How much compression does a saw need to fire on starting fluid?

[777funk]

Out of many small engines I've worked on in my lifetime, I have the first engine (my son bought an old Poulan 2000 as a project) that I can't get to fire regardless of what I do.

It seems to have low compression, but shouldn't it at least think about firing with ether?

The reed valve plate seems to seal. I'm not sure about the cover that the plate sits on and the carb to the plate on the other side. Those gaskets are questionable... but still with ether shot directly into the cylinder (muffler off) and from the carb throat as well, it should fire.

There is verified bright blue white spark with the plug out of the cylinder. We checked the flywheel key to be good. It did seem to have water in it when purchased. We drained it out and washed it out with gas.

Anything else?

 

[Efisher26]

Don’t use too much ether, no oil in it, it might fry the rings. Use gas with oil mix and spray it in, 120psi+ depending on the saw, maybe alittle less


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[777funk]

Don’t use too much ether, no oil in it, it might fry the rings. Use gas with oil mix and spray it in, 120psi+ depending on the saw, maybe alittle less


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Wouldn't it at least pop a little even with low compression? By the resistance of pull rope, I'd guess it's about 70 psi. Johnson outboards from the 60s seem to run good on 70 psi.

I agree on the ether. Once something fires over, I don't use it at all.

 

[Efisher26]

It’s all about the cylinder size relative to compression. You may also have a leak in the lower end.


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[777funk]

That makes sense. Maybe it's just so low on compression that it's a no go.

 

[champion221elite]

If you don't have a compression tester, try the hillbilly method. Hold the recoil handle and see if the weight of the saw causes it to fall and pull the rope from the recoil. If it drops quickly like thump, thump, thump you've got low compression.

 

[Brent Adams]

One more thing to check.
I know you said you saw good fire, but check the plug wire where it goes across the cylinder fins. My dad gave me one of those he gave up on. Turned out that the plug wire had gotten up a little farther than it should(normally theres just enough space for it at the end of cylinder where fins taper in) and the fin had left an indention in the insulation. Good enough to show spark with no load, but when plug wire is connected to plug as installed, it shorts and no fire in combustion chamber. I solved that one by wrapping with rubber electrical splicing tape and rerouting wire to get away from fin.
You May also have a ring stuck from water. Ring Will rust. Piston will oxidize. That Will cause very low compression, and may damage the cylinder of you do get it to start. Id pull the cylinder and clean it up. Doesnt take a lot of time, and you would know for sure what you had. I had a Homelite XL that got water in it. Took me a bit to get all the etching out of the piston from not much water.

 

[Poleman]

I’ve got saw to run at 85lb but was very hard to start and lacked power.
I say a minimum of 100 lb to run and 120-130 to perform ok but not outstanding. 140-150 should be normal( depending on elevation, which does play a part). Sometimes the compression seems low but you can feel how it acts on pull over.... if there’s a pop at the top and the starter force quickly stops is a good sigh. When you pull the starter and there’s very little resistance and the internal continue to spin after you starting pull is a sure sign of very low compression.
Some homeowner type saws are lower compression for ease of starting for the home owner. Pro saws are all higher compression.
A saw may also have spark, fuel but when compression is low it my not fire because any pressure is bleeding off rapidly. Ie. no ignition due to low compression.

 

[Old Mac Guy]

A few years ago, I was reading an "Instruction Manual Wizard Power Chain Saws" published by Lombard for their Model 3. This saw was first called "Wizard", later called "Wood-Lot Wonder". On page 8, I read this:

"(H) Should Engine Lack Power.... Check compression. Remove spark plug and place compression gauge in cylinder spark plug hole. After cranking the engine several times, the gauge should register at least 55 lbs. or more. If compression is faulty, replace piston rings."

I know that some of the earlier engines can run well at 85-90 lbs. psi, but I've always figured around 100 psi is a good standard for 2-cycle engines from the 1950's. But 55 lbs.??!! Wow!!

That information about the Lombard model 3 engines is good to know when someone is looking at an old saw and sees only 55-60 psi in the cylinder.

Maybe more modern saws are different, but I'd say if your Poulan is not firing with ether and you have at least 55 pounds compression, the problem is elsewhere.

 

[Poleman]

I think the older saws (or the ones I’ve messed with) all had a spark you could weld with. Most modern saws have a weak or very negligible spark. It may be due to electronic ignitions, different and changing advanced curves, spin over rate etc.
I’m with you on the older saws for the most part. There are a few that will fight back.

 

[777funk]

One more thing to check.
I know you said you saw good fire, but check the plug wire where it goes across the cylinder fins. My dad gave me one of those he gave up on. Turned out that the plug wire had gotten up a little farther than it should(normally theres just enough space for it at the end of cylinder where fins taper in) and the fin had left an indention in the insulation. Good enough to show spark with no load, but when plug wire is connected to plug as installed, it shorts and no fire in combustion chamber. I solved that one by wrapping with rubber electrical splicing tape and rerouting wire to get away from fin.
You May also have a ring stuck from water. Ring Will rust. Piston will oxidize. That Will cause very low compression, and may damage the cylinder of you do get it to start. Id pull the cylinder and clean it up. Doesnt take a lot of time, and you would know for sure what you had. I had a Homelite XL that got water in it. Took me a bit to get all the etching out of the piston from not much water.

Good thinking. Will check the plug wire and make sure there's not a problem with the installation that we're missing. Likely it could use a new ring. There's light scoring on the top of the piston but less than saws that have run fine. But compression seems low by the feel of the rope. I'd guess less than 80.

 

[777funk]

Tore it all the way down and it has a cracked piston ring. $5.25 on ebay and hopefully she'll run for the first time since buying.

 

[777funk]

New ring and compression is great. It seems to run like a top.

Looks like with very little compression, even ether will not cause it to fire. First time I've seen a no compression no fire (at all) in a saw.