I have an identical problem that I noticed yesterday for the first time on a Poulan that I bought a couple years ago, and it has the blade on. I'm thinking wildbill has the answer for me.
The last time I used it, it ran extremely irregularly and roughly - like it was going to die. Almost felt as if the fuel was getting dumped in without appropriate regulation. As you can tell, I'm no lawnmower mechanic. I also noted at that time when I restarted it after emptying the bag, that it would backfire and sometimes sparks would come out. Anyway, I finished mowing, embarrassed whenever someone came by looking at me wondering why I was using a mower that fiunctioned like that. Now the next time I try to use the mower, I get the behavior described by the poster aabove. It pulls smoothly about halfway and then it jerks backwards in a way that is actually very dramatic and hurts my shoulder. It didn't start after a few pulls and it is too risky to my body to keep pulling the pull cord.
So is this in fact probably a broken key between the flywheel and pistons (or whatever). And secondly, is this probably why this mower has been running so bad. Or is it two separate problems.
Thanks.
Rod
