Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

Hi, I'm trying to repair a Stihl blower for my community pool to have a backup.

Life guards told me it was making "rough sound" when running & then stopped altogether. It was very hard to cycle with pull start. I poured some 2 cycle oil down spark plug hole & after a day, it pulls ok now. Wouldn't fire with fresh gas. Compression is 80 after pulling a 2-3 times. Took off carb & muffler & piston looks ok. Cleaned & blew out carb without breakdown, put back together, tried some start spray, still won't fire. My multi meter shows voltage when I spin fly wheel around on fly wheel component (don't know what to call it - has wire to spark plug & shunt wires to 0/1 switch.

When I pull the starter rope I hear a knocking noise. Next step is to pull off the fly wheel & housing & look under the piston to see the noise.

I'm still wondering why it won't fire at all with starter spray & run with bad sound as before.

Any ideas are welcome.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

I would also check the blower assembly on the other side of the fly wheel..Ive seen them come loose before and clunk around.
Make sure you clocked the rings properly when putting it back together 80 is a little low for a 2 stroke.

Tom

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

Also check the spark arrestor.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

Yeah,,I throw all my screen type spark arrestors right into the trash can where they belong.

All you have to worry about is cleaning out the exhaust port once every couple years or so.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

Thanks for suggestions.

I'm not sure what to do to check spark arrestor!

I went ahead & tore it down. The piston looks pretty good except for 2 vertical scores on side of piston. The scoring on cylinder wall stops before the exhaust & carb ports.

I think the noise was from one of the crankshaft bearings. I dug out some metal shards & the bearing spins freely now. I suspect the shards were from the scoring in the piston.

So wondering if it's worth putting a new set of rings in it & see what happens. I see a new head & piston online for $30.

The piston rod seems to have some play where it connect to crankshaft you can move the piston over & back about +/- 15 degrees. I'm wondering if this should be really tight or not. This would also have been a source of noise.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

Spark arrestor is just a small square screen usually that is under a small cover with a couple screws right at the end of the muffler.Stihl uses one bolt type and the arrestor is round sometimes,,just cut it off it gets all clogged full of carbon and oil.

The piston shouldn’t be really tight,,the rings actually are what takes up that small gap and seal for compression.Sounds like u didn’t loose alot of metal all the way around the piston or walls....some minor scoring is acceptable especially by the exhaust side on a 2 stroke,,Ive seen 2 cycle engines run with horrible looking pistons.

If you have a digital caliper I would see if you can find the specs for that piston and make sure its within spec also but not necessary.

I would cross hatch hone the jug quick with some 600 to 1000 grit sand paper and put new rings in it for $30 especially if the jug comes with it...would also be a good idea to do a leak down test and make sure the crank seals are still good...you can make a leak down tester out of a harbor freight compression tester tool and an air compressor.

Also for crank bearings,,do a quick check and grab a hold of the piston rod,,make sure you dont have any up and down slop in there,,there will be a little side to side but up and down is a sign of bad bearings,,either in the rod end or crank,,also see if the flywheel has play up and down.

Tom

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

I have had problems with a few residential hand held blowers that were used commercially.
Some bolts unscrewed under the flywheel till they eventually fouled on it.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

I agree with Bert,,I had jug bolts come loose a few times also..vibration is a problem with some units.

Re: Stihl blower, comp>=80, knocks when crank rope pulled, won't fire 2 cycle spray

As for the piston & rod, they are both on needle roller so have no latteral support.
Thus they will move sideways.
This is not a problem and allows the piston too self align in the bore.
What they can not have is any movement along the bore,
So you put your finger on the piston lightly pressing down.
with your other hand you slowly rotate the crank.
The piston must go up & down smoothly with no play.