The first indication is that it won't run for crap. I don't know the technical reason, but suspect it upsets the air/fuel mixture - probably a vacuum thing. And somewhat like you indicated, the problem isn't particularly noticable at idle, but when you open it up (in my case, anyway) it would break up and miss fire. There was a lot of black smoke and the plug did foul on that side. It should be easy enough to check. But that seems like far more a remote thing than just gumming up. I'd hate to see someone tear stuff apart when they may not have to. But, if you're exhausting the posibilities - exploritory surgery is an option. Good luck.
