Exhaust valve is open when that happens.
Does it mean...
A. the valve isn't closing
B.The spring pressures arent right, again, leading to A.
Again what camshaft and what spring part number and what pressures did they tell you they set them at?
If it was loading up on fuel it would eventually blow the Muffler case.. that wouldn't cut out like lean...it would blubber the tone would get real low