Stihl FS-61 / won't run at high speed

mvronMay 24, 2014

I have a Stihl FS61 trimmer. I have replaced the carb with a Walbro WT-38-1. The original carb had the choke lever on the air cleaner cover. This carb has the choke lever included on the carb - the little tube on the side. I have been trying to adjust the high speed jet to no avail. I have replaced the flange gasket also. When the trimmer starts at idle, the trimmer "chokes" and dies when increasing the speed. . I leave the original choke lever - the one on the air cleaner - open and use only the choke on the new carb when attempting to start. It dosen't seem to matter how the high speed jet is set. Appreciate how the high speed should be adjusted starting from the beginning. Idle jet closest to the engine, high speed jet closest to the air filter. Is this correct? I removed the fuel filter from the tank so fuel flow should not be a problem. Replaced the fuel line. The on/off fuel lever was broken so I re-drilled the fuel opening in the on;off lever and made sure it is completely open when being re-installed. Ideas would be appreciated. Could it be that the flange gasket is not installed correctly?

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yes, the high speed mix screw is closer to the air filter.
How many turns out do u have both screws?
Gun it and if it bogs and looks like it is going to flood, Let it settle back to idle, turn in the HI screw a little more like 1/8 and keep doing that. By using a non original carb on my fs65 and 80's mine are not the standard 1.5 (or whatever) turns. my low is 2 and high is 1/8 turn.

When you get it close to running right, you might have to adjust both because there is speed midrange where it draws gas from both high and low. Your case, your idle (if currently high enough) may go lean. etc. So as u reduce the high speed fuel and it wont idle any more, give it a little more gas for idle, etc.

    Bookmark   May 27, 2014 at 12:36AM
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No matter how I set the high-speed jet it always almost dies when the throttle is increased. I set the highspeed to 1 1/2 turns from stop and the low speed to the same. It's really hard to start - but when it does the low speed runs too fast and the high speed dies when the throttle is increased. Trying to slow down the low speed does nothing to get the high speed to run - does nothing to help with the hard starting problems. .Like I said, I'm running the machine using the choke on the new carb - and leaving the choke on the original air cleaner open..

    Bookmark   June 2, 2014 at 4:48PM
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First, i didn't say to slow it down, i said as u adjust the HIGH FUEL MIX it will affect the mid range mix. if you are talking about "..So as u reduce the high speed...", that is AFTER you get it to run high and only need to trim. Given, ideal would be 1.5turns for both screws but when u r hacking like i said...

" does (run,) the low speed runs too fast." vs your original post: "When the trimmer starts at idle," is misleading.

Does the gas cap have a hole in it? if no hole, leave the gas cap loose when u are testing. It IS a gravity feed carb...

    Bookmark   June 3, 2014 at 7:07PM
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Before taking the trimmer to the dump, one last attempt.was made. Reset both jets to 1 1/2 turns, loosened the gas cap. It started and ran perfectly. Could have been the gas cap. Now I need to build a usuable handle for the choke lever on the new carb. That little tube is completely useless. I don't know why the carb makers couldn't make a useful handle on that little tube. Many thanks to "out_sider" for all the ideas. It was much appreciated.

    Bookmark   June 4, 2014 at 9:15PM
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