Chainsaw dies when hot

[Steve Tucker]

Ok, since you guys did so well with the STRUNK chainsaw thing, I figured I'd give you crack at another problem I've been having.

I've got a Craftsman 16" chainsaw that dies when you really get into some heavy cutting (read that, you run it at full bore under load for an extended time). When you try to re-start it, it just won't. Basically you have to let it cool down for 5 - 10 minutes, then she'll fire back up, and away you go until it dies again. However, it will start and run fine if you are just cutting light stuff.

Now for some other pertinent info:
I'm running 40:1 mix, as suggested by the manufacturer, and the mix is brand new (new gas, new oil).
The plug is new, and not fouled.
The ignition module is new.
The low speed jet is set approx. 1.25 turns out from full in, and it idles fine most of the time.. however sometimes it will die when idling, but will restart with no problems.
The high speed jet is set so that it is just a little rich at wide open (i.e. the motor goes from that high whine to a little grumbly)
The fuel line is not mushy, and the in-line fuel filter is not plugged, so the carb should be getting no shortage of fuel.
The fuel tank cap is brand new.

Lastly, I'm pretty handy with motors (was actually a small engine mechanic during my college years), and after tearing apart the carb, and finding nothing, I'm stumped (no pun intended ) The only thing I haven't checked is the exhaust for carbon build up.. and I intend to do that the next chance I get.

So, if you've seen this before, and can point me in the right direction.. I'm all ears!

Thanks in advance!

[treeclimber165]

I have experienced the same thing from time to time with various equipment. The only thing I could ever figure out was that the saw was too hot. Some saws just aren't designed to cool as well as others. I live in Florida (read: HOT) and the only thing I found helpful was to not leave the saws sitting in the sun. The fuel vaporizes in the fuel line and vapor-locks the saw. The next time it happens, try loosening the fuel cap for a second. Release the built up pressure, re-tighten the cap, then re-start. Other than that, you might just be over-working that poor lil craftsman saw.

[Darin]

Many times the manufacture is too cheap to insulate the carburetor from the cylinder head. Does the carb sit on top of the cylinder head on your model? If it does you probably are expieriencing what the treeclimber is talking about. Vapor lock. My old Pro Mac 610 was notorious for that. Just cheaply built. I had to have a second saw sitting there to fire up while the other cooled down. And switch them when I ran out of gas. How did I ever make money? With a Mac as my main saw. Those were the High School days.
Darin

[sonny]

My sugestion, Is to check the compression, prior to running, then after getting hot or dieing out check it out right then.

[Steve Tucker]

Ok, the plot thickens...

I went over to the local Sears parts depot, and had a long chat with the chief small engine mechanic over there. Here is what he had to say:

1.) It appears that the saw I have has a horizontal Poulan power head, but with a small twist. Poulan makes their pistons with two rings, and the Sears version only uses one (makes the saw cheaper?). This equates to more wear on that one ring (not good). I'll explain the "not good" in a bit..

2.) He suggested taking a compression reading when the saw was cold. Run the saw until it dies when it heats up, then take another reading, and compare the two. He suspects that the compression from seals on the crankshaft, the ring, etc.. will go down as they expand from the heating.

3.) Now here was the clincher, he explained that as compression is lost in the crankshaft area, this results in an incomplete "charge" of fuel / air mix being transferred to the combustion chamber. This in turn results in loss of power due to the mixture leaning out, and consequently the saw running hotter. This cycle repeats itself (leaner, hotter, leaner, hotter) until the saw just dies. Then, when the saw cools down, it starts up again and leaves the user baffled as to what the heck is going on..

4.) Now, back to the "not good" part concerning the single ring... With that single ring, all the wear from running is heaped on that single ring. When that ring wears, the gap on the ring end will expand. A new piston ring / cylinder will give about .010" of ring end gap, but anything in the .020" and up for end ring gap will cause marked loss of compression and will result in the saw running poorly, and potentially dying when hot.

Okay.. Solution?

I bought a carb overhaul kit, a new fuel filter, and fuel line when I was there (cheap money). This will completely eliminate the fuel system completely as a potential source of the problem (bad diaphragm, dirty filters, pinched tubing, etc..). Once I get that installed, I'm going to test the saw and see if the problem still exists.. if it does, then the next step is to check the compression as the mechanic (and one of our forum members) suggested.

If the compression changes dramatically, I'll have to open the powerhead up and check the ring end gap. If the ring end gap is above .015" or so when cold, and the cylinder wall is not scored (it looked fine when I checked it through the exhaust port) then most likely just changing the ring should breathe a little more life into the saw.

I'll keep you posted on what happens...

[Deere John]

Thanks for the details Steve. I knew about the vapour lock, as I was 16 too and used mini-Macs to attack things that were way too big. But you related a few things I didn't know.

[davefr]

It's amazing you found a Sear's person that could comprehend 2 cycle engine theory. The theory sounds reasonable. I guess it goes to show that Sear's is really after absolute lowest cost and is willing to have the manufacturer cheapen whatever they can at the sacrifice of product quality.

