Bad coil? Won't fire but sparks fine... (Poulan 42cc)

[jstluise]

I was given a Poulan Pro 42cc (18") saw that had been sitting for multiple years. Of course I couldn't get it to fire, so I suspected a carb issue. Good compression and I could see a spark when I pulled the plug (also tried another plug I had).

I tried priming the carb manually with a squirt of fuel, but nothing. Also tried priming it directly into spark plug hole, nothing. Next some starting fluid through the carb as well as the spark plug hole, and still nothing. Now I'm leaning toward an ignition problem.

I pulled the plug and started opening the gap little by little to test the coil. Eventually I got as wide as possible (1/8") and I still had it arcing. I figured if it was a bad/weak coil, it wouldn't arc that distance.

Not sure what else to try at this point. Any ideas?

 

[Guido Salvage]

Did you check the compression to see if it is adequate for the saw to fire? Did you try a different plug? Not sure what the gap should be on the saw, but .020 - .025 should work. You only need fuel, spark and compression to make it fire.

 

[jstluise]

Did you check the compression to see if it is adequate for the saw to fire? Did you try a different plug? Not sure what the gap should be on the saw, but .020 - .025 should work. You only need fuel, spark and compression to make it fire.

The compression is 90 psi, which I thought was okay, but maybe not? This is a dry and cold compression test, throttle open...pumped up to 90 psi in 4-5 pulls and held there.

I did try a plug that was working in another saw, but no luck.

I figured if I can see the spark (and it is strong enough to arc 1/8"), then I would get at least some kind of fire when I used ether...but that isn't the case.

 

[bluesportster02]

most saws wont run at 90 psi

 

[Guido Salvage]

The compression is 90 psi, which I thought was okay, but maybe not? This is a dry and cold compression test, throttle open...pumped up to 90 psi in 4-5 pulls and held there.

I did try a plug that was working in another saw, but no luck.

I figured if I can see the spark (and it is strong enough to arc 1/8"), then I would get at least some kind of fire when I used ether...but that isn't the case.

You will probably need north of 125 PSI to have that saw run. Pull the muffler and have a look at the piston, you should see vertical lines where it is scored. Avoid the use of ether, it washes the walls of the cylinder. A shot of mix on a saw is all you need to make it fire if you also have spark and adequate compression.

Personally I would hold out for another freebie and hope it has bad fuel lines and a good top end.

 

[zogger]

plugged up exhaust

 

[Guido Salvage]

plugged up exhaust

I don't think it is going to make any difference at 90 PSI...

 

[jstluise]

Well Guido, you're on the money. Pulled the muffler and checked out the very scored piston. Looks like this saw is going to be sold for parts. At least I didn't pay anything for it!

This can't be normal wear, can it? Seems pretty bad...thinking someone straight-gassed it.

Thanks for the help!

 

[zogger]

I don't think it is going to make any difference at 90 PSI...

Most likely, if that is the real max on compression

edit: and with that scoring! Just saw it, hahaha

Nope, she ain't gonna run...

 

[Guido Salvage]

Not normal wear, straight gas or major air leak. Looks like they ran it till it died.

 

[Chris-PA]

It happens to so many of those.

 

[hamish]

Not normal wear, straight gas or major air leak. Looks like they ran it till it died.

Take a look again, that's a bunch of carbon scoring.

 

[roosterhog]

I was going to fix a guy's poulan wild thing one time. You could pull the rope out without even having the saw lift off the ground. Almost no compression. Looked like your cylinder as well with a lot of carbon scoring. I think they had too much oil (too rich). The guy said, "We'll I always wanted to put a lot more in. Oil is cheap and I'd rather have too much than not enough." He fried it.

I think another thing that happens to a lot of saws is this: people leave them full of gas and put them in the barn or garage for let's say 1 year or more. Most of the gas evaporates but the mix oil stays in the tank. Then one day a tree falls over in their yard so they gas the ole saw up without pouring and rinsing that slop out of the tank or even worse, they just crank it up with whatever is in the tank. Now instead of 40:1, they have like a 3:1 mix. They fire up the ole saw. They cut for awhile and then it quits on em or won't restart because carbon has fried the cylinder due to the richness of the mix. Then they gripe and say "Poulans are sorry! I'm going to get a Husky!" Then they'll pour the ole slop out of the Husky next time because they paid more for it and want tot take care of it.

Right now I have two saws on a shelf that have sat there for 2 years. There is nothing but mix oil in the tank which will have to be rinsed out before use.

 

[Guido Salvage]

Take a look again, that's a bunch of carbon scoring.

Posting on my iPhone, I enlarged the picture and now see a lot of buildup in the exhaust port. Chunks of coal are not kind to a saw.

 

[jstluise]

Thanks guys for the lesson. I didn't realize that too much oil would do this. I definitely know it's not good for a saw and would foul a plug, but didn't know about carbon scoring.

I noticed on the gas cap it says to mix 40:1, instead of 50:1 like I've always seen. Is there a reason for this?

Rooster, I think your scenario is likely. This saw came from a farm. But also the old fella who lived there was known to not use the correct fuel (one time put diesel in a mower) so I thought he might have ran straight gas in it. What would a piston/cylinder look like that had been straight gassed? (Compared to too rich of mix)

 

[roosterhog]

Thanks guys for the lesson. I didn't realize that too much oil would do this. I definitely know it's not good for a saw and would foul a plug, but didn't know about carbon scoring.

I noticed on the gas cap it says to mix 40:1, instead of 50:1 like I've always seen. Is there a reason for this?

Rooster, I think your scenario is likely. This saw came from a farm. But also the old fella who lived there was known to not use the correct fuel (one time put diesel in a mower) so I thought he might have ran straight gas in it. What would a piston/cylinder look like that had been straight gassed? (Compared to too rich of mix)

Most poulan stuff (or Craftsman stuff made by poulan) runs ona 40:1 mix.

In short, a straight gassed piston would looks fried like yours but without all the black crud. I saw a web site (I think it might have been linked from this site - I guess you could google it) that for rc airplanes. It had all kinds of engine failure pics from everything you could think of (straight gassed, plug too hot, too lean, too rich, etc.). Very interesting stuff.

There is a site called Acresinternet for saws. If you go through the manufacturers and look up the model number of a particular saw, it will have all sorts of good info like fuel mix ratio, spark plug and gap, carb type etc.

 

[BigMoneyGrip]

Did you check the compression to see if it is adequate for the saw to fire? Did you try a different plug? Not sure what the gap should be on the saw, but .020 - .025 should work. You only need fuel, spark and compression to make it fire.

All of these events must happen at the correct time, or it won't start. Let one get out of phase and you can pull on it all day and it won't do anything.