could be something like the coil, but considering you've seen the filter plug, might clean the carb inlet screen if it has one [should] and try popping the top of the carb to see how much sediment is in the bottom of the bowl - a "little rust" can cause "little spaces" to plug up when things vibrate for awhile, as in a few minutes of running with the attendant fuel flow agitation, the fact that the accelerator pump works doesn't mean jets aren't plugging up
Bill
