It turns out that on some of these units the rigid pilot air tube was too close to the center of the burner motor housing so too much air was being forced into the tube when the blower wheel is centered in the housing. just moving the wheel a hair to compensate would eliminate this problem. Replaced many of these motors and wheels before and have never had this.....many hours burt on this one but at least it working.
Good ol ENG A. A fella and I had cleaned up the burner and accessories on a direct fired mua. It was doing the same thing. Turns out he had hit one of the differential air pressure tubes. It would attempt ignition, the pilot solenoid would open but no cigar. Makes troubleshooting a little difficult at times.