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Fuel starvation, can’t remember if you said you have changed the fuel filter in the tank.

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Can you get a pic of the top of the piston, ie from the exhaust with the piston a bit lower. 
i know there will be carbon, but I,m wondering about detonation. Looking at the edge of the piston.


another thing to look, if you can. While the piston is lower in the bore and light is coming in the exhust, look through the spark plug hole and see if you can see any scoring. 
you can also get a different angle looking at the top of the piston. Not that you will easily get a pic this way.


the fuel in the air filter, does sound like wrongly set meatering leaver or diaphragm on carb then gasket.  if you look at the pic you will see the basic way they go. 
if you have a zamma carb you should be able to find your carb here for more detail



edit it maybe running rough because of scoring, and you could be getting a reasonable compression due to fuel and oil



Edited by Wonky

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Nice pictures - pictures always good for adding info. Extra saw history helpful too. 

As you say age isnt a big concern, pictures suggest the insides are in decent condition.

The light scuffing above top ring I've seen once before - it was on a saw i did porting on, assumed I'd left grinding grit in there. Probably fine  brick / concrete grit in your case, suspect its just a visual tell-tale. Worth noting: sometimes the top few mm of the cylinder increases in diameter a smidge (it's a manufacturing thing, honing to top of a closed cylinder is difficult) - its a good place for crap to get trapped.

Compression numbers, thats low at 115psi but depends on your tester. Compare your numbers to what you get on another 'good' 2 stroke with same tester... Two stroke mix down plug hole or flooding the engine will give higher numbers through partial ring sealing - like your picture through ex port which looks shiney / oily.

Personally, I'd keep going with carb cleaning / fuel investigations before anything more drastic, because that picture says the piston is happy. But keep in mind that compression number is low.

Fingers crossed for you.




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thank you for so many  helpful replies people.

very embarrassed to say, pump diaphragm and gasket order wrong.

can’t believe it :( so simple 

almost didn’t want to admit it.

moving on.... some pictures for you.

 Looks pretty smooth right?

I don’t know if I want to dismantle the bellow to get a look at inlet side unless you guys think I need too? 
might try a bolt in the decompression valve hole to see if that is causing loss of compression? Is that a done thing? 

I don’t ever remember moving or tilting effecting idle so guessing it’s not crank seals? 

thank you so much for help and support people! 






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Well that piston looks a lot better than I expected as well as the cylinder.


And thanks for the honest reply about the carb. It was for simple clarification  I posted the ipl pic of the carb even thou adw said about it first. 



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That's sounding hopeful !

Re carb - I've done the same thing with the metering diaphragm before now, round and round in circles trying to sort that one.


The pictures all suggest piston, rings and cylinder are happy. If it now runs properly I'd be looking for an excuse to dismiss the low compression number. Might be the compression tester isn't really the right one for a small 2 stroke engine. ... Might be the decomp valve, blanking it off with a bolt and sealing washer is the cheap solution, common practice. 

Leave the rest of it assembled, less cleaning and less chance of getting crap inside the engine.


Thanks for update btw.


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Hopefully it will be the fuel sorted now! 
gave the air tube inlet things a real good clean today too they very thick with petrol/dust gloop.

took the bellows off as I could see dust/ gloop in them where I’ve been disturbing stuff and have them cleaned them up good with soap and air.

took decomp valve off, pretty dirty tried getting carb cleaner in there and open closing loads and I duno if I imaging it but I think it’s closing better. Ordered a blank plug too tho to retest with. 

thanks so much again guys.


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There should be zero dust in the inlet tubes, if this has entered the cylinder and mixed with fuel you have lapping paste,,  this will wear the ring and remove the nikasil lining off the inlet side of the cylinder, this may well be why you have low compression, is so it will never run right, I would be lifting the cylinder before wasting too much time messing with the externals.

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