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I believe you said you have the service manual, if not get a Mercury manual from a dealer. Once you have a OEM manual clean and set your carbs as outlined in the fuel section of the manual. After you have done the carbs, follow the step by step instructions in the Timing,Synchronization and Linkage adjustment section of the manual and you should be good to go. Most of the problems with outboards are simply because they are not set-up properly. If memory serves, your 90 is a 3 cyl. motor.
6 cyl til 86 and 3 cyl starting with 87 ..... I have a 75hp which is the same engine , shakes and shudders at idle but real smooth once the rpm build up .
i run two 85 hp mercs on two different boats i like the one on bass boat so much i put on on the house boat. it could be as simple as the ball./ mine would run fine in nutral and it would rev up but in the water under pressure i8t would just go dead. it doesn't sound like you'll have to tear it apart. just see if you have fire at plugs when spinng it over. may need new battery or starter. if it won't turn over. if you have fire at plugs compression and fuel it will run. they are great motors. not enough info in thread to ell you much more. sorry hope this helps
i outran the coast gaurd with bas boat.lol i wasn't catching anything so i went across state line up river to get some quart beers. saw the coast gaurd a bigger boat slow down to turn around i was already a mile ahead of him. i went on to truck.lol a freind who i was fishing with lost his hat. he said my hat. i said i'll buy you another on i aint turning around. lol