McCulloch Pro Mac 800 problem

[boda65]

The last time I was cutting wood, my PM800 started loosing power. Out of the wood, it seemed to be plenty rich, but at WOT it would clear out then start screaming as if it were way too lean. I suspected the pulse line or intake boot was cracked.
The pulse line looks good and seems pliable. I looked at the intake boot from inside the intake passage and don't see any tears. I would have to take the whole top of the saw off to do a good inspection of it. I do see the piston looks scuffed The scuffing is on the skirt but not on the top part of the piston. I checked compression and got 160 psi so I'm not going to worry too much yet.
Any ideas for other causes for this? Dirt in the carb perhaps? Crank seals?
Jim

 

[Brian13]

Can you do a vac test and confirm the seals are still good? Sounds like a possible air leak.

 

[boda65]

I do have a vac pump, but have never done a vac test on a saw. I have to make a plate to cover the intake and exhaust ports, correct? With a nipple on the carb plate to attach the vac to.
I don't mind tinkering with my relics, but I hate it when one of my main saws is down. I work 6-7 days a week and am currently home-schooling my 9 yr old. Kids basketball Saturday mornings and church on Sunday. Don't leave much saw time. I got so much more done in 2009. We had a lot of 4 day work weeks and several mandatory weeks off. However, I'll take busy over economic collapse any time :msp_smile:

 

[Brian13]

More or less yeah, you can also take a bicycle tube and put it between the muffler and exhaust port rather than make a plate. I have seen a plug that will go in place of the spark plug with a nipple on it for the vac pump and then just seal the intake with a plate( I put the bicycle tube as a backing on mine to help seal). You might even be able to just put a piece of tube between the carb and intake port to seal it as well. If it has a impulse line you can just use that to draw your vac, that is what I used for my 044.

 

[stinkbait]

What kind of shape are your AV mounts in? If they have a lot of play in them that can put a lot of stress on the intake boot. The boot could have a crack but be unnoticed out of the wood at wot. Then when you put the bar in the wood and put pressure on the AV mounts the engine could flex enough to open the crack in the boot. I have had this happen to me. Saw runs fine not cutting but put the bar against some wood and apply a little pressure and you have yourself an air leak.

 

[boda65]

I'll try to make some covers at work tonight so I can do a vac test. If it looks ok statically, I'll flex the engine around to see if that causes a leak. The AV mounts are good, this was a NIB saw and has very low hours on it. I did notice the little boot around the choke rod was cracked, but it seals with the choke in. I think it's better to use stuff rather than let it set.

 

[rmh3481]

It could be anywhere from your mix tank to your intake bellows so your going to have to tear it down and look. Dont forget your hand held gas tank, the one you mix your fuel in, how old is the mix? Pour some in a glass and look at it. I saw a beautiful Poulan 655 scored from a slurry of water, alcohol, gas and oil last year around this time. Condensation can get into some strange places and mixed gas just wont last these days.

Also check the tank vent and all the lines. If the carb diaphragms are stiff or off color replace them and maybe the filter too. If there is an auto zone or a good parts store close you can borrow a brake bleeder (vac tester). If you want to cheap out on the pressure test you can use a large horse syringe and a bucket of water. Block off the ports and hook the syringe to the impulse line or through the spark plug fit with a nipple. Pressurize the case by bearing down on the plunger (5-10psi is enough). Dip the case in a bucket of water for a few seconds and watch for bubbles. Turn the crank with your fingertips while having a friend working the pressure to find any flat spots.

 

[boda65]

The mix is good, seldom sits for long.

I did a vacuum test this morning, pumped it up to 10 inches of mercury. It dropped about 1 inch every 45 seconds. Moving the tank around to flex the intake boot didn't really seem to make a difference.

I can't do a pressure test right now, as I don't have a regulator to get the pressure down that low. I don't like how the vacuum drops, but still doesn't seem like that would be bad enough to show as a sudden performance change...

 

[belgian]

Did you take a look at the exhaust side ? I have a feeling it's not going to look good....

