Re: max RPM, 100hp 1989 inline 4
I believe mine is now working (knock on wood) as designed. The kick-in point for the other 2 cylinders is 1800 RPM. If you run your motor slowly up to around 1800 rpm and then increase throttle slightly and then decrease throttle slightly and you notice some roughness in the motor while doing, this is normal as the motor is supposed to have a rough spot as it transitions from 2 to 4 cylinders. If you are getting above 3000 rpm, I feel fairly sure (but not positive) that you are on all 4 cylinders. I don't believe mine would get anywhere near that on 2 cyls. If you listen carefully as you pass through 1800 rpm you should also be able to audibly hear a difference as the other 2 cylinders kick in.
When I first got my 2+2 it ran awful. It would "spit" back every few seconds at idle, and I was advised this was from too lean a mixture (or the inability for enough fuel to get through the carbs to make the mixture rich enough). The previous owner had owned it for 3 years and he had been bad about leaving old fuel in it. I had a pro Merc shop do everything to that motor to try to fix the lean spit. They rebuilt all 4 carbs, rebuilt the fuel pump, replaced the reeds and replaced all fuel hoses. After the mechanic got it all done, he was still unhappy with it. He likes to let one idle and walk away and work on other stuff for long periods, and on my motor, it would idle fine for awhile then maybe spit back or maybe stop dead. It had a new control computer, so that wasn't suspected. This Merc mechanic had lots of years of experience and he just "knew" that the problem was still fuel. He went through all 4 carbs a second time and went through the fuel pump a second time and checked all fuel lines again to make sure no air was getting in. One of the mixture controls on one of the carbs seemed to have almost no effect even after 2 rebuilds, so he was pretty sure there was a fuel problem but he couldn't pin it down. He finally called the techs at Mercury and explained everything he had done and asked them for advice. After explaining the situation to the Merc tech, the tech asked the mechanic if Mercury QuickClean was being in this motor. The mechanic replied no. The Mercury tech advised that the mechanic let us take the boat and that we should run the motor enough to run 2 tanks of fuel through it with Mercury QuickClean mixed with our (fresh) fuel at double the recommended strength. The Merc tech said these motors just run better with QuickClean and that double strength continuously was okay. We were bummed and skeptical but we took the boat as recommended and after running less than 10 gallons of fuel through it, that motor was running like brand new, with no spitting. Ever since then I have been sure to run the fuel out of the carbs and leave the tank almost empty at the end of each season so as to have fresh fuel when run next time, and I now always run QuickClean at double strength in that motor and it runs very well and smooth.
I don't know if this will help you or not, but that's the saga on what it took to get our 2+2 running well... Best of luck to you...
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1989 SeaRay, Merc 100HP 2+2, plus a few older boats and Mercs: Mk 58A, Merc 500, Merc 650, Merc 1000SS, Merc 1100SS (continuously learning and confused but having fun)
