Hello,

My port motor is running 350rpms slow or behind the stbd motor. Computer scanner says everythong is good. Furl injectors are goong to be professionally cleaned and other filters checked. Any other ideas or experiences with this that the problem could be?

Thanks

What's the condition of the props?

Sounds like you may be down a cylinder. I had an unplugged one and it was down 350-400 from starboard. I had just had a tune up done. I chased props by having them tuned (just had it done and thought pitch was off) and then all the other standard gremlins I thought it was. No codes. Ran fine. Down 400. The rear starboard cylinder wasn’t fully seated.

worth checking and best of luck

Same here. Had number 8 plug wire fall off once.

Prop is good. I feel its a cylinder thing too. I will habe to double check all connections etc. Sometimes never think to do the easy things.
Thanks

I would start with the injectors. When they start to lean out or not flow correctly I've had motors lay down on the big end. Fix one thing at a time or you may not know what caused the problem to begin with. Also check throttle rigging.

thank you

I had one that was lagging by a few hundred RPM compared to the other one too. I chased the issue for a while. It turned out to be the Crankshaft Position Sensor. Others have had them go bad without throwing a code as well.

496HO: would IAC cause this? 1 engine lagging intermittently lagging 100-400 RPM
See post #26, where I found my issue.

Man if only there was a source of information on the 496mag HO...you know.....a guy named BUP.

He would likely tell you the following checkpoints:
Cam Position Sensor
IAC
Battery Voltage
Then Cool Fuels, Rails and Injectors tested on a stand to verify flow rate.
Anything else and you are just throwing parts at it praying something works.....

got details...so fuel pump has a pressure regulator (kinda looks like thermostat a bit)...that was made with paint on the inside of where it sits which ends up sticking to regulator. paint ruins regulator then paint gets into fuel injectors and clogs or can ruin them. Merc replaces fuel pump/regulator and injectors for free. so they are replacing all this on both motors now. some sort of recall warranty. the new fuel pump cap they send does not have paint in it now. this caused the cylinder to run hot (lack of fuel) and the gasket area around the bad cylinder burned it up. everything involving the cylinder is fine so that good.