I was running stacks on a 550 with a mac muffler and it was livable but I didn't get it ideal. I had the stock 38 pilots in, seemed to like the screws out, and was experimenting with mains up and down around 115's. The mid and top were excellent, so I didn't play with the needle. It basically acted both lean and rich around idle, was hard to figure out. It was a little weak down low, it didn't like/need choke to start, had black plugs, and ran worse with 40 pilots, so it was acting too rich at idle with 38s even with the idle screws leaned all the way out. Also, it was getting backfires out the carbs when cold, which I think is a lean sign, and the throttle stop would have to be turned in quite a bit at first to hold idle when cold, and you'd have to back that screw off bit by bit for the first 5 or 10 miles. I went back to the airbox, and it resolved the severity of the idle climbing and the popping out the carbs, and is stronger down low, as expected.
