A rich transition would do that, but I cant believe he used the same carb from gas to e85....on gas it would puff a cloud of black smoke during the occurrence....kinda easy to see on a pipe rack and freaky odds that two carbs would have the same problem.
I think it may be a common problem which could exist in both carbs. It's sometimes difficult to convince yourself that "less" is better as opposed to "bigger is better". It's not always obvious.
I recently replaced a well used engine with one having basically the same parts/combo. That changed the carb tune-up to the point that I originally thought I screwed something up in the build! Must have been the ring seal.
But as they say...."Leave no stone unturned." If expected fixes don't work, you have to start with the unexpected ones.
Does over-rich E85 cause black smoke as bad as gas?
