Burd,
Bad crank seals can cause your symptoms, but usually on older, high-hour saws. When hot, the worn seals expand, allowing vacuum to leak around the crank, instead of helping to pull the incoming charge into the cylinder. However, if your saw is not old, and you've faithfully run the proper fuel/oil mix, I would not suspect crank seals, first.
Try it again when cold--some saws (especially ones with poor crank seals) can be hard/impossible to start when hot--some call this "vapor lock," but again,
any vacuum leak, be it from a leaking impulse line, carb gasket, etc..., will cause a hard/no start on a two stroke, as two stroke crankcases must be airtight. And when the saw is hot, a cracked line or gasket can leak, though it did not leak when cold, thus causing the "hot no-start." (To rule out a leaking carb gasket, tighten carb hold-down bolts. If confident enough, remove carb and examine carb gasket for tears, cracks, etc..., reassemble, tighten firmly--but don't strip carb hold-down threads, and retry. Note: prior to carb removal, cover/seal off carb throat and blow away all debris with compressed air, so it does not fall into engine when carb removed.)
My theory:
I think Kevin j may be be right, when he says:
Take off some plastics, look for fuel lines loose, impulse line loose, cracked or worn etc.
The "impulse line" uses the variation in crankcase pressure, caused by the stroking of the piston/crank, to "pump" fuel from tank to the carb (presumably through the action of a fabric-type diaphram, acting as a "one way/check valve"--but don't quote me on that part.
Regardless of the theory, however, you need a nonleaking impulse line for the saw to run properly.
For example, I have a 25 (?) y.o. Homelight 360 (probably a near-professional saw, by today's standards? Opinions vary, but it's powerful and reliable, though heavy) that my Dad gave me and, last year, the "impulse line" developed a crack. After running perfectly for years, suddenly, one day, it revved wildly, running lean, and actually got hot enough, in the few seconds before I could shut it down, that it continued to run even AFTER I KILLED THE IGNITION. Essentially, it was "diesling"--running with no spark. And it was running
fast. I've had my two stroke dirtbike do this too, when I run it out of gas for winter storage. The lean mixture let's it run hot enough to start firing without the spark. Had to stall the bike in gear.
The reason I mention this is because, IF your saw revved wildly before it died or was shut down, that would IMO, definitely be indicative of a too-lean mixture, which would be consistent with a cracked/leaking impulse line.
The reason being is that, counterintuitively, a lean mixture causes overheating, and a too-rich mixture will cause a a two stroke (or fourstroke, for that matter), to run cool.
And a leaking "impulse line"
will cause a lean mixture, as it did on my Homelite 360. After it continued to rev crazily, with the ignition switched off, I had to (quickly) choke it to death. And after almost total disasassembly (everything but the internals), I found the impulse line hose, that hooks to a nipple on the crankcase (with no hose clamp, IIRC) had cracked, defeating the impulse line's ability to "pump" enough fuel to the carb.
A new piece of line, reassassembly of the carb I never needed dissassmble in the first palce (had I known what the problem was before I tore the carb down :shut: ) and it ran fine, and runs fine, to this day.
Check every line's connection you can find, and check every inch of every line for cracks.
Also, saws have a weighted end on the fuel pickup line, in the tank. I've had the line get "hung up" in my small, homeowner's Poulan Micro XXV, such that the end of the line was above the level of fuel in a near-full tank--result: air in the fuel line and a no-start. The solution is to rearrange the fuel line so it rests on the tank's bottom. When you tip the saw, in theory, the weighted fuel line end "follows" the fuel (because Gravity tells it to) so that, again, in theory, the fuel pickup is always submerged. That's what that "rattling" sound is in your saw, if you "shake it like a Polaroid picture" (apologies to Andre 3000).
Priming the carb directly, (air cleaner off, safety glasses on, last will and testament filed with the attorney) with the proper gas/oil mix (not "straight gas"!) will prove whether you have a fuel-delivery problem or not. Take a clean Frenches mustard bottle (with the threaded nipple on it), put some fuel/oil mix in it, and dribble a quarter-thimble of fuel into the carb throat, then try to start. Hold the throttle open while dribbling--if it runs, briefly, you know you have a fuel-delivery problem. A leaking impulse line, or an unsubmerged fuel pickup line are two possible causes of a fuel-delivery problem.
Good luck.
Peter
