Stihl FS80RE Won't Stay Running

[Nelson Ogden]

A friend recently gave me a Japanese Stihl FS80RE trimmer (or brushcutter as Stihl called it). This is the old version with the TK slide carburetor. He hadn't used it in 15 years. I replaced the fuel pickup, fuel lines, primer bulb, carb gaskets, and both carb diaphragms. Also disassembled the carburetor as much as possible for a thorough cleaning. For cleaning, I sprayed every part with carb cleaner, plunged each passage with a fine wire, and carefully blew out with compressed air.

After doing all of that, it starts easily but won't stay running. I went back through the fuel system, checked all of my work. Also disassembled the carb, re-cleaned, and this time, I removed the adjustment needles during cleaning. I was very careful to note how far out they were so I could put them back in the same spot since presumably, the adjustments haven't been touched since last time it was running. Still have the same problem.

I did notice that if I keep pushing the primer bulb once the motor is running, it will stay running longer. Seems like the carb just isn't drawing fuel in properly.

I was careful to install the fuel pump diaphragm in the proper order according to the illustrated parts list as follows: outer housing/shell -> diaphragm -> gasket -> inner housing/shell. Also made sure the tiny o-rings between the fuel pump and carb body are in place.

Anything else I should check?

 

[KYsawman]

There is a check valve in the shindawa fuel lines inside the tank underneath the filter. I am pretty sure that Shindaiwa made the trimmer for Stihl. I have had some trouble with the replacement pump gaskets, the little flaps that act as the one way valves try to curl up. Also check to make sure needle clip is in place on the slide. Is the impulse passage clear through the intake manifold?

 

[Nelson Ogden]

Thanks for the tips. It appears the primer bulb assembly contains a check valve and it seems to be working (fuel goes through in the correct direction but not backwards).

The little flaps on the pump gasket look OK but that's still a possibility. The old diaphragm looked like thin rubber and the new one looks like reinforced plastic. Maybe a rubber diaphragm would be better. I can't reinstall the old one; it disintegrated when I removed it.

I didn't see a needle clip on the slide, though the needle seems to be firmly inserted in the slide.

I followed the impulse passage all the way from the pump through the carb to the engine and all passages are clear. I didn't go into the engine -- should I? When I pull the starter, I don't feel anything at the impulse hole on the engine, but I don't know how strong the impulses should be.


There's a new twist. I found that if I lay the unit on its right side (muffler down, gas cap up), I can start it and it will stay idling. If I slowly pull the throttle and bring the engine speed up, I can then turn it right side up and mow with it for a while. The instant I notice the speed dropping, I immediately release the throttle and turn it back on its side and it will sometimes stay running. Then I can slowly bring the speed back up and mow again (for 30 seconds or so). The gas tank is full and I verified that the fuel pickup is staying submerged the entire time; even when it is right side up. With it running, I played with the adjustments (going as much as 1.5 turns each way) and didn't notice any change in how it operates.

 

[Larry B]

Check your tank vent. Run the carb through a carb dip for a couple days or an ultrasonic cleaner. Some of the Japanese carbs have tiny passages. Check all crankcase bolts are tight. Small crankcase leaks will do strange things. Look for oil at the case seams. Double check carb fuel pump surfaces are absolutely clean with no residue from old pump valves. The TK carb have a non serviceable check valve. When they fail you get weird probs. The old slide carbs are very different from the cube carbs on most stuff now. If you look around the web there are threads where guys put a new cube carb on stuff with old slide carbs.

 

[Nelson Ogden]

Finally got it running but had to replace the carb. I bought a used Shindaiwa S-25 carburetor on eBay. I didn't do anything to it other than to swap fuel pumps. I re-used my old fuel pump because the fuel pump on the Shindaiwa unit had a larger fuel inlet nipple and I didn't have the right size fuel line to fit it. Also, I was curious whether the problem was my pump or whether the problem was in the carb. Since my old fuel pump is running on the replacement carb body, I guess the problem was that check valve in the old carb body.

I did read another thread where people describe drilling out and plugging the check valve. I'll keep my old carb and if I ever have problems with the Shindaiwa unit I am using now, I'll try drilling out the valve as others have suggested.

So thanks again for all the good suggestions! I used the unit for about 15 minutes and it ran perfectly.

 

[MACHINE]

Check your needle settings.


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