372xp won’t start.

[Craigb]

I’ll pull the exhaust off tomorrow and have a look and I’ll put the compression gauge on a saw I know works great to check it. I’m wanting to cover most things on this thread in an effort to provided an easy to find trouble shooter for anyone rebuilding a 372. Starting from the top down. I’ll put pictures up as I go and hopefully the collective knowledge on 372s can be captured.

[spudulike]

Great, lets check out the method of measurement......we have seen so many use a car gauge and register low compression when even a saw with 200psi will register 75psi. 

If the test shows the saw is at fault, 75psi is VERY low so the top end will need to be removed and the piston/cylinder inspected for damage etc.

On heating the plug.....I did that today and bingo, a flooded saw becomes a running saw again!!

[Craigb]

The plot thickens. So whoever had the saw before me put the exhaust gasket on first then the shield. Also the piston is in good nick but has two rings. It isn’t oem as the saw came with the original piston and cylinder which were scored, but they had one ring. I’ll take the top cover off to see if I can identify which brand the new cylinder is.

[Craigb]

Here’s the plate

[Craigb]

I’ve fettled the oem piston up and got the transfer and scoring out of the cylinder.

[Craigb]

Also I’ve just found a second exhaust gasket that was between the shield and the muffler.

[spudulike]

The gasket should be between the muffler and heat shield plate and none between the plate and cylinder.

 

I wonder if a circlip has disintegrated between the piston and cylinder fitted or the big end is breaking up. 75psi is almost non existence as 150-175 is more normal on one of these saws. 

 

The best option is to clean the OEM cylinder and to fit a new Meteor/Hyway piston. Re-using a seized piston is never a good long term option. The piston skirt does improve compression and does cut down the piston rock at BDC.

[Craigb]

Meteor piston coming in the post. I’ll inspect the bottom end when I take the cylinder off.

[adw]

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.

[spudulike]

Show us the damage to the piston. The front looks OK so it is possible the circlip has come out, the end has broken off or the big end is kicking out white metal. Get this diagnosed before putting it back together again.