Hi, I have a 79 model 115 merc inline 6, that I bought about 2 years ago. It is on a 17 foot Mustang, and has only ever revved to 4900, trimmed right out, with a 15 inch prop. I have had the carbs serviced, new fuel pump, distributor serviced, new plugs and leads, but no different. A friend has the identical boat and motor, and gets 5600 rpm with a 17 inch prop. He thought that maybe the motor was dropping cylinders up high, but, if you drop off plug leads, one at a time, while going along, the motor loses performance. It seems to idle really well, good compression, and will rev quite hard in neutral. Can anybody help? Thanks in advance<br />Biff