Author Topic: Clone saw carbs a cheap troublefree substitute , on brushcutters  (Read 283 times)

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Offline joe_indi

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The problem with most OE carbs for brushcutters are they are designed to run very lean and chances are they get out of order rapidly at times.
I had a couple of Stihl brushcutters, FS 55, 85, 120 200 all with carb issues that customers abandoned because of carb replacement.
So i had a carb of a clone saw and had the bright idea to try it on a FS 55.
So serious ( initially) modification was required. But once you know your way around it, it's not such a big issue.
The first issue was the impulse. On the saw carb the impulse is connected through a tube. But the mold of the pump section has the areas for the internal impulse and all that has to be done is drill holes or cut paths, and block off the connector ( i removed the connector and plugged the hole)
Checking whether the new impulse works is simple. Remove the metering diaphragm, tighten the carb onto the engine, with spark plug removed and metering lever pressed pull the starter. If impulse works the fuel is pumped out. I then closed up the metering chamber and primed the cylinder and started up. Started first shot. But setting L and H  is not too simple because the fuel delivery is different between a saw and a trimmer. But once you get it right it really works and with oodles of power potential! So i opened up the muffler a bit , filed of a bit of the exhaust port to slightly advance the timing, and , that FS55 became a different animal altogether!
So, i did it again on another FS 55 and a third!
Then i thought if it can be done on a FS55, why not the old FS85 i had, worked great. So i tried it on a FS120 still great and so was the FS200.
The obstacles ( i saved for last) changing the throttle shaft and in the case of the FS 120/200 the choke shaft. Some careful vwork with a round file is needed , that is all,but the efforts are worth it. One issue i had was on the 55 and 85 i had to do without the primer that sits on the carb. But, old Stihls like my FS160 never had one. An extra pull or two are all that are required . I made the mistake of trying a 3 tube carb of a saw that has priming option. I tried it on the FS120. Other than pumping up the fuel into the carb earlier that primer sticks like a sore thumb because it plays hell with the tuning. So i decided to eliminate the primers. Fuel consumption has not increased, but the power delivery is much better. The biggest surprise was the 55, it's like a little monster in its all out form.
In case anyone is hooked i could upload videos to my Google drive and post the links here
Also photos

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