ORIGINAL: carrellh
Jeff,
I'll go 'against the grain' here.
If it has been running fine (for about four gallons of fuel) with the existing fuel/plug/exhaust, I see no reason to start making drastic changes.
Try a new A3 plug (since you have one) and see if it cures the issue.
I agree. I have used an A3 in my OS 46AX with no issues, and it runs very well on 15% nitro.