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.