McCulloch SP81 Compression Question

[gene]

Hi All, I have an old SP81 that my father bought new in the 70's and that I use from time to time on larger live oak.
The saw recently had a problem crop up in that it wouldn't stay running at idle and high speed power was down a lot, finally the saw quit and wouldn't re start.
Spark was fine, changed plug and condensor. Spark still fine.
I haven't had to adjust the carb for several years since I rebuilt it last time (Walbro SDC). It seems fine. I tried running the needle in (1 only, high speed) and then back out to the previous setting to clean any junk out. No help. (I have a new fuel line in the saw)

Pulled gas tank/carb mount, cleaned and resealed carb to cylinder boot and installed a new crankcase pressure line and gaskets.

Still it won't start for anything.

I checked compression pressure with a good gauge and am seeing 25psi w/o oil and 30-35psi with WD40 sprayed into the cylinder. My Husq 42 pulled about 50psi no oil. (Yes, I had the throttles wide open when doing this).
Now, in the past when trying to start the SP81, if I didn't have the compression release in, a jerk on the starter rope would almost not turn the saw over, now it turns over pretty easy with, or without the compression release.

There dosen't seem to be any noise around the compression release when it is closed, just when open.

Do I have some broken rings?

Would the fact that the saw hasn't run in a couple of weeks make the compression pressure that it develops a lot lower than normal without anything being broken?

Is this thing worth fixing due to the wonderful avaliability of parts at every convenience store?

thanks in advance...

 

[MacDaddy]

Its time to rebuild it or sell it to me Ive been looking for one of those

 

[McC]

It sounds like you had an air leak, which was causing the fuel mixture to be lean. This probably wore out your rings. That is why you have low compression. If you rebuild, be sure to replace the crank seals, and all other gaskets with new parts.
Scott

 

[Al Smith]

Rings are real easy to check.Pull the muffler off.Squirt some light oil, diesel fuel,Wd-40,in the exhaustport .Pull it it up on compression.If it's got bad rings,it will be bubbling all around the exhaust port.Hopefully,you haven't delaminated the chrome lining.Yes,it's worth fixing,in my opinion,but,then again,I like Macs.

 

[gene]

Parts for the SP81

OK, I don't have a problem with pulling the thing apart to take a look inside. I think I must have a broken ring or two as there is little scoring inside the bore that I could see (Steel bore, not chromed, this is an OLD saw).
The question is where do I get the seals, gaskets and any internal hard parts that I may need. I also need a blowup of parts numbers for the saw.
Anyone have any ideas???
LOTS of dead live oaks are a waitin....

 

[McC]

I've had good luck getting parts from Randy Duncan. They know what you need, and get it to you quickly. (937) 927-5665
Scott

 

[ozflea]

Yes Randy Duncan is your man as i have just placed an order with myself.
After just placing a local call from downunder to him. Just might take a little longer to recieve.

 

[ozflea]

Yes the SP81 had a habbit of turning your fingers into something very sore if you forgot to push the decompressor valve in, worked on many of them could be broken rings or plain just needing a rest and or rebuilding.
Sold many of these for cutting pit props used in underground mining out here in Oz.
Not a bad felling saw with a 16" or 20" bar and super chisel chain in 3/8".

The one eyed Mac Man from Downunder.

 

[gene]

Yeah, it is a vibrator. Loud as all hell too, but indeed powerful....I never could get the thing to turn over well w/o the compression release before, and I know that feeling well when you yank the rope w/o the compression release, sort of a painful tingling...
Now....it's like a trim saw to pull it through....

 

[Al Smith]

I also will attest to the little saws ability to make your fingers hurt.Both it,and my 6-10,will snap back worse than my 125.If you check threads from Wiemedog ,you will find the name of a dealer in New York state,that has Mac parts,by the ton.The sp 81,is my next restoration project.I thought they all had a chrome liner,hmm,I learned something .I,ve so far been able to find all the Mac parts I have needed,but sometimes it's been a challange.

 

[Lewis Brander]

New York Parts source:

Hello to all. The NY parts source is Robert V. Johnson, PH # 607-638-9297. I hear he has a ton of parts and old Macs laying around. Happy Holidays. Lewis.

 

[gene]

Thanks

Thanks much. I will give these guys a call and see what I can do with the SP.

 

[Lewis Brander]

Compression Test results:

Gene, did you ever retest the compression? I use an automotive screw in type gauge and my saw read higher than that. I usually add a teaspoon of regular 2 stroke oil to the cylinger and crank her over a few times and do the test (with plug wire grounded). Just wondering if you got any higher readings. Lewis.

 

[gene]

Hi Lewis,

No, I never did retest the compression in the Mc Culloch, I did retest in the Husqvarna 42 and got only slightly higher compression numbers and this was after a pretty good run of the saw.
I am going to lay the low numbers down to the type of gauge I was using which is a pro automotive gauge with an extended hose between the cylinder connection and the gauge itself. There is a bit of volume in the hose in addition to the normal cylinder volume and the small piston is just unable to raise the pressure to the level that would be found in the cylinder if only the plug were there. In automotive or motorcycle piston displacement ranges, this additional volume added to the combustion chamber wouldn't affect readings bu mych, but with these tiny chainsaw engines, it's pretty significant.
Anyway, the SP81 has some sort of obvious compression defect. Previously I was almost unable to pull the saw through unless the compression release was pushed in; a yank on the starter rope could be painful to the fingers. Now, the thing pulls through like a little trim saw, with or without the compression release, just remember here that this is an 81 cc or 5 cubic inch saw.

 

[sedanman]

Make sure your compression guage has a working schrader valve in the hose (this is on the end that screws into the plug hole) I use an 'automotive' compression tester on everything including small chainsaws. 5 or 6 pulls is usuallt enough for accurate results. If you are using the wrong hose say one from a cylinder leakage tester without the valve, you will NOT get accurate test results.

 

[gene]

Sedan, I think we're both right, the schraeder valve is at the gaugehead part of the tester, not at the end of the hose that has to be used to connect to the sparkplug hole and then to the tester. The piston is having to pump up the pressure in the hose and in the cylinder before it ever gets to the valve.

Glad to know that it is indeed inaccurate on small displacement cylinders.

Tool is a KD 8 Pc Universal Compression Test Set. This is a professional grade tester and was not cheap, maybe I should shop at harbor freight instead....