Me too. Finally found the posting for the similar problem on a 38. Peggy posted the Tech Tip.
Somebody rub my nose it it so I don't forget next time.
https://www.stovebolt.com/ubbthreads/ubbthreads.php/ubb/showflat/Number/670601/


"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."