Low Power/RPM's Merc 7.4 MPI Z302
 

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Re: Low Power/RPM's Merc 7.4 MPI Z302
 

I had a very similar issue last summer with my 502 powered Z302 and found that the timing was way off. I had recently installed the motor and the person who set the timing come to find out used a faulty timing light. I was only able to reach 4k on the tach but idled perfect and would rev fine while it was not under any load. I would check your timing and follow the spark plug cables to make sure they did not get them reversed while performing the recent tune up.

Re: Low Power/RPM's Merc 7.4 MPI Z302
 

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Re: Low Power/RPM's Merc 7.4 MPI Z302
 

When you did the plugs and such, did you check the fuel filter and the fuel/water seperator? some other things to look into, i'd suggest running some enzyme fuel tank cleaner in with the gas. Sediment can form in the tank and can crud up the lines. Star-Brite makes the enzyme cleaner, and it works great. Have you checked/cleaned/replaced the fuel injectors? Maybe a seafoam treatment?

Re: Low Power/RPM's Merc 7.4 MPI Z302
 

New plugs just last week and did add a bottle of Sea Foam to tank. I did not change out the fuel/water seperator, which I will do before this weekend. What is the process to check/clean/replace any of the fuel injectors?

Re: Low Power/RPM's Merc 7.4 MPI Z302
 

Just a thought. What elevation are you running at? Is this boat new to you? (the way is is acting is new or you don't know) I run a 1996 Z252 7.4L with a bravo 3. If I was trying to turn a 26 prop at 5500 feet mine would act the same way. I have 24's and they work great. Reed

Re: Low Power/RPM's Merc 7.4 MPI Z302
 

I had the exact same thing happen to mine this past weekend in northern Wisconsin.

Some things that you might want to check,

Distributor cap and rotor
Plug wires and coil wire-Check for rust if they are still original
Anti-siphon valve on top of fuel tank and fuel line
Knock sensor operation
Timing
Fuel quality

I got into mine and found the cap and rotor heavily corroded. Also found the coil wire was rusted and seized to the cap. I removed the anti-siphon valve and found the spring was broke and partially cocked and not allowing the check ball to open fully. I checked the fuel line from the tank to the water separator and it was good. I also replaced the knock sensor as a precaution because I've heard of these going bad. Once I have it up and running again I'm going to verify timing which I would highly recommend! With all the issues I found I was amazed that it would even run!

You didn't indicate that you had any engine miss so I doubt that there is an injector problem. Start with the basics and move up from there.

Good luck!