2 stroke petrol engine starts & dies

[markpayne]

I have a strimmer & hedge trimmer, both driven by a 2 stroke petrol engine. They both start but as soon as they get hot they die. They will tick over for a while longer but as soon as I touch the throttle they die. Each year I change the plug & air filter, plus when I have finished using them I drain & run them dry. They are now about 5 years old.

Please can someone advise me what the problem is / might be.

Yours frustratingly,
Mark

 

[^neo^]

first try some fresh petrol in them, it does 'go off' after a while and seems weird thats it's happening on 2 different machines.
Then there should be 2 little screws on the side of the carburettor, have a fiddle with these and see if it helps.
If not, send them in for a service.

 

[wotan]

Make sure any engine gaskets are in good order and tight, two stroke engines rely on crankcase compression to transfer the mixture into the cylinder.

Wotan

 

[noseall]

Following on from wotan regards compression:

...once the valves warm up they can then allow compression to slip particularly if they are not seating properly, i.e. sooted up.

 

[Stivino]

Yeah, check the valves first

 

[mattylad]

Are you also putting 2 stroke oil in?

 

[Richard C]

Check the exhaust silencer (steel mesh filter) isn't all cruded up.

 

[wotan]

Following on from wotan regards compression:

...once the valves warm up they can then allow compression to slip particularly if they are not seating properly, i.e. sooted up.

Most two stoke engines only have one valve, usually a reed valve, occasionally a poppet valve, on the intake side, make sure this is seating correctly, otherwise there will be a loss of crankcase compression, and some blowing back through the carb.

Wotan

 

[noseall]

[quote="wotan";p="1245793Most two stoke engines only have one valve, usually a reed valve, occasionally a poppet valve, on the intake side, make sure this is seating correctly, otherwise there will be a loss of crankcase compression, and some blowing back through the carb.

Wotan[/quote]

Check any valves....all valves.....see if you can find a valve!

 

[Richard C]

Sorry to burst your bubble noseall & I’m not sure if that’s what the earlier contributors meant but many performance 2 stroke engines do indeed have valves as wotan states; but I thought they were on the exhaust to scavenge wasted fuel rather than on the intake

I very much doubt you will find one on a garden strimmer though

 

[Thermo]

take the air filter and run it without the air filter on and see what happens.

as for old petrol, i dont believe in this. many of my tools lay out of use and start up first time after several months. I started up a strimmer that i know has been standing for 4 years a few months ago and it started and ran fine, with the old petrol in it.

 

[sparkiedave]

they can do that when the piston rings are worn, I have a chainsaw that does it, but it is about 16 years old!!

 

[Burnerman]

Your unhappy machines probably just need a carburettor adjustment to keep them happy....as these things are needed to run at all sorts of crazy angles, the carburettors can't have a float bowl of any size to provide them with fuel, so they use crankcase decompression to provide a flow of fuel from the tank to the carb. The pulse provided by crankcase decompression is governed by 'flap valves' incorporated in the carb diaphragms and after a while these tend to fail. Anyway, if your machines have any mixture adjustment screws visible, they may be marked 'L' for low speed and 'H' for high speed running. Turn the screws in very gently until they stop, and then turn them back out again 1 turn - this should get the machines going again. Small engines will always be a bit temperamental with slow running because the flywheels are so small. I don't think you'll find any sort of valves in these small machines - reed valves on the inlet side are only really used when the primary fuel mixture is drawn directly into the crankcase rather than down a piston port, and an exhaust valve is only incorporated on racing engines, where it effectively raises the height of the exhaust port - giving maximum RPM but no facility to tick over at all. Anyway, all two strokes can be brought back to life so long as they have good compression, a clean air filter and a clear exhaust system, so good luck with yours! Cheers John

 

[TicklyT]

.....as for old petrol, i dont believe in this.....

It does happen, but probably over a year or so rather than a few months. The stuff sold as petrol is a mixture of hydrocarbons. The more volatile fractions will evaporate first, leaving you with the heavier fractions that don't vapourise so easily, so it's like trying to run the thing on paraffin.

Even more so with two stroke mixture - the fuel evaporates leaving a residue of oily gum in the carburettor that was once the lubricating oil. If you're very unlucky, or you've used a castor oil based lube, you can end up with a hard varnish bunging up all the jets and small airways in the carb.

Worn pistons, rings, main bearings and crankshaft seals usually, but not always, cause more problems with starting than high speed running. I remember an old BSA Bantam with worn main bearings - it would start fine, but when it warmed up, the clearances opened up so much the cam no longer opened the magneto points.