I think this is where somebody in the know about motorcycle engines would rattle of some slick sounding jazz about pressure, vacuum, air/fuel mixture, humidity and all that... Unfortunately what I know about the inner workings of a motorcycle engine would fit inside that pop you heard.  Now that I have wasted your time, lets see what some of the real mechanics have to say.  

If you go gas it up & the miles indicate it wasnt time to go on reserve, then yes, the exhaust was likely plugged with rust & crappola. Ive seen cars just shut off going down the road, coast to a halt, hit the starter & fire right up & the cataclysmic perverter needs replaced, junk moves around & the exhaust gasses carry it onto the ceramic, plugging the little holes & as soon as the engine is off, the pressure is off & the stuff falls back down, allowing it to flow. Never heard of an exhaust clogging, except for catalytics, but I can say its not possible.

A cloud of sh...t can really change how the bike runs.

Yeah.  It's probably affecting your volumetric throughput.

I still can't believe you said that, Verslagen.

You know it looked like dirt. I think a dirt dobber built a nest in there and it just took some time with the motor running to get it out of there.
looks to like it would have ran hot though. i didn't seam to be. I remember thinking after I got it running that it didn't seam to have as much power as a old 450 suzuki I use to have. but after the pop it showed me it was a 650....