McCulloch PM850 rebuild

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The photo above sort of let's the cat out of the bag but in the process of working on this saws I noticed the flywheel was a 12 fin rather than the 20 fin that should be on an 850. Just a bit more evidence that this saw had been tinkered with at the factory. Another trip to the attic and I found a suitable 20 fin flywheel.

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Coil installed (just a normal electronic ignition coil), I use a 0.010" feeler gauge to set the clearance and tighten everything down. If you look carefully at the back edge of the fuel tank/air box you can see scratches from a screwdriver installing the top/rear screw that holds the flywheel cover in place. One more piece of evidence how this saw has been worked on before.

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Speaking of which, I had to add a couple of holes in the flywheel cover. I made a couple of templates some while back to help position the holes in the right location.

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In the interest of full disclosure, I still had to file a bit to get the holes lined up so the screws would go in. That said, I had to file the front one as well indicating the new block was slightly different from the original.

Mark
 
I didn't want the muffler coming loose so I made sure to bend the tabs of the locking plate over to fix the screws.

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The grommet in the rear of the handle was shot so I substituted another that I found somewhere along the way.

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At this point I got a little carried away and put the rest of the saw together without any photo documentation.

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Mark
 
I did put gaskets & diaphragms in the carburetor since I didn't want this one to run out of fuel when it needed it most. I also had to replace the metering needle since the tip of the original was deformed and would not seal completely.




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Back together, I only had a 24" bar and chain readily available so that's what I put on.

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On Saturday morning I made a little time to go to the local brush dump and give it a test run...no action photos since I was by myself but here is evidence that it did work as it should. The log was hackberry, the action was a lot of fun, and I would swear I could feel the saw getting stronger with each cookie....until I started trenching the earth beneath my log.

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Unfortunately, Kevin's 7-10 did not do so well...….runs great on the top end but idle is still all over the place.

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One last tip, if you are working on 10 Series saws with a full wrap handle bar, do yourself a favor and modify a scrench to simplify adjusting the chain or removing the
clutch cover.

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If everything goes according to plan, I'll box the 850 up tomorrow and send it back on Tuesday.

Mark
 

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Good thread, Mark. Real nice flow and great pics as usual. I wonder if you could scroll through it fast enough it would turn into a video? LOL

And fwiw, I'm still going with a weird timing issue at those low RPM's on Kevin's 7-10. 'Bout the only components that haven't been swapped out unless I missed it somewhere.
 
That was a great thread!!!!! I have a Mac 250, that you got me dimensions for the dogs years ago, so I could build a set,,, gonna have to pull that beast off the shelf tomorrow, and make some chips!!!! Thanks for the memory!
 
I did put gaskets & diaphragms in the carburetor since I didn't want this one to run out of fuel when it needed it most. I also had to replace the metering needle since the tip of the original was deformed and would not seal completely.




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Back together, I only had a 24" bar and chain readily available so that's what I put on.

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On Saturday morning I made a little time to go to the local brush dump and give it a test run...no action photos since I was by myself but here is evidence that it did work as it should. The log was hackberry, the action was a lot of fun, and I would swear I could feel the saw getting stronger with each cookie....until I started trenching the earth beneath my log.
Job well done Mark
Great right up to , its very helpful while im doing the oil pump on my 850
Thanks
peter

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Unfortunately, Kevin's 7-10 did not do so well...….runs great on the top end but idle is still all over the place.

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One last tip, if you are working on 10 Series saws with a full wrap handle bar, do yourself a favor and modify a scrench to simplify adjusting the chain or removing the
clutch cover.

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If everything goes according to plan, I'll box the 850 up tomorrow and send it back on Tuesday.

Mark
 
with the manual oiler rod ,how fat out of the hole by the rear of the saw does the rod come, the rod went under the old mount , does it still go under now ive got the new mount on
how did you put it on and the tank
mine only pushes in about 1/2 ''
i think mine may be hung up or twisted maybe
 
with the manual oiler rod ,how fat out of the hole by the rear of the saw does the rod come, the rod went under the old mount , does it still go under now ive got the new mount on
how did you put it on and the tank
mine only pushes in about 1/2 ''
i think mine may be hung up or twisted maybe
my pm850 was running nice and now its pissing out bar oil in a stream when wot , like turning the hose on , and it want idle
any ideas , saw was running sweet
ive just replaced the oiler in the tank and didnt touch the settings but was oiling good, ive also changed the manual oiler
 
Might as well tear it all the way down and see the extent of the damage. I noticed a couple of things as it was coming apart, first off the cap for the manual oil pump was cracked and popped off when the fuel tank came off. I couldn't have run that way very long as there was not a lot oil spilled and crud collected around the manual pump.

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The rear anti-vibe mount will also have to be replaced if this saw will be put back to work.

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Many times, particularly of the saw has not been previously molested you can remove the 4 hex head 8-32 screws that help hold the crankcase halves together. In this case the saw had been molested and one of the screws on top had been rounded enough that the 1/4" socket could not get a grip. Not to worry, a 6mm socket is smaller by just enough that you can force it on with a few light hammer taps, a hand held impact wrench come in very handy for an occasion like this since you apply some down force with the hammer blow in addition to twisting the fastener.

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With the last fastener out, the crankcase pops open and the piston, connecting rod, and crankshaft come out. The piston looks even worse when you can see it all.

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Hopefully the cylinder will be salvageable.

Mark
dang that's worse than my other 850..poor thing

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