Troubleshooting a Chainsaw Mill Motor

Chainsaw's getting cranky when it's running hot — what's up with that? Find out here — this forum knows chainsaw motors. April 20, 2011

We are running a pair of 088's on a 68" bar and cutting some beautiful WO. We had mechanical problems last fall and both power heads have had a great deal of attention from our mechanic who has over 40 years experience.

The main pulling power head was a complete rebuild when the inside main bearing needed replacing. This unit came from Procut sawmills in Canada about '95 and was ported and polished out of the box. Since the rebuild it seems to have a little less compression when you pull on the starter cord but sounds hot and runs strong.

The problem is that it only runs cold. After the first run it gets hard to start and after several runs it just won't start. The mechanic cleaned out the gas tank vent and it got a little better but it is still not right. If you can get it started it is very sluggish until it gets up to speed. We have ordered a needle and diaphragm kit for the carburetor and I suspect I need to adjust the low rpm carburetor screw. Any suggestions would be appreciated.

Forum Responses
(Sawing and Drying Forum)
From contributor R:
Sounds like the carb or a cracked/stuck ring.

From the original questioner:
The rings are new, so hopefully it is not that. I guess that would be something to check after the carb rebuild.

From contributor X:

I don't have a whole lot of small engine expertise, but the symptoms don't sound like anything in the carb. Have you had it checked for cracked head or jug? Hairline cracks can't always be seen they sometimes can only be seen by having it magna fluxed. Hope it's the carb for your wallet's sake.

From contributor F:
A bad crankcase seal can cause those symptoms on a two-stroke also.

From contributor B:
I've been around the block on many types of engines in 30 plus years of working on them, but I need better info on your specific engine to give sound advice. I am unfamiliar with the term "88", and am assuming that is the engine. Please write in again, tell me 2 or 4 stroke, how many cylinders, which type of ignition system, what type carburetor, what hp you are expecting, basically any info you can give.

From contributor X:
An "088" refers to a Stihl model 088 120cc (I think) 2 stroke chainsaw. The predecessor 084 is the same basic saw. There's plenty of 4 stroke blowers and trimmers, but as far as I know, 4 stroke chainsaws are still not widely available. I wish they would get the technology down to where a 4 stroke could be the same relative size and weight as a 2 stroke though. Lots more torque, much quieter, and they would last longer.

From contributor B:
I ran a chainsaw mill a couple of years ago, used an ancient Homelite engine that ran like it was going on testosterone - a real ripper. But it was too noisy, and took too long to cut through the logs we had.

2 strokes are finicky. One of the problems we encountered was an intermittent lack of power. It ended up being the points were unreliable. With the installation of an electronic ignition chip, that was eliminated; starts and runs like a monster (provided you pull the cord, and not let it pull you) every time. Another time I had a problem with the high speed air/fuel adjustment; burned up two pistons before I found out that you need to set that by running at speed and pulling the choke out some, about 1/2 way. If you get more power, you are running too lean; bye bye piston. If it bogs down some, but not bad, you are okay; if it falls flat and dies, you are too rich. Some carbs you can adjust with screws, others you need to adjust the main metering needle adjustment. It is very important to get this setting correct. Also, make sure you have enough oil in the fuel mixture; a little too much is way better than a little too little.

Last, get a good spark plug. I hate Champion plugs for automotive purposes, but they are the best I have found for small engine use. Set the gap right. I hope this helps.

From contributor I:
Another problem could be the gas. The new ethanol gas has been causing a lot of problems with the older small engines. Once the gas gets hot in the carburetor it tends to vapor lock. We have one gas station close to our area that still sells the pure old fashion gas. And you can tell the difference in power between the two. Try a higher octane mixture and see if that helps.

From contributor O:
Check the coil. When they heat up they can cause problems similar to what you are having.

From the original questioner:
Thanks everyone. The Sawing and Drying Forum is a really great tool. These other possibilities are exactly what I was looking for. Sorry about the shorthand, yes it is a Stihl 088.

My mechanic suggested that the carb needle might not be working right, running too lean for hot start and the choke too rich for hot start. I tried hot starts with it in "run" position and full throttle but that didn't work. If it starts at all, it starts in the "half choke" position. Intuition tells me that it is not the carb.

Contributor X - your idea of a hairline crack in the head sounds possible, I guess we will check that sooner or later if nothing else works. What about that crankcase seal Contributor R? Is there a test or symptom to see that in a two stroke?

Appreciate the input Contributor B. It does have new plugs but I will be taking them out today for inspection. It does have a new ignition module. Thanks for the helpful pointers on carb adjustment. I run the Stihl synthetic oil and feel pretty good about that.

I am running 93 octane, but I'll check to see if it has ethanol which I believe boosts octane so could be in premium gas as well. Thanks for the thought about the coil. My neighbor has a spark checking tool I will use the next time we get the motor hot. Thanks again everyone. I'll let you know what we find out.

From contributor B:
Since two stroke engines work both sides of the piston, air leaks at either end have an effect on fuel ratios. The crankshaft seals need to seal both pressure and vacuum, and if they don't then the air/fuel ratio is unreliable. It is certainly important, but again diagnosable by whether or not you find the engine running lean at higher speed. With all due respect to the suggestion, I really doubt it is your primary trouble. I do believe it is a fuel related problem, but I've learned all too many times that when I'm smart enough to know it's fuel or ignition, and am sure of it, I'm wrong. You may take a second look at your fuel volume available at the engines as well, both when cold (working) and hot (not working). Somehow there's a problem between the two, and that's the key to diagnosis here. I'll be keeping an eye out here, looking forward to seeing your solution.

