I just experienced the very same thing with my 81 490 at the Boise Inter Am yesterday. Bike has a fresh top end, plug looks good, looked at the top end with a fiber optic scope and everything looks like it did a couple races ago, just fine. I started the bike up first thing in the am to warm it up and it ran just fine. Since I was in moto 7 the bike was shut off. Started up normally before my moto and as I was headed to the line it got very, very sick sounding, rev'd slow with no power and several friends commented that it smelled very hot, silencer core was very dry and ashen in color which is not how it normally looks. Compression is still good but there is no spark. Did you correct it by replacing the PVL?
Fritz