Thread: Stihl won't start

With the two strokes the problems stem from fuel, spark and compression in roughly that order. Most likely is you've got dirt in the carb or a fuel saturated diaphragm. Replace or clean the air filter, often a problem with saws. Next check and if needed replace the spark plug. If the plug is fouled replace it. Do you leave fuel in it when not in use? Try cleaning the carb and if it still won't run install a carb kit. Does it have good compression? If not replace the rings. Lot's of other things could be wrong such as detached cc vacuum tube, sheared flywheel key, clogged muffler, magneto defect, bad plug wire and so on.

Modern fuel does not take nicely to being left in the tank or carbi.

Originally Posted by rob streeper

With the two strokes the problems stem from fuel, spark and compression in roughly that order. Most likely is you've got dirt in the carb or a fuel saturated diaphragm. Replace or clean the air filter, often a problem with saws. Next check and if needed replace the spark plug. If the plug is fouled replace it. Do you leave fuel in it when not in use? Try cleaning the carb and if it still won't run install a carb kit. Does it have good compression? If not replace the rings. Lot's of other things could be wrong such as detached cc vacuum tube, sheared flywheel key, clogged muffler, magneto defect, bad plug wire and so on.

Thanks for your reply Rob,
A coupe of things I forgot to add in my original post, that being:
1. I replaced the spark plug
2. The retired mechanic replaced the rings (not the piston)

Your reply will certainly help when talking to the small motor guy I will be seeing.

Cheers,

Nev.

Originally Posted by cqvillas

Thanks for your reply Rob,
A coupe of things I forgot to add in my original post, that being:
1. I replaced the spark plug
2. The retired mechanic replaced the rings (not the piston)

Your reply will certainly help when talking to the small motor guy I will be seeing.

Cheers,

Nev.

Try blowing out the air filter (reverse direction of course) with an air compressor blow gun. Be careful not to hole the filter. If that doesn't work I suggest cleaning the carb.

When cleaning an air filter with a compress the safest way is to blow at right angles across the filter rather then directly at the filter. There should be enough air flow to clean but not damage the filter.

I doubt it will be the air filter. Those big saws (like my old 076) can run with a half inch layer of sawdust on the filter. It just makes them run a bit richer than usual.

One of the things that can cause what you are seeing is a leaking crank case or impulse tube.
The impulse tube should be replaced periodically if it hasn't be done.
Did the mechanic perform a crankcase pressure test? This should be SOP after changing the rings.

You say it doesn't like running on its side - will it start and run in the upright position?

Did it bog down before it stopped. Mine did today as it was being pinched in the cut by the slab I was cutting. First couple of slabs were holding up a nice wide kerf behind the saw so I forgot to keep checking to see if it needed wedges. Anyway that would not stop it from starting after you pulled out. Clean carby and fresh fuel?

Carby needs work.

Use fresh fuel.

Thanks for your reply BOBL,

The air filter in the saw is virtually brand new & quite clean.
The saw runs well when upright, but I did notice a bit of fuel leaking when it was on its side & I'm sure this has something to do with it stopping but can't be sure.
The fuel wasn't the freshest but wasn't that old either.
The retired mechanic did say that these old big saws can sometimes boil the fuel in the carby, whether this is true or has anything to do with it stopping I don't know.
I'll reply when I get a diagnosis from the saw doctor.

Cheers,

Nev

Originally Posted by cqvillas

Thanks for your reply BOBL,

No worries

The air filter in the saw is virtually brand new & quite clean.

Great

The saw runs well when upright, but I did notice a bit of fuel leaking when it was on its side & I'm sure this has something to do with it stopping but can't be sure.

OK, in this case it could be the movement of the fuel filter in the tank. On very rare occasions the filter moves in the tank and ends up sitting the level above the fuel. You can check if this is a problem by filling the tank completely and seeing if it keeps running. All fuel tubes including the impulse tube to the carby should have been replaced when the carby was serviced - if not then running the saw on its side may open up a small crack in one of these tubes and cause the problem you are seeing.

The fuel wasn't the freshest but wasn't that old either.

I never keep regular (lowest octane) fuel for use by chainsaws for more than about a week. I buy highest octane I can get and keep it for no more than 4 weeks
I used to put the old fuel in my car and top off the tank immediately but now we have diesel vehicles so I can't do that. Recently I bought a whipper snipper and I use the old fuel in that.

The retired mechanic did say that these old big saws can sometimes boil the fuel in the carby, whether this is true or has anything to do with it stopping I don't know.

It's very unusual for this to happen.

I used to into model aircraft with similar fuel systems, plastic tank, flexible fuel lines with a clunk pickup, fuel either pumped to carby by impulse tube operated pump or more commonly, system pressurised from exhaust (glowfuel, not as volatile as petrol based fuels so very little chance of blasting the tank contents).

Problems like you have been experiencing, (start run OK in normal position, cut out when orientation changes and unit has run maybe 5-10% of tank contents through creating an air gap) were generally related to a pinhole, nick, cut etc in the fuel supply tube within the tank. Not an issue with a full tank as the defect is submerged in the fuel, not a problem initially when you change orientation as it is still submerged, but at some time during the run, normally early on, the defect emerges from the diminishing fuel reserve into the air space in the tank and sucks small amounts of air, which causes the fuel in the line to foam resulting in a lean run and eventual stopping. Often the hardest part of the system to get to, so the last to be checked. New plumbing in the tank and the problem generally disappeared and a very quirky motor became quite quite predictable and reliable.

For a quick check try comparing viable run times in vertical position and horizontal position, a significant difference often indicates a leak in the tank internal plumbing.

Thanks for your input malb, I will certainly be giving this information to the mechanic as well.....

Cheers,

Nev