Take the chainsaw back under their satisfaction guarantee. They don't post a time limit in their stores.

P.S. Make sure the chainsaw is cold when you return it and try and perfect the Clark Griswald look in Xmas vacation!!!

[homelite360]

whell i guess if the technition has been around for a while he has had to work on quite a few. or they gave him a book on what will go wrong with these saws after you run X number of tanks of gas through it. If you ask me i think that they should take their junk back and give you new junk. but have fun harrasing the sears people

[Darin]

I wasnt aware of the single ring craftsman and the 2 ring poulan. There was another thread in here that somone was talking up a craftsman. I also am amazed you found a qualified tech. AMAZING!!!! I have ordered parts for craftsman saws before (when I was in the repair business) and never recieved right parts or info (I once even recieved a RCA VCR part). I had to tell the customer that I cant fix your saw but bring your VCR in.!? Good luck.

[Mouse trap Engineer]

Hi Folks,
Man! you guys have really dug into this Poulan thing and i think thats they way to solve things. But in my opinion you have overlooked one critical detail, Take all the covers off and clean the cooling fins. The saw may not be transfering out heat due to impacted cooling fins! Please don't take this wrong the Sears Tech has a good point BUT i will bet you he has never actually witnessed that particular failure in person. That is tech school talk and its good but these guys overlook so much it's not even funny.
I myself prefer Stihl and Husky, but i have used Poulan saws of my Dads and also I own some Poulans that have been excellent saws and give good service, I have a 3400 that was sold by Sears over the years i have changed most of the cover back to green due to breakage etc, this saw has TWO piston rings and came from Sears that way.

[Mouse trap Engineer]

Hi Folks,
Man! you guys have really dug into this Poulan thing and i think thats they way to solve things. But in my opinion you have overlooked one critical detail, Take all the covers off and clean the cooling fins. The saw may not be transfering out heat due to impacted cooling fins! Please don't take this wrong the Sears Tech has a good point BUT i will bet you he has never actually witnessed that particular failure in person. That is tech school talk and its good but these guys overlook so much it's not even funny.
I myself prefer Stihl and Husky, but i have used Poulan saws of my Dads and also I own some Poulans that have been excellent saws and give good service, I have a 3400 that was sold by Sears over the years i have changed most of the cover back to green due to breakage etc, this saw has TWO piston rings and came from Sears that way.

[BillW]

Is this saw old enough to have points & condenser?
Have had some old ones that had smilar symptoms...Repl the condenser & no prob.....

Too low Compresion will not get you enough fuel to run/start

[Steve Tucker]

Ok folks.. I've finally gotten to the bottom of this whole saga.

As I stated previously I overhauled the carb, and replaced all the fuel lines, as well as the tank fuel filter. While the saw did start quicker, and seemed to run smoother, alas.. the "dying when hot" problem reared it's ugly head when I took it to task and attempted to cut some oak logs.

From there I took the advice of multiple readers, and proceeded to compare the cold and hot compression readings. Prior to firing up the saw for the log cutting test, I had taken a compression reading. The cold reading was ~30 psi with a quick leakdown to 0 psi. This didn't give me a good warm fuzzy feeling...

The hot reading was pretty much the death knell for the saw.. the reading was simply non-existant.. zip, zero, nothing.

After the saw cooled down, I ripped it apart, all the way down to the piston / ring. The single ring was chewed up pretty badly.. most of it corresponding to the exhaust port area (the side you couldn't see by looking in the exhaust port). The cylinder walls were scored mainly on the exhaust port side, but also to a much lesser extent on the side opposite the exhaust port. The "top" and bottom sides of the cylinder were still smooth, and showed very little wear.

Ok, analysis of the whole thing from what I've seen.

1.) The removal of the second ring from the Poulan design was a disaster waiting to happen. Unfortuantely this is a case of beancounters having more political clout than the engineers. (Yes, I'm biased.. I'm an engineer!)
2.) The power head was set-up in a horizontal stroke position, causing uneven wear on the cylinder walls, piston and ring. Essentially what happened is the area of the wall that is not on the same plane as the horizontal (ie. the exhaust port side and port opposite it) do not retain the lubricating properties of the oil in the mix during storage (it seeps off due to gravity).

Epilogue:
The saw may have cut it's last log as a comlete unit, but I'm glad to say that it has become an "organ donor" for a friend's saw who had the same make and model. His bar had gotten damaged, so that got replaced, and I set him up with the covers from my old saw (his were rather beat). It's good to know that somewhere it's still cutting wood..

And yes, I bought another saw. Don't ask which type..

[sonny]

Piston scored on intake side, bad air filter & or dirt, going thru the carb.

[HUSKYMAN]

If a single ring engine is designed properky then the saw can last a long time. The Husky 55 has a single ring and is a very durable saw. Poulans just suck