372xp won’t start.

[spudulike]

Just now, adw said:

That machine should at least fire with some fuel down the inlet, if you are not getting primary compression in the crankcase the fuel will not be sent up the transfer  ports so no fuel to the plug, I still think we have a large air leak and possible main bearing issue.

With 75psi??....it would need around 130psi bare minimum to fire and run. The 372 bottom ends are pretty solid. The seals tend to be pretty good from past experience. The only thing I ever found was very slight air leaks down that sleeve that goes round the crank on the clutch side or a lightly leaking clutch side seal but never had a major leak. 

The compression is non existent, unless the gauge is a car one and the compression figure is much higher but hasn't the gauge been tested on another saw??

[Craigb]

For clarity this saw arrived with me with a new  piston and cylinder on it and it came with the original piston and cylinder in the box. The piston you can see in the pictures with two rings is the new one. The original piston and cylinder that came with it was badly scored on the exhaust side. Whoever had it before said it started with full throttle once but they couldn’t get it to fire since. It came with a new spurious coil in the box with the blue one on the saw. Judging by the double gasket on the exhaust the previous owner didn’t build it correctly when they put the new cylinder on so who knows what I’ll find when I strip it back. 

[Craigb]

No the gauge hasn’t been tested yet.

[spudulike]

So the compression reading of 75psi may be 170psi? What is the gauge...make, type etc If you lift the saw up by the starter handle, does it tumble to the ground of hang there on each compression stroke?

[adw]

I was going by the new cylinder Steve ( even if it is a Chinese copy ) I would expect at least a pop from it, anyway let’s see what we find when it’s in bits, and why did the original piston seize ? Like to get my hands on it, 5 minutes to do a pressure/ vac test before the cylinder is lifted.

[spudulike]

Yup, if you get duff info then you get duff diagnostics. I was working on something going severely wrong with the top end causing the massive drop in compression - a ringless piston would make 75psi. 

As you said, that 5 mins in your hands usually tells a big story and usually gives you 75% of the story in most cases.

The pressure and vac test is the foundation to any good running machine and starts any decent diagnosis on an old machine like this. 

[adw]

I get duff information every day of the week.

Ive been using these machines for years, and my father before me

I mix all the fuel myself

I buy the best oil money can buy

I have twenty other machines running on the same fuel

i read the operators manual every night before going to bed

It must be a Friday night machine

I know my rights 

etc etc.

[Craigb]

Sorry for the confusion. I think we could all agree if the three of us were looking at the saw in person there would be no ambiguity. I’ve had very limited time to solely focus on this saw. I think you are right in saying I’m getting false reading with my compression gauge because the compression feels ok judging by the force needed to pull the rope. You can indeed hold the saw by the pull start and it slowly drops at intervals relative to the stroke if that makes sense. It definitely doesn’t just plummet. I’ll vac and pressure test it. I know the vac tester is good.

[spudulike]

OK, sounds like you are using a compression gauge for a car which will have a much heavier rated schrader valve which isn't a problem for a 1.0 litre engine with electric start but put it on an engine 20 times smaller with a rope and pulley........!!

I would still hazard a guess that the main problem won't be an air leak. It may have one but I have only found them ever leaking slightly around the clutch seal, never a major leak. The 372 is a solid machine.

If the thing has a spark and sounds like it has compression, the thing to check is the fuel system including the impulse line that has been known to come off.

It is always worth doing the pressure and vac test on a wreck as you don't want to get it going only to take out all the work you have put in but don't reckon it will have a big leak unless someone has done something silly.

I would be giving the carb a look at, diaphragms are OK, the diaphragm and gasket fitting order is OK, the gauze strainer for blockage, the needle valve for sealing and pressure check this once back together. Check the fuel line (pressure test it), fuel filter and impulse line is OK and connected.

 

[Craigb]

1 hour ago, spudulike said:

OK, sounds like you are using a compression gauge for a car which will have a much heavier rated schrader valve which isn't a problem for a 1.0 litre engine with electric start but put it on an engine 20 times smaller with a rope and pulley........!!

I would still hazard a guess that the main problem won't be an air leak. It may have one but I have only found them ever leaking slightly around the clutch seal, never a major leak. The 372 is a solid machine.

If the thing has a spark and sounds like it has compression, the thing to check is the fuel system including the impulse line that has been known to come off.

It is always worth doing the pressure and vac test on a wreck as you don't want to get it going only to take out all the work you have put in but don't reckon it will have a big leak unless someone has done something silly.

I would be giving the carb a look at, diaphragms are OK, the diaphragm and gasket fitting order is OK, the gauze strainer for blockage, the needle valve for sealing and pressure check this once back together. Check the fuel line (pressure test it), fuel filter and impulse line is OK and connected.

 

I dread to ask but what would you hazard a guess at? This saw better not be a lemon.