Since it is a used unit the causation can be more troublesome . You have pretty well checked , cleaned or replaced the normal areas of concern . The ignition switch may be faulty as you have indicated which can be bypassed . Secondarily and more likely is that your coil is heating up and breaking down . They are inexpensive on eBay . I would also advise since it is used to ensure your fuel filter and air filter and gas cap vent are clean and check the cylinder and headbolts are tight roy- bilt and ryobi have fastener issues Good luck Bud !

There are some youtub and good listed instructions on rebuilding carbs. Generic and specific, which I have found, close enough for most small 2-cycle carbs. The most challenging part I found was trying to replace the small fuel screen in the carb. This taught me to clean the screen w/o removing it with less psi from compressed carb cleaner or compressed air. I also find the fine screen being restricted and hard or cracked diaphragms as being common problems in the small carbs. Below are 2& 4-cycle youtube and text examples different, which works similar enough. Digital Pics before pulling your fuel lined and in disassembly stages could save you from pulling your hair. LOL & Good Luck! http://www.youtube.com/watch?v=VGq1wuBNM00 http://www.ereplacementparts.com/article/4582/How_to_Clean_a_2_Cycle_Engine_Carburetor.html

Having lot the same problem with a Husky 128 LD.about 3yr old. Last season found primer bulb cracked. replaced it and trimmer would not prime. Found fuel lines had basically disintegrated. Replaced, still wouldn't prime.figured carb was gunked up. This season I replaced the carb and gasket. Replaced the fuel lines with non-permeable lines. Replaced fuel lines. Using no-ethanol fuel with synthetic 2-cycle oil. Fuel is fresh and blew fuel filter out with carb cleaner. Now primes real easy, starts real easy. Runs for a minute or two at open choke. With about a 30 second warm up a half choke. Then dies especially when I give it some throttle for some cutting. Will restart but still won't run very long. Any suggestions. First year and half of service was great. Seldom had to run full throttle. Had a Troy built 2 cycle and 4 cycle before this was so much better. Now my Troy built mower with a Honda engine that's a whole different story!

A youtube is at the site's address below on, "How to Remove a Trimmer Clutch". In this case, I can see a torque wrench is not necessary due to the clutch's hub is screwed down over the clutch and will keep the clutch tightened with a spanner wrench from spinning off. The spanner wrench will work for the Poulan's possibly but not as safe as the designed clutch wrench attached. "This article explains how to remove the clutch from most line and string trimmers. Tips are provided for removing tricky clutch drum screws. Here at eReplacementParts.com, one of the most common questions we receive about lawn trimmers is how to remove the trimmer clutch. Trimmer clutches need to be removed when replacing the clutch drum, replacing other clutch parts, and when the trimmer starter needs to be accessed for repairs, service, or replacement. Removing the clutch drum is a little tricky, because turning the clutch drum screw will only turn the piston unless the piston is somehow bound up. Detailed steps and pictures are included in this article that explain how to remove a trimmer clutch and how to keep that trimmer piston from spinning. Tools needed to remove a trimmer clutch: �h needle nose pliers �h screwdrivers �h #25 torx driver �h 3/4" socket and wrench �h adjustable spanner wrench �h a piece of nylon rope" http://www.ereplacementparts.com/article/1771/How_to_Remove_a_Trimmer_Clutch.html