Cylinder problems.

[spudulike]

The tester is only a cheapo gunson but tried earlier on a 026 which is old but a good saw it read 160.

 

The 3120 is a runner but was cold.

 

I think i will order a new aftermarket pot and piston as it does/will need one and then try a comparison.

 

If it is the latest Gunson with the long brass connector and schrader valve then 150psi sounds just fine - my 346XP with much lowered squish pulls 165psi!

 

If it is the older version with the purple packing then 150psi is ok but a test with a schrader type will verify the full compression - the older version is good with smaller engines on strimmers and small saws as the end valve is much more sensitive!#

 

TBH 150psi on an OEM P&C is fine - don't mess with it, you may well not improve on this!

[Mesterh]

Its the newer type.

 

I will do as you say and keep the oem until runs v poor.

[spudulike]

Its the newer type.

 

I will do as you say and keep the oem until runs v poor.

 

Then 150psi is good - can't imagine that 3120 is being pulled over very fast either - the OEM P&C is Mahle and the Chinese one will never be as good - if it aint broke - don't fix it.

 

Go buy a buggered saw and fix that:lol:

[Megatron]

IMO you want to be looking for 140+ psi on a chainsaw. Anything less there is usually a problem. 150 should be fine but if you have reason to believe your piston and cylinder are deteriorating then pull the muffler to check. Compression doesn't tell the whole story... the saw in the picture was pulling 170psi.

 

The rate at which you turn the engine over (within reason) shouldn't influence the compression reading. There is no need to peg the throttle open either as the saw will just draw air through the exhaust port instead of the intake. Just keep pulling the rope until the compression reading won't go any higher.

 

I believe there are two types of compression gauge. Those that have the schrader valve at the end of the hose (what you want for testing saws) and those that have the schrader valve next to the gauge. Using the latter would mean the volume of air in the hose would count as part of your combustion chamber volume and you would consistently get lower than expected readings. Wouldn't make that much of a difference with a car engine cylinder, with a much larger combustion chamber volume.

[spudulike]

That old scherader valve on the end of the pipe or by the gauge is a load of:001_rolleyes:

 

The vast majority of gauges have some sort of valve near the gauge but this meerly lets out the air after taking the reading and has no bearing on the pressure read.

 

The critical valve is the one in the bit that screws in to the engine - some have a soft spring schrader valve and these are fine but can read a little low, some have a stronger spring schrader valve and may not even take a reading off a small engine and the others use a brass bullet like valve with an o ring on it - these tend to take a higher reading and are good for smaller engines such as strimmers etc. The compression in the engine has to be strong enough to open the valve and this causes some difference in how they work!

 

All I can say is the guy is using the newer type Gunson using the soft sprung schrader - I have one of these and 150psi is a good reading on this gauge - I have got the same on an engine with a new piston!

 

I don't mean to find fault in others comments but have three types of commpression gauge plus one that didn't work so have compared all and all have their own benefits and have taken note of how each works so speak with much experience.

 

The guys 150psi is fine when measured with this gauge.

[spudulike]

Pulling the muffler and looking at the piston is excellent advice and one of the first things I do on all saws - almost quicker to do than a compression check.

 

One good thing to do is to check a known good reasonably new saw with your gauge and see what it reads and then use this to understand if your own saw is ok or not.

 

One thing to add is that I had a Poulan Countevibe 3400 once and couldn't get over 130psi but found out that this is quite normal so different saws vary and some domed topped saws have incredible compression - got 220psi from and old Jonsered once:biggrin:

[Megatron]

That old scherader valve on the end of the pipe or by the gauge is a load of:001_rolleyes:

 

The vast majority of gauges have some sort of valve near the gauge but this meerly lets out the air after taking the reading and has no bearing on the pressure read.

 

The critical valve is the one in the bit that screws in to the engine - some have a soft spring schrader valve and these are fine but can read a little low, some have a stronger spring schrader valve and may not even take a reading off a small engine and the others use a brass bullet like valve with an o ring on it - these tend to take a higher reading and are good for smaller engines such as strimmers etc. The compression in the engine has to be strong enough to open the valve and this causes some difference in how they work!

 

All I can say is the guy is using the newer type Gunson using the soft sprung schrader - I have one of these and 150psi is a good reading on this gauge - I have got the same on an engine with a new piston!

 

I don't mean to find fault in others comments but have three types of commpression gauge plus one that didn't work so have compared all and all have their own benefits and have taken note of how each works so speak with much experience.

 

The guys 150psi is fine when measured with this gauge.

 

There is a type of compression tester that does not have a check valve at the end of the hose. Hence, the volume of the hose is added to the combustion chamber volume.

Compression readings are only good like you say at comparing with another known good saw, measured with the same gauge. As a result of the many variables involved including the opening pressure of the valve etc