periodic low/no compression on 357xp

[mattyboy]

Hello,

 

About 3 months ago I had the pot and piston replaced on my 357xp after the last one went kaboom. It works absolutely great now apart from on 2 occasions when ive been using it around about the middle of the day I've gone to pull the pull cord and it feels like there is zero resistance(like the is no compression at all). This is soon resolved after pulling on the pull cord about 10 times and the compression feeling seems to come slowly back after each pull. Its only happened on 2 occasions in the last 3 odd months but ive not been using it every day at all, maybe 2 or 3 days a week. Just wondered if anyone else has any ideas on this or happens to anyone else. Im a bit worried that it might just brake at some point randomly half way through the day which would be annoying! got back up saws but i love the way my 357 cuts over everything else!

 

thanks

[spudulike]

Hello,

 

About 3 months ago I had the pot and piston replaced on my 357xp after the last one went kaboom. It works absolutely great now apart from on 2 occasions when ive been using it around about the middle of the day I've gone to pull the pull cord and it feels like there is zero resistance(like the is no compression at all). This is soon resolved after pulling on the pull cord about 10 times and the compression feeling seems to come slowly back after each pull. Its only happened on 2 occasions in the last 3 odd months but ive not been using it every day at all, maybe 2 or 3 days a week. Just wondered if anyone else has any ideas on this or happens to anyone else. Im a bit worried that it might just brake at some point randomly half way through the day which would be annoying! got back up saws but i love the way my 357 cuts over everything else!

 

thanks

 

Is the pot and piston the OEM unit or a Chinese Copy?

 

Few things to look at: -

 

Is the decomp sticking in and is the lack of compression with the decomp in or out?

 

Could be a sticking ring - if it holds in the piston then the saw could loose compression.

 

Is he saw an EPA version with automatic decomp?

 

Have you taken the muffler off to check the state of the piston through the exhaust port?

 

The saw will develop less compression when hot - you should expect 15psi less when hot than cold.

 

Have you measured the compression - worth doing so you are dealing with fact - 150psi cold is good, 170psi is better

[mattyboy]

Im not sure if the pot is an OEM one, think it was from Italy and is nickel plated or plated with something, it wasnt the dirt cheap one the next one up I seem to remember.

 

I never push the decomp in, always leave it out.

 

The saw i dont think has automatic decomp, but to be honest not totally sure what this is. Its a 2010 model.

 

Ive not taken the exhaust off, i may do that now.

 

So if its a sticking piston ring is this a serious thing? Would it benefit from a new ring?

 

Thanks for your reply spud

[Stubby]

If you have a de comp button then its not auto de comp

[mattyboy]

If you have a de comp button then its not auto de comp

 

aah makes sense

[spudulike]

As Andy says, if you have a blue or black button on the side of the cylinder, then the decomp is manual and the saw isn't the EPA model which is a good thing!

 

The cylinder, if Italian is a Meteor one and is a decent quality item and will be Nikasil plated - harder then Chrome and better.

 

Best look at the piston - if the original pot seized, if the reason for this happening hasn't been remedied then it may have gone again - the saws are good performers but the inlet manifolds can be a bit problematic if not sealed well.

There was a modification in the early days from a plastic manifold clip to a metal one so make sure the inlet manifold is held on to the cylinder with the metal clip.

 

It may be worth investing in a £20 compression gauge!

[mattyboy]

so you think it sounds pretty serious?

 

where can i get a compression gauge from?

[Rich2484]

so you think it sounds pretty serious?

 

where can i get a compression gauge from?

 

Halfords. Gunson high gauge petrol engine tester. £25 when I got mine a year ago.

[spudulike]

so you think it sounds pretty serious?

 

where can i get a compression gauge from?

 

I wouldn't say that until you know if the piston is fried - it takes 5 mins to remove the muffler and shine a light up the exhaust port.

 

The piston should be a nice light grey colour and slightly wet looking - bit like the underside of a fish:thumbup:

 

If there are any deep vertical scores then you have some problems. If you are unsure then get a photo up on here and let us see what you have.

 

It is difficult to know what the issue is without seeing the saw.

 

The gunson Hi gauge is what I use but would check the piston first.

[mattyboy]

Does anyone know what size the bolts are on the exhaust are, the ones where the deep holes are. My 4mm allen key doesnt seem to do anything to it. I dont have a 5mm is this the one i need?