You might have stumbled over what many of us stumbled over on our first \6 valve adjustments: it's easy to get so wrapped up in paying attention to the clearances that you miss the changeover in the middle of the engine, where two intake valves are next to each other. So you're going along, adjusting alternate valves to 0.020" and 0.010", and you get half the adjustments backwards. Result is rhythmic idle and puff/chuff out the exhaust. Front to back or back to front it's E - I - E - I - E - I - I - E - I - E - I - E.
