1999 Mercury 90HP problems

Please someone help me!!! i recently sold me precious motorcycle for a boat, after painting and refitting the boat. I took it out trusting the sellers word that it does 39 miles an hours. It's a 1975 '17 wellcraft v hull CC. great hull condition. Fitted with a 1999 Mercury 90HP. but when taken out on the water it wouldn't get up to speed , It does about 8 miles an hour, and shuts off after a minute of running and has a hard time starting. since then I replaced all spark plugs, clean out the carbs, new fuel line, bulb and fuel tank. but no change. it will run in high rev out of the water on muffs but backfires every 3 secs. checked all coils and all three have ignition. please help someone.

Re: 1999 Mercury 90HP problems

Troubleshoot in a logical manner. Start with a Compression test to ensure mechanical integrity of the motors cylinders. All cyl's should be over 100 and very close to even, original compression was right about 125 psi.

Next check the ignition system, using an adjustable gap spark tester set to 7/16 inch, do all cyl's have a bright blue sharp 'crack'? Also is the ignition timing lever smoothly able to move with the throttle lever all the way from base idle setting to max advance at WOT? With the spark plugs out and all plug wires grounded, using a timing light and remote starter button, check WOT Timing to ensure full advance at WOT.

Lastly, the fuel system. All lines and fitting clear and tight, no restrictions, and no leaks, air sucking into a line before the pump causes many problems. Is the fuel pump developing enough pressure and flow? Are the carb's clean, floats adjusted, linkage set according to the factory manual?

All the above must be done and work in harmony for your motor to perform correctly. Take it one step at a time and it will all come together in the end.