ryanb790,
A hung or burnt intake valve will allow the firing cylinder to pop back through the carburetor. If this is happening as the engine idles, I would pull the valve cover and look at the valve train. Adjusting them would be a top priority too! A compression check might turn up a bum cylinder.
Be sure your plug wires are arranged 153624 and that the cap is clean inside and out with no cracks. My guess would be a valve issue. Keep us posted.
Stuart
