Stihl Trimmer Ticking Me Off

[Iplayfarmer]

I picked up a Stihl FS36 trimmer the other day for 20 bucks (I got four new bottles of 2-cycle oil in the deal too). It seems to be in pretty good shape except it wouldn't start. If I squirt anything into the carb (carb cleaner, sea foam, gas mix) it starts and runs like a top for about 3 seconds until whatever I squirted runs out.

I've had the carb apart 3 times now. Every orifice I can find is clean and open. I've put a new carb kit in it. I've checked the fuel line. It's open and clear because it will actually siphon fuel if I let the carb end of the fuel line drop below the fuel tank.

What's left to check?

I don't have a problem working on things when there are obvious problems. It's when I've done everything I know to do and everything looks like it should work that I get frustrated. I'm there now.

 

[Mac624]

have You checked the crankshaft seals?? As You are squirting gas into the cylinder, You are using another gas ratio then the carb can deliver.

How is the compression??

 

[irvingj]

I'd check the seals, too... if they aren't tight, the engine won't be able to suck air & fuel into the crankcase on the compression stroke. (As piston travels up, it creates a "negative" pressure in the crankcase.)

This will leave no air/fuel mixture available to pass through the bypass port (and into the cylinder) on the subsequent power stroke.

But it will run just fine if you bypass the 2-stroke process by squirting fuel into the carb.

Try putting low-pressure (5-10 psi) air into the plug hole and listen to see if it escapes past the crankshaft seal(s).

Sounds major, but really not that hard, usually, to replace the lower crankshaft bearing & seal. Two-stroke engines are wonderfully simple. I'm guessing this engine also uses a reed valve, which also needs to be checked.

Have some fun! Rip it apart! Just have some crankcase seal available for reassembly (see a motorcycle shop).

 

[Iplayfarmer]

How is the compression??

Compression is about 50 pounds. That does seem low to me. I am still trying to understand, though, how the engine can run so good when I prime the carb and be just dead when the prime is used up.

I'd check the seals, too... if they aren't tight, the engine won't be able to suck air & fuel into the crankcase on the compression stroke. (As piston travels up, it creates a "negative" pressure in the crankcase.)

This will leave no air/fuel mixture available to pass through the bypass port (and into the cylinder) on the subsequent power stroke.

But it will run just fine if you bypass the 2-stroke process by squirting fuel into the carb.

Try putting low-pressure (5-10 psi) air into the plug hole and listen to see if it escapes past the crankshaft seal(s).

Sounds major, but really not that hard, usually, to replace the lower crankshaft bearing & seal. Two-stroke engines are wonderfully simple. I'm guessing this engine also uses a reed valve, which also needs to be checked.

Have some fun! Rip it apart! Just have some crankcase seal available for reassembly (see a motorcycle shop).

I took the plastic cover off the bottom end of the engine. The seal looked good, but I'll try the compressed air trick. Here's hoping I have a regulator that will put the pressure down to 5-10 psi. I also looked at the reed valve. It looked like a reed valve. What are signs of problems with the reed valve?

Also, what is crankcase seal? I have some red RTV (high temp). Is that good enough?

 

[Thomas]

Does the muffler have screen,for that might be plug w/burnt carbon.

Maybe a bad plug.

 

[oliver28472]

Ditto the screen. Throw it away.

 

[Iplayfarmer]

Would a bad plug still run when the carb is primed with fuel mix?

 

[kthompson]

I don't think it would be true in your case but have a Stihl leaf blower that was dependable and then would not crank. Took a while to notice "dirt dobbers" had filled the exhaust holes of the muffler. Did find before I got to trying fuel in the carb.

 

[Gator6x4]

Does the carb have a fuel primer bulb? Can you pump the bulb and get it to fill with fuel and stay full? If not then the white plastic piece the primer bulb is attached to is defective. If you remove the bulb you should see what looks like small brass rings or ports on the white plastic piece. Inside each of the holes is a plastic diaphragm if a hole gets in the diaphragm the unit will act as you described. DO NOT USE AIR PRESSURE TO CHECK TO SEE IF THE DIAPHRAGMS ARE OKAY. Hold the unit up to a strong light. I know awful small hole. Some people damage the diaphragm by cleaning the hole with a piece of wire.

 

[2manyrocks]

Agree that you should check fuel primer bulb--seem to be a source of problems.

Is the vent on the fuel cap working? Even if the fuel line is good, is there some kind of leakage in the connection where the gas line goes into the gas tank? Also, maybe there is a strainer inside the gas tank that has come loose from the fuel line and maybe the line isn't actually picking up gas inside the tank?