 

[Brian13]

Instead of pressurizing the case if you dont have a regulator, you can take a little bit of oil on the seals and see if it sucks it in. Otherwise a bike pump might be able to be used instead of the air compressor.

 

[boda65]

Well, I just tried twice to get photos of the exhaust side, keeps coming out blurry. Photography is not my forte apparently. :msp_laugh:

The exhaust don't look too bad, some visible lines but nothing that catches a fingernail.

This saw literally only has a few hours of run time. It has had nothing but Stihl ultra oil at 40:1, same as my others. Ran great until the last time I used it. Still has 160 psi compression, so I don't think anything is harmed too bad at this point.

 

[belgian]

Ok, it was just an assumption...overlooked the 160 PSI thing.

I have a PM800 that was nearly impossible to tune right ; it was either too rich or wouldn't accelerate. Switched the Zama carb with one of a 7 series I believe and problem solved. Is yours a tilly or a Zama ?

 

[boda65]

I have a PM800 that was nearly impossible to tune right ; it was either too rich or wouldn't accelerate. Switched the Zama carb with one of a 7 series I believe and problem solved. Is yours a tilly or a Zama ?

It's a Zama. Prior to this, it accelerated like crazy, 4 stroked with no load, and cleared up in the cut.

 

[056kid]

My guess is an air leak after the carb, or a fuel related problem. My sp81 did the same thing. I would tune it rich, run a tank it and it would be screaming, sometimes after re fueling it would not start without some fuel down the carb. After it got running again, it would run normally and safely. After the last day I cut with it, I noticed that the compression had gone down some, say from 180 to 160. I dont know the culprit of the problem, but like I said in the beginning, I bet it is an air leak.

 

[boda65]

Sorry for the slow updates, got a lot of irons in the fire right now. (excuses, excuses).
Put 5 psi pressure to it today. Found some bubbles coming out where the intake boot attaches to the jug. Took it apart further to investigate. I noticed the piston will actually rotate in the bore a little bit, as if the wrist pin is loose. This can't be good. I can't think of any circumstance where a piston can rotate even a little bit and be good.
This is all puzzling to me. This saw is, for all intents and purposes, new. I have used only Shell premium gas mixed with Stihl ultra oil. I listen for all my saws to four stroke under no load. It wasn't making any unusual sound when it started having problems. I am pretty discouraged at this point. I'm wondering if it was just a bad one from the factory. Is it too late to mail in my warranty card?

 

[promac850]

Sorry for the slow updates, got a lot of irons in the fire right now. (excuses, excuses).
Put 5 psi pressure to it today. Found some bubbles coming out where the intake boot attaches to the jug. Took it apart further to investigate. I noticed the piston will actually rotate in the bore a little bit, as if the wrist pin is loose. This can't be good. I can't think of any circumstance where a piston can rotate even a little bit and be good.
This is all puzzling to me. This saw is, for all intents and purposes, new. I have used only Shell premium gas mixed with Stihl ultra oil. I listen for all my saws to four stroke under no load. It wasn't making any unusual sound when it started having problems. I am pretty discouraged at this point. I'm wondering if it was just a bad one from the factory. Is it too late to mail in my warranty card?

I bet the wrist pin is not loose... I think there is a small amount of play in the needle bearings in the piston, the pin rides on those bearings. The pin is pressed into the connecting rod. Don't let any of that discourage you. It might be as simple as a new gasket.

 

[rwoods]

Keep you chin up. promac610 is probably right. Gasket, screws loose or need a new port boot. These parts are still out there. Got to remember that the NOS parts maybe as old or older than you saw so you have to watch rubber parts. Ron

 

[boda65]

PM800 back in service!!!

Finally got the saw back together yesterday. Went out today to try it out and it runs great! I can't believe such a small leak as this had that it would have such a big effect on it. As for the scoring on the piston skirt, upon closer inspection, they are just lines, nothing that catches a fingernail. Here is a video of it in some oak. Thanks to all for your advice and opinions.

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