Symtom of rev limiter malfunction?
 

I have a weird just off idle stumble on my 509 BBC with 750 double pumper. Starts fine, idles fine. As I start giving it throttle, it stumbles and farts a bit. Once past 3/8th throttle, it goes away and runs fine. Almost like it was getting too much fuel as the accelerator pump squirted fuel. Thought it was mis-cammed or mis-adjusted accelerator pump. Still there despite messing with it (changed from pink cam to orange cam). Am thinking I may be barking up the wrong tree and thought it may be ignition instead. Have a very old MSD 6A that MSD converted to a 6T 30 years ago. This conversion added a second box, which plugs into the 6T with a short cord. Into this second box is plugged the chips to limit the RPM. As the engine is brand new, I have not taken it to the 6000 RPM that is the current rev limiter chip to see if it cuts out. Thoughts? Thanks!

If it truly is getting too much fuel it will have a black puff. Does it do it rolling into the throttle slow or a quick hit? It's hard to tell by your description. My opinion would be that the problem wouldn't be at a point like you describe if it was a rev limit problem, but you could check by just pulling the limit chip I would think.

Curtis

Most of the time a stumble is not enough fuel.