Compression tested most of my saws, could use some help with the results

[badhabit]

Hey guys I decided to compression test most of my chainsaws. Wondering if I could get a little feedback on the numbers posted. I ended up testing 9 saws using an Autozone compression tester with the valve thing in the end.

Jonsered 801 - 225 psi
Stihl MS290 - 175 psi
2 McCulloch PM605's - 175 psi
McCulloch Super PM610 - 168 psi
McCulloch PM10-10A - 165 psi
Jonsered 630 - 155 psi
McCulloch 7-10A - 145 psi
McCulloch 10-10A - 135 psi

Honestly I was kind of surprised on the difference between the highest and lowest numbers. The 801 is an old saw, I was surprised it tested so high. It is a beast to pull on and hurts pretty good if it kicks back. Also surprised the MS290 posted a 175, thing has cut a ton of wood and has a ton of hours on it. It is still on the original piston/cylinder/rings.

Also the Mac 7-10 testing at 145, is that low for that model of saw? I feel like it should be up higher with some of the other saws. It's my second biggest saw engine wise but has the second lowest compression. It does have a decomp on it but I made sure it was not engaged while testing. Not sure if that was having a negative effect on the result via leaking air or something.

Wondering what psi = trouble? Is anything lower then 130 time for new rings/piston/cylinder? Any of the above numbers look way off for that model of saw?

Anyhow any feedback given is much appreciated. Apologies if some of this info is posted up elsewhere in the forums.

Thanks,

Jake

 

[cpr]

There's no set number. They all look to be in the "useable" to good range. Yes, the 7-10a could be higher and the 10-10a is getting tired, but if they both still run well for you, I wouldn't rip them down unless you need a project.

 

[wigglesworth]

Sounds like there all due for a rebuild.

You might get by with new rings on the 801 though...

 

[kgip2k]

I am no saw mechanic, but if they are still running, and cutting good, I would leave them alone.... Maybe start looking for parts to see what you can get, but if they work I wouldn't worry much... Maybe a little more oil in the lower comp saws

 

[Adirondackstihl]

Sounds like there all due for a rebuild.

You might get by with new rings on the 801 though...

 

[Chris-PA]

It is only a rude test. Keep in mind what the test is doing - you're pulling the engine over with the recoil at low RPM and measuring the peak pressure, and from that trying to infer how much is leaking past the rings. It's useful, primarily if you know what it used to be or what other saws of that model do, but it is nowhere near what the peak pressures are when the saw is running. Under higher pressures and temperatures the rings may seal differently, port timing and combustion chamber volume varies from engine to engine, etc. And one guy may be able to pull the recoil faster than another.

It is somewhat more useful than reading tea leaves or goat entrails.

 

[Cantdog]

Compression figures vary greatly by P&C design and of course also how worn out they are. Generally I find the larger, older reed valve motors don't run as high compression when stock as the the more modern saws. For instance your 630 blowing 155 is pretty normal for any stock 630/670/625 with a fair mount of use and still a good runner with a modern saw motor...flat top piston etc......the 801 is also right around where it should be at 225 psi...these 80 cc saws were designed to run on the leaded, high octane gas of the day, so came with a fairly high domed piston to create pretty high comp ratio/figures...these were also some of the most highly developed stock saw motors of the time and though rather heavy by todays standards still go through the wood pretty darn fast with amazing torque some 30-40 yrs later....my 80 pulls a very consistant 235-238 on the gauge and has never had the top end off.....it's all in the design.......On the other hand my big 'ol Mac 1-52 with a very fresh piston and rings only pulls 135 psi and runs as good as it ever did (my BIL bought it new in '62 and used it only three months pulping an island)......the Mono is also very low hr saw that was the wifes grandfathers and it struggles to get to 100 psi but again, starts runs as good as it ever did.....slow and loud....

 

[FATGUY]

some of those are remarkaby high.

 

[Mastermind]

I'm a fan of serious compression........just sayin. :msp_thumbup:

 

[Chris J.]

"...
Jonsered 801 - 225 psi
Stihl MS290 - 175 psi
2 McCulloch PM605's - 175 psi
McCulloch Super PM610 - 168 psi
McCulloch PM10-10A - 165 psi
Jonsered 630 - 155 psi
McCulloch 7-10A - 145 psi
McCulloch 10-10A - 135 psi
..."

Those compression readings look good to me. Hopefully the saws aren't gunked-up with old congealed mix and/or carbon.

 

[wigglesworth]

I'm a fan of serious compression........just sayin. :msp_thumbup:

 

[redoakneck]

Great post, I was wondering about compression also. Just got a ms-660 and it pulled 153, looked at the piston and rings and looked close to getting a ring stuck and had a lot of wear. Tore it down and put a new meteor piston and rings in, cleaned the cylinder, and did replace the base gasket. After the NEW piston pulled only 145:msp_sneaky:


I know it will take a few tanks to get the rings seated, but how much higher do they usually get , 10 psi at most??????


