Hi All, I've been asked by my neighbour to have a look at his chainsaw, which won't start. It isn't that old, it has a spark, and the plug gets wet after a few pulls of the cord with the choke out. Yet it really isn't even showing the SLIGHTEST sign of attempting to start. Not even with a whiff of Easy Start. It's a cheapie MacAllister one, but I'd have thought it might at least attempt to fire! I'm not familiar with it, so I don't know what it felt like to start when new, but compared to my Husqvarna of the same size, the starter cord is much easier to pull. True, it does get easier again with the plug out, but there doesn't feel to be as much compression as I'd have *thought* there ought to be. Does anyone have a feel for what sorts of numbers I should be seeing if I screw my compression tester into the plug hole and give it a few pulls? 140-180 PSI, maybe?
Checking compression on a small 2-stroke
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Petrol?
I'd expect it be around 150 so yes
But no experience of testing them
I'd expect it be around 150 so yes
But no experience of testing them
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Is the plug clean, better still, try a new one. Some spark plugs have a resistor in (that suppresses interference) but those can go out and stop the plug from sparking in the high(er) pressure environment of the closed cylinder.
From my days of 'messing' around with Two-Stroke powered machines a new plug solved most starting problems.
Does the machine have points or does it spark depending on where the magnet in the flywheel is relation to the piston position - that is not so easy to determine. Could the flywheel have slipped on the crankshaft putting the timing out?
From my days of 'messing' around with Two-Stroke powered machines a new plug solved most starting problems.
Does the machine have points or does it spark depending on where the magnet in the flywheel is relation to the piston position - that is not so easy to determine. Could the flywheel have slipped on the crankshaft putting the timing out?
If you don't have a gauge......
Stick your thumb over the plug hole
Crank the engine over smartly and you shouldn't be able to stop the gas escaping.
Whilst you are there, invert the machine, crank it over and blow out any fuel that will have collected in the crankcase.
John
Stick your thumb over the plug hole
Crank the engine over smartly and you shouldn't be able to stop the gas escaping.
Whilst you are there, invert the machine, crank it over and blow out any fuel that will have collected in the crankcase.
John
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Anything above 110 psi, should be acceptable, but a rough check would be to put your finger tight over the plug hole, and turn it over - it should blow your finger away. Poke a pencil down the plug hole and make sure it fires, near TDC.
[EDIT] You can sometimes detect the point of firing, using a cheap, portable MW radio, tuned to where there is no station.
Last edited:
I meant to add.....
If you pop off the exhaust silencer you can see the piston, rings and barrel condition.
Any scratching here and the engine is in bother.
John
If you pop off the exhaust silencer you can see the piston, rings and barrel condition.
Any scratching here and the engine is in bother.
John
Thanks. I've tried three plus (one of them nearly new). I'm pretty certain it's not the plug, though like you, they are my first port of call! Timing could be out, I guess, but I can't see any evidence of the flywheel having moved. The lacquer blob on the nut is still intact.
If you don't have a gauge......
Stick your thumb over the plug hole
Crank the engine over smartly and you shouldn't be able to stop the gas escaping.
Whilst you are there, invert the machine, crank it over and blow out any fuel that will have collected in the crankcase.
John
Thanks, I'm OK for a compression tester but I will try inverting it and giving it a few pulls.
That'll be my next job, if compression is down. To be honest, I'm reluctant to get involved too deeply, as there is only likely to be a bag of cooking apples in it for me!
Yep, that’s a knacker!
Lubrication and/ or air filter failure big time.
John
Lubrication and/ or air filter failure big time.
John
He's got "form" for this sort of thing. I've seen him mixing 2-stroke before, and no matter what the ratio is supposed to be it always just gets "some". Sometimes you can't see across the garden for smoke, other times... well... you've seen the piccy!
Depending on the make, piston and barrel kits are available at really competitive prices (Stihl, Husqvarna etc) - but if it's uncommon or some B&Q thing then it's scrap.
People just don't listen and learn!
John
People just don't listen and learn!
John
Depending on the make, piston and barrel kits are available at really competitive prices (Stihl, Husqvarna etc) - but if it's uncommon or some B&Q thing then it's scrap.
People just don't listen and learn!
John
It's a cheapo MacAllister. I'd have to see if there was bore damage as well, so it's really just not worth it.
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