550s are about 41% bigger than the stock 390s. Set you throttle switch over points at something like 20% low and 50% hi. Then if you have a stock MAS try setting you low throttle settings from 1000-7500 at -35%. If you have a 2G MAS try -20%. If you have a severly hacked 1G MAS then 0% to +25% at 1000, 0% at 2000 and maybe -5% the rest of the way up. You probably need to change your spark plugs by now also, if it has been running extremely rich. Pull them out and see if they are covered in black.
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