Just cut with it and it runs fine, my tech tach crapped on me so it is running rich until I can get a new tach. Compared to my well broke in dolmar 7900 it is a toss up. I hope this saw pulls better once broke in, if not going to get it ported.

 

[chadihman]

I'm a fan of serious compression........just sayin. :msp_thumbup:

Yeah man Thats why I bought my lathe.

 

[Trx250r180]

just run your saws rich ,eventually you will create a domed piston out of carbon and get 250 psi

 

[Nickolas]

Ya know I been discussing this with a few people on a few saws I have. Now I have a few old McCulloch saws from the sixties and they are all around 125 psi. One guy said it was low and time for a ring job and another said it was typical for these older saws. Well I can say that they all have plenty of power. The Super 250 especially is a real work horse. That saw will cut circles around any other saw I have. So for me I'm content with them. They all start with ease and no issues with anything. The way I look at it is if your happy with the way they run then I wouldn't worry about it. I will only do a rebuild on a saw if it drops below 100 psi otherwise I don't see any sense in it if it's till running good. Now if you do this kind of work for a living than you may want your saw in tip top shape so that could be a different scenario. Anyway for me and what I have 125 psi is all good.

Nick

 

[milkman]

This thread made me curious, so I went out and used my el cheapo compression tester on my 026, came up with 90#. I don't know the history of this saw, so I thought it's probably time to refresh it. Just for kicks I pulled the plug on the 346 and it came up with 90# also. Guess I know where the problem lies.

 

[badhabit]

This thread made me curious, so I went out and used my el cheapo compression tester on my 026, came up with 90#. I don't know the history of this saw, so I thought it's probably time to refresh it. Just for kicks I pulled the plug on the 346 and it came up with 90# also. Guess I know where the problem lies.

Make sure your tester has the valve in the tip or it won't give accurate results. Something about the pressure not building up the way it should and will give low results. With both your saws in the 90 psi area you might not have the right type of compression tester.

 

[badhabit]

Ya know I been discussing this with a few people on a few saws I have. Now I have a few old McCulloch saws from the sixties and they are all around 125 psi. One guy said it was low and time for a ring job and another said it was typical for these older saws. Well I can say that they all have plenty of power. The Super 250 especially is a real work horse. That saw will cut circles around any other saw I have. So for me I'm content with them. They all start with ease and no issues with anything. The way I look at it is if your happy with the way they run then I wouldn't worry about it. I will only do a rebuild on a saw if it drops below 100 psi otherwise I don't see any sense in it if it's till running good. Now if you do this kind of work for a living than you may want your saw in tip top shape so that could be a different scenario. Anyway for me and what I have 125 psi is all good.

Nick

Ya I know what ya mean all of my older saws run pretty well, even the ones with low compression. A lot of the saws with lower compression run harder then ones with higher compression. Even on the same model of saw. My Mac 10-10 with 135 psi runs better then my Mac 10-10 with 165 psi. As long as it is running well that is what matters.

Still curious how low of a number is to low psi wise and would signal a need to look at the saw.

 

[Anthony_Va.]

Great post, I was wondering about compression also. Just got a ms-660 and it pulled 153, looked at the piston and rings and looked close to getting a ring stuck and had a lot of wear. Tore it down and put a new meteor piston and rings in, cleaned the cylinder, and did replace the base gasket. After the NEW piston pulled only 145:msp_sneaky:


I know it will take a few tanks to get the rings seated, but how much higher do they usually get , 10 psi at most??????


Just cut with it and it runs fine, my tech tach crapped on me so it is running rich until I can get a new tach. Compared to my well broke in dolmar 7900 it is a toss up. I hope this saw pulls better once broke in, if not going to get it ported.

My 660 blew 155 when it was brand new. After broken in, it still blew 155. Sometimes the rings seating won't give anything in compression. But usually it does give maybe 5psi at the most from what I've seen.

Ya I know what ya mean all of my older saws run pretty well, even the ones with low compression. A lot of the saws with lower compression run harder then ones with higher compression. Even on the same model of saw. My Mac 10-10 with 135 psi runs better then my Mac 10-10 with 165 psi. As long as it is running well that is what matters.

Still curious how low of a number is to low psi wise and would signal a need to look at the saw.

I don't like anything under 130psi. Thats getting a little low in most saws. But if it's still running like it should then I'd keep running it. When you can feel it getting ragged out a little then it's time for a freshening up.

I have a stock 260 that blows 170psi.
My 372xpw will blow about 182psi, and my 441C is 190psi.