Thread: Chainsaw question - engine dies

I have a Dolmar PS221TH top handle pruning saw. Awesome piece of kit, until now.
It starts and runs fine for about 15-20 second and then dies. Just before it dies, if you give the manual fuel pump a push, she will keep running for another 15 seconds. So its seems to be starving fuel.

Air filter is clean.
Fuel filter is new.
Spark arrester has been ultrasonically cleaned.

Will a fault in the carb typically give the 'run, die, run' symptom? Should I be looking for a blockage or a knackered diaphragm or reed valves?

I have never pulled a carb apart, so be gentle

Perished fuel hose is another contender I would look at first.

Does it run one of those vacum powered diaphram fuel pumps inbuilt to the carb? If so the vacum hose or diaphram have a wee hole in them.

Carb Diaphragm or air inlet leak in inlet manifold

Change fuel filter too

Google the vacuum pulse supply to the carb diaphragm, a leaking/split inlet manifold can stop the vacuum pulse getting to the carb diaphragm

breather hose to tank , normally just a hose with a self taper in the end or a metal with a cut down side to let air in . If stored on sde so fuel runs back down breather and it then evaporates it gums up air slots . happened on my solo when stored on side for mths ,so now store upright on rag for oil leak .

A little while ago after cleaning one of my chainsaws, when I put the top back on it pinched the fuel line so reduced the fuel supply.
If ya lucky an easy fix

have you tried a remix of fuel???? I double dosed my newer saw and it would run but not rev..the wee fuel filter was clogged....

Fuel hose cracked or holed

Before you go replacing parts or fiddling around to much give it a nudge with new gas. I've found chainsaws can be really finicky if they've been sitting a while and the gas is old - easy fix if that's the issue.

Gas is fine as I run it in two other saws without issue.

I put aside my fear of small engines and pulled the carb off after watching a couple of youtube videos. Carb looks all good under 3.5x magnifier. Was super clean in side. I was kind of hoping that the gauze filter in the carb would be blocked, but no. Not to say that there isn't a hole in the diaphragm, but I certainly could see one.

Fuel lines next, but being a top handle saw, I have to pretty much pull it to bits to see.

Was the diaphragm soft and flexible? As 7mmsaum says it’s a very common failure. The other big one is a crankcase leak from the crank seal or joint.

Originally Posted by 2post

Was the diaphragm soft and flexible? As 7mmsaum says it’s a very common failure. The other big one is a crankcase leak from the crank seal or joint.

Yea, everything was nice and soft. They looked brand new (even though I know they are not).

Nice and soft might be just as bad as hard if it is the diaphragm pump type. The diaphragm plate should be perfectly flat so it seals, if it isn't it won't work...

Now to be devils advocate here, I have a cheap 2 stroke generator here that I've been blaming the carby on. Would start and fire right up, stagger for about 10-15 seconds then cark it. I did every check I could think of, including replacing the carb which didn't change a thing. Then I spied the compression tester and hooked that up - only pulling 50psi max. Hmmm...

Taking the thing apart (it is effectively brand new but died after around 5 minutes of running and the owner just chucked it back in the box about 7 years ago leaving the fuel in it) the piston rings have a tiny roll pin that stops them spinning, one of the pins had backed out and dragged a score mark down the bore below the transfer port. So that's something to check - compression and vacuum testing the thing to make sure it's not a mechanical failure before you start replacing parts randomly chasing something that seems like the cause but isn't (like I just did haha).

Is the compression test the same as a 4 stroke?
Vacuum testing?

Originally Posted by hotbarrels

Is the compression test the same as a 4 stroke?
Vacuum testing?

Compression is the same as four stroke, reading the max cylinder pressure that the cylinder and piston can produce. Vaccum testing is a little different, usually you blank plate the air filter end of the carb and the exhaust port and with the piston at the bottom of its stroke pull a slight vacuum with a hand pump tester (screwed into the spark plug hole for the one I have access too). This checks basically everything soft that seals the crank case, crankshaft seals and any gaskets and rubber boots to make sure that there isn't any air leaks into the crank case leaning the engine out.

Well, thats the theory I have on it - there are other tests that people do that a supposed to show leaks but some of it seems like black magic and snake oil to me to be honest...

And that compression test I did on that little generator - that was after about three odd weeks on and off including waiting for bits ordered from China so not really my finest work. Seems that as far as I can figure someone had been into it a little before me and rewired it wrong which meant no spark (despite the owner telling me he hadn't done anything with it and just packed it away).