From contributor R:
I agree with Contributor B that it is probably a fuel issue, but I put the seal and ring possibility out there because you stated you had crank bearing problems and it is not uncommon to nick a seal or crack a ring on rebuilds. Oft when people, including other small engine shops, bring me two strokes to repair that have had everything else tried, it is a stuck/cracked ring or bad seal that is the culprit.(Such will have the same symptoms as you described). You said he is experienced so I'm assuming he has covered the basics.

From the original questioner:
I'm a bit new to these diagnostics but the compression test held at about 50 pounds. The hose to the carb had been clogged with some trash from the vent membrane but the symptoms improved when that was cleaned out but not by much.

I'm having difficulty grounding the spark tester to test the spark after a hot run when it is acting up. It will only reach to the carb and a screw that holds the pull cord grate so I ran a copper wire to the muffler and a few other screws but I still can't get the tester to show spark even though the engine starts fine.

From contributor L:
I'm running two 880's on an 8 foot bar. The compression on both of those 880's is 120-130 PSI. They are both about a year old. Hope that helps. Sounds like low compression to me.

From the original questioner:
So if I tested the compression when it was hot and there was a cracked in a seal or the casing it would be even lower, yes? Would the stuck/cracked ring give the same symptom?

From contributor E:
When you check for compression the piston has to be above the port hole's on a 2 cycle or their will be no comp. On a 4 stroke the valve's are in play, then I believe as far as the crank case goes you would have to have a way to put air in the crank case? Then the piston would have to be below the port hole's to hold compresson in the case. The best way is to just change them - not a big job. You should be able to change the PTO from the outside and the mag side. You would have to pull the fly wheel off and probably the coil to get to the seal.

From contributor X:
I thought the compression test on a chainsaw was the same as any internal combustion engine. Screw the gauge in and give it a couple of good sharp pulls. Whatever the highest reading is, is what the compression is. A good compression gauge will not zero itself, it will hold the highest reading attained.

In your case you want to test it cold and hot. 50psi is telling you the chamber has a leak somewhere. Whether the seal, head, gasket, crankcase - it's leaking somewhere. Eliminate the simplest/least costly first if you're having to guess like I would have to. Otherwise it's time to take it to an experienced small engine tech before you crap it out for good.

Sort of akin to a parachute failure; a total failure is the best kind to have - the partial failure is the worst malfunction. Same with your saw - better to think of it at this point as in a total failure before it possibly wrecks itself for good, because running it in a partially failed condition for long periods like this is most likely damaging something more every time you run it. Time to break away from the partial failure and pull the cord on emergency chute (the tech).

From contributor B:
50 psi is way too low. A two stroke engine builds and uses its compression above the ports, and should get at least 100 psi if it is a healthy engine. This sounds similar to an engine I ran too lean and ruined the piston. The piston gets too hot and melts down, scarfing metal to the cylinder wall as it does. The piston material from the cylinder wall then binds to and smashes into the ring grooves of the piston, which then destroys the cylinder surface from lack of clearance. I hope I'm wrong. When it happened to me, it took all of about 30 minutes of run time to destroy it all. Best of luck, I guess removing the cylinder head would be my next move if it were mine.

From the original questioner:
Thanks guys. I had mentioned the change in compression as soon as I got it back from the rebuild but the mechanic never picked up on it so I thought it was not important. Armed with this new information I will initiate a new discussion with him.

From contributor R:
A bad seal won’t lower the compression on a compression test. The seals are below the compression chamber. They just cause, for descriptive purposes, a vacuum leak and make it run really crappy. To check the compression right on any engine you need a compression gauge with a valve. It will have a button just under the gauge to release the pressure built up in the gauge and reset the needle. You will also need to pull the starter 3-4 times to get an accurate reading, You have to offset the volume in the hose and gauge.

From the original questioner:

Yes the compression tester has a valve and took about three pulls to get to a steady pressure. My mechanic has my respect and way more experience and certification. He says that there are no spec's on the compression and no adjustments either. He also mentioned that the polished exhaust port will lower compression. He thinks the governor spring in the carb may be rotted out and it may need the entire carb replaced. I'm hoping to exchange the two carbs to see if the power heads swap symptoms.

From contributor R:
Was the compression only 50lbs stable?

From contributor Y:
Compression varies, but on larger saws 150-155 most of their life. Less than 120 and it's time for a top-end rebuild. Smaller saws are higher. If you measure it, make sure you use a tester with the shrader valve in the tip - if not the compression will read low.

The saw needs to be wet inside from fuel mix then have the throttle trigger taped wide open of cause make sure the ignition is either off or the spark plug lead earthed to test for compression pressure. Then pull over.

From the original questioner:
Ok. I don't know about the compression but the thing sounds like a bat out of Hades. I decided to swap carbs to see if the symptoms changed with the carbs. We took the first carb off the powerhead with a problem and found what we hope is the problem.

A welch hole is a hole made in the machining/manufacturing process and is later plugged. It is on the side of the carb opposite the fuel pump diaphragm, the metering chamber. What we found was a tiny little clear looking washer thingy that I am told is a dislodged piece of sealant that was applied to the welch hole plug. It was stuck in the metering lever. Will run tomorrow and let you know if that was it.

From the original questioner:
That was it. A lever in the metering chamber in the carb was wedged open by some trash, thus it started well cold but would not idle or restart after it was heated up. Thanks for all the input to everyone.

From contributor B:
Outstanding! So glad to be wrong.