Small Engines (Lawn Mowers, etc.): Snapper 2 stroke snowblower problem, combustion theory, internal combustion

Question
Hello,
    I've been asked to take a look at my brother's snowblower. It's a snapper single stage with a 2 stroke, model # 3201. The problem is that after approx. 30 to 45 seconds of running it will start to pop (backfire) through the carb every 5 seconds or so. I am a wiz with engines and internal combustion theory having been in engineering for years, building personally tricked out drag bike engines that do amazing things on pump gas and repairing just about anything mechanical when others would toss it away and deem it unfixable, but I have to admitt that this one has me baffled. At first I thought maybe there is a problem with the intake reed but quite frankly after giving it some thought, the machine develops power and torque nicely so I know there is compression and the plug isn't overly wet or fouling either. It starts up right away and even though the carb isn't adjustable, I've tried playing with the float height figuring I might get some results either way trying to lean it out or make it run a little fat, leading me in some direction. I've come up with nothing at all. Then I thought that I'm looking in the wrong area since I'm getting enough compression to build a nice amount of torque without smoke or severe blow-by. It must be ignition related figuring that i'm getting an occasional detonation or firing while the piston is at or near the bottom of it's stroke and the intake port is exposed. So today I yanked the flywheel looking for a set of points hoping that they would be causing spurious firing. Not the problem since this is a straight magneto with no points. The flywheel was on key and everything looks fine. The only thing I noticed was some skuff marks where the plug wire was touching a cover in 2 spots but there is no visible degredation of the insulation on the plug wire. The coil/iron-core assy itsef looks fine as well except for some rust on the exposed plate stack. I am stumped. Do you have any ideas at all? Obviously I'm trying to avoid changing the coil/core assy if it's not bad since around here, as a rule, they will not take back electrical parts. I remain teachable.  

Answer
Hi Mitch,

Sorry, this has stumped me too.  You have been doing some thinking and checked all that I would have.  I suspect ignition timing or an electrical defect somewhere.  I think a new coil may be the only option.

Let me know how you make out,
Michael