I've had a couple of bouts of SVS. It starts with a noticeable exhaust sounds (especially at idle), and vibration at certain engine RPMs.

This latest time I could not shake it with "the cure" until I read a post from Doc.

I first sprayed a de-carboning spray into the carb at fast idle and then gunned it a while to blow out as much carbon as possible.

Then I sprayed in Deep Creep for 30 seconds at high idle and then choked it with Deep Creep to stall while spraying. Let it sit for overnight and ran it and sprayed it for 30 seconds again.

I left the air cleaner off and drove her to work (30 minutes). Parked and sprayed for 30 seconds and stalled it while spraying.

Ran it for lunch and sprayed again when I got back. Sprayed it again when I got home.

Did that for about four or five rounds and it cleared up. I can tell it's gone because the vibration from SVS is gone and the engine is running smoother.

Hope this helps.

EDIT: Poor quality gas and low compression conspire to cause carbon build-up in the heads and gunk up the valve stems. I purchased an extra set of heads with 2000 miles on them and they had a lot of carbon over all the combustion chamber surfaces and the valves - especially the intake valves of each head on the carb's side.