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?

 

[irvingj]

Iplayfarmer- all of these suggestions are valid, but I would still go on the assumption (I know, that can be dangerous..) that you're not getting fuel delivery from the carb, but other things are OK.

Go ahead & try a new plug; can't hurt. As you say it runs when "primed" I wouldn't worry about the muffler yet, but it's true they will clog up.

I'm guessing you have one of those small Walbro carbs and you put in a carb kit- did the kit come with a diaphragm?

It sounds like your fuel line TO the carb is OK, as you indicate fuel will flow out when it's disconnected.

If it can't get fuel from the carb, which is what sounds like is going on, there are a few possibilities:

1. The diaphragm uses the pulsing pressure in the crankcase to flex back & forth and "pump" fuel through a checkvalve. If the diaphragm has a hole, or is not seated/sealed properly, it won't funtion correctly. There is also that "bleed port", an opening into the crankcase area behind the carb to expose the diaphragm to CC pressures. Also, the carb gasket needs to have a good seal.

2. The cyclical pressure in the crankcase is caused by the movement of the piston up & down, creating both a negative and a positive pressure in the crankcase, alternating between the two. Piston coming down (power stroke)= CC positive pressure, going up (compression)= negative. If there is a compromise in crankcase seals, you won't get the necessary pressure differentials to operate the diaphragm, and if the negative pressure isn't strong enough, it won't pull fuel & air in through the reed valve. But again, as it DOES run --as long as it has fuel-- I'd be betting the reed valve's most likely OK.

The bottom crankshaft seal is usually the culprit, but there are a couple of other seals as well; you'd have to take it apart to see for sure. As far as the reed valve, if it looks OK and seems to seal (can you blow through it backwards?) it's probably OK. Look for burning, cracking, or coking.

3. Any compromise in the seals/gaskets can be a problem... suggestions about the primer bulb, if it has one, are good.

As your machine will run when primed, I think you've already eliminated a lot of other stuff....

Sorry this was somewhat rambling! Keep looking for why the carb isn't delivering fuel.

 

[4Kings]

Compression is about 50 pounds.

50 lbs of compression is not a good sign. You could have a scored piston. Pull the muffler and take a look.

 

[Lil_FarmerJim]

I would agree with IrvngJ, I've had these problems with those little carbs and it can be a major pain to fix. General cause is lack of constant fuel flow. The primer forcibly pulls gas from the tank into the carb and returns the over flow back. When you push down on the primer, does it seem hard to push, does it refill quickly or slowly? Did you replace the fuel lines on the primer as well as the main to the carb?

Also did you replace the fuel filter? That is part of the whole system as well. With out a filter it will not maintain a constant draft on the tank. The echo's filters are decent filters since they don't have the metal ends on them that fall aport in ethenal mixed gas. If you have a filter that has the metal button at the end replace it. It will clog the carb in a heart beat and make it a paper weight when the stuff hardens.

As with any advice that is free take it for what its worth.

 

[The Toolman]

Probably to late here, but most older weedeater problems can be traced to a spilt in the fuel line in the tank. I've had to replace the lines on my sthil trimmers a coupla times over the years...

 

[Iplayfarmer]

Probably to late here, but most older weedeater problems can be traced to a spilt in the fuel line in the tank. I've had to replace the lines on my sthil trimmers a coupla times over the years...

In this case it needed a carburetor kit. I gave in and took it to the local small engine shop. They cleaned the carburetor with some kind of jewelery cleaner and put a kit in it.

It runs great now.

 

[irvingj]

Probably an ultrasound tank, Iplayfarmer-- great way to remove crud from teensy places. Glad you got it up & running!

 

[Iplayfarmer]

Glad you got it up & running!

Me too.

After I took the trimmer in, I saw this thread about soda blasting. Next time I may do the soda blasting thing to clean a carb. The concept seems sound, and it beats poking around with a wire or tip cleaner.

 

[irvingj]

Wow- neat idea!!

Ultrasound works great, but I don't have one; last time I took my 750 carbs to my motorcycle shop, I got hit with $30 for the "soaking" (hm-m-m pun there?)....

Well worth it, but the soda idea is now on my list- thanks!

 

[catfishdaddyengineering]

I have replace primer bulbs on 3 Stilh weed trimmers that were all stored inside. The just don't like the ethanol and waste away. New bulbs have lasted awhile now so they must be a different plastic that is ethanol resistant.

 

[RandyT]

the 50 pound compression check tells me you are done.