<VV> Run On Engine

[Grant]

Re: Occasionally between shifts, the engine will accelerate on its own.
 
This is a SWAG, but it sounds like something is keeping one or more carbs or linkage from returning to where it should sometimes. I have found a few trouble-prone carburetors with the accelerator pump installed backwards or having lost the clip connecting it to the lever. This can keep the carb linkage from moving or can lock it up. Blipping the throttle can dislodge it until next time. Since this occurred after a rebuild, that should be suspect. A sticking carb can affect the one connected to it and keep both open when they shouldn't be. I have also seen carbs that had the pump springs reversed, missing, and replaced with incorrect ones and pump cavities with part of an old pump cup or scrap of gasket in the bottom. Might be worth a check. Good luck in your quest.
The Carbmeister