I hope to get my 90hp tower started this weekend ,Reinstalled power head to the lower unit and hooked up all the wiring . Tested for spark which looked good an strong and compression tested , I have two gauges one gave me a reading of 150psi across all 6 cylinders and one was only 90psi across all 6 cylinders , what would be a good reading after honing and new rings? Should I mix oil extra rich for this start up or do anything special ? Will only run it for a few minutes and then will sit until April when I mount it on the boat and test run . Thanks for any suggestions.
Sounds like one of your gauges is way off. I'd say the gauge that's reading 150 psi is likely more accurate. The important part is that the readings are perfectly even across-the-board, and both gauges confirm that.
Run the engine on 25:1 premix. If you are oil-injected, run extra oil in the fuel. You should also use a fuel stabilizer, since it's gonna sit for a while. Drain the carbs after running by pulling the brass main jet plugs.
Get a new gauge, A good healthy Merc should show 120 psi across the board, but I think yours is fine. Double the fuel mix for the first couple of tanks and no pinning the throttle until you get some hours on it.