Completed tune up over the winter and went to start up both motors. Port motor will not go above 1800 rpm. I double checked plugs, wires, firing order, didn't touch timing. Everything is correct. Motor starts very smooth and runs smooth, but will not go above 1800 rpm, almost like something is holding it back. Throttle is full open. I did notice that the alternator appears to not be charging the battery. Both batteries are brand new. Do I need a scan tool to troubleshoot this, or is there another way?

Thanks.

How is your fuel filter? The engine MAY be starving for fuel.

Changed fuel separator before the season. I think that is the only filter.

Alternator can cause the issue or it could be the ignition control module on the distributor is bad. Also check your distributor that the gear does not have any small cracks in it if you do pull the ICM off.

Timing not advancing? Kinda what it sounds like to me. Could be wrong...Have you thought of swapping ignition modules from the good running motor to the not good running motor?

I pulled the distributor off when I sealed up the intake manifold. I installed it back in the same position. Do I need to set base timing again because I unplugged the ICM?

you need to set the base timing because you pulled the distributor not because you unplugged the icm

If you pulled the distributor, you definately have to check the timing.

+1
You need to find the ECU plug and pull the cap off, short pins A&B. then rev it up to +1500rpm and set the timing to specs. It is either 6 or 8 BTDC, can't remember.

Well, I broke down and bought a scan tool. No fault codes in the ICM. I set base timing and ran motor. It now goes up to 2000 rpm but that's it. I also replaced the fuel filters again just for good measure. One thing I did notice is that the outboard exhaust riser was not as warm as the other three risers. Going to pull all the plugs tomorrow and see if any are fouled. Any other ideas? Any tests I can run with the scan tool? I bought the mercruiser digital scan tool for mefi 1-5.