I can see your logic chain as to why you think the oil in the venturi, which obviously shouldn't be ruled out. I was curious as to why you automatically ruled out ignition. I understand the EI timing is probably constantly adjusting given the atmospheric, engine power settings, etc condition. What if the mag is firing before the EI? Doesn't matter how "hot" the EI fires if the burn is already being initiated. That's why I was mentioning the data plate so he can check the mag timing.
I think by throwing out your chain of thoughts in internet troubleshooting might make the victim of the issue have an "AHA" moment, or help other people build on your thoughts/suggestions. Sometimes hearing someones thoughts on a solution can jar your memory into thinking of something else that is helpful. Or building on one idea/solution. Hell, the true is same working on these issues in person!