed and cleaned or replaced.

Anyway, I readjusted the throttle cable and pulled the H needle out and shot some cleaner thru it. Put H at 3.5 and it Jumped up to 10200 rpm and it seems, sounds like it is running like it should.

Sounds like you fixed it. 10.2K is probably hitting the ignition rev limiter. What kind of cutting head do you have on it?

This dam thing is way too complicated for me as a string trimmer. I needed something more powerful than my fs45 that broke the timing key but this fs90 is about ridiculous. Is there a regular 2 stroke that has some power to it. Thanks for the help.

Just about all of the Stihl Pro series of weedeaters are 4-mix. If properly cared for they should run for years trouble free.

 

[billyboy]

I never figured out how to remove those kind of plugs or how you fix them if you take them out with a pick. Does a random stihl dealer have those in stock and are they different for every model of carb. Seen a few before and they all look about the same size. Think I may take it to a dealer anyway to see if the valves need adjusting. I have a auto string advance head. Looks to be .80 or .90 size.

 

[SteveSr]

I never figured out how to remove those kind of plugs or how you fix them if you take them out with a pick. Does a random stihl dealer have those in stock and are they different for every model of carb. Seen a few before and they all look about the same size.

If you are talking about the welch plugs found in the carb metering chamber it is best to leave them alone... unless you are dam sure there is a blockage under them.

Think I may take it to a dealer anyway to see if the valves need adjusting.

Valve adjustment is simple. Set the engine to TDC on the compression stroke (both valves closed) and check/adjust. The kit including measuring gauge is part number 4180-007-1005. Should be less than $5.

 

[billyboy]

I’ve cleaned a few carbs and I always avoid those plugs. I’ve read some folks use fingernail polish to reseal. Maybe I’ll try that valve adjustment. Got all winter to check em out. Didn’t read the process too closely but noticed they had some kind of special stop for the spark plug hole.

 

[SteveSr]

Well, free time and parts finally converged to finish up this project. Here are a couple of notes:

1. The existing C1Q-S174 carb appears to be same as the previous C1Q-S131 with the exception of tamper proof hex screws instead of the former's limiter caps. I never could find the Stihl Tech Note that introduced the S174 carb with its initial settings. If any of you have access to this I would appreciate a copy. It would appear that the initial settings of L=1.25 and H=3.5 are approximately correct.

2. Tuning was accomplished by attaching a string head with .090" round line to put enough load on the engine to keep it off of the ignition limiter. The engine "H"screw was then tuned for maximum RPM at WOT. Since this is a bypass jet carb the tuning wasn't sensitive. I think that I ended up with H=4.25 which was just a hair on the rich side with an outdoor temp of 60 degrees.

3. Some measurements were done with the as-found 0.100" square line (.140" diagonal) and the Stihl recommended .090" round line. Both lines were cut to the maximum length allowed by the shield mounted line cutter. With the Stihl round line the engine ran at 9600 RPM. With the square line it only made about 8800 RPM for a drop of about 800 RPM. I don't know what this equates to in HP but I would consider this a significant drop considering this is a 4-stroke engine and not a 2-stroke.