It is getting fuel but has a weak spark and will not start.

DO NOT , I repeat do not use starting fluid on a 2 stroke chainsaw or trimmer. you will score the cylinder or the piston. When trying to diagnose a hard to start motor just give it a small squirt of mix in the carburetor. If it won't fire you have a compression problem or an ignition problem. when you test for spark make sure you have a good plug, that is well grounded and not in bright sunlight which can make it hard to see the spark. if you have spark, do a compression test.

Poulan pro pp4218a or any of their chain saws all SUCKS!

Eugene's comprssion check suggestion is great. Check for a clogged air filter. Next, try a little quirt of quick start spray (Ether). Still NO GO? If she fires but doesn't run, then you've confirmed that the spark is OK. If the saw has sat for several months, you could be dealing with "sticky", congealed fuel. Get a bottle of SeaFoam. Flush out the old gas. Add 1 oz of SeaFoam and fill the tank with fresh gas. Give it another shot of quick start. (I recently had a bout of "sticky" gas in my tiller. I got it running on quick start, but it quit after a half hour and wouldn't start - even with quick start. So the compression had gone flat. Removing he head I found the intake valve suck open - STICKY GAS !!!)

A pull the muffler and either a struck ring or badly piston and cylinder.
Alternatively you can do a compression check as you need at least 100 psi for it to start.