01 Mercruiser 5.0 seems to have lost Horsepower

[viper1216]

Bought my boat at the beginning of this year. When we first went in, she ran awesome. I was getting like 30-34mph easy and she jumped up on plane at 3/4 throttle. Lately, it just doesn't seem like it has power anymore. Takes a while to get up on plane, and maxing out at about 26-27mph at 4500 WOT. I looked through the service records that were given to me...and now that the "new boat" syndrome has worn off, I am reviewing a bit more closely. It had an "8 cylinder tune-up" done in November of 2018 at the old owners Marina...whatever the hell that includes. Not sure if it was just plugs or filters and everything. So I'm thinking maybe fuel filters are clogging up or something. Would that cause HP loss? Engine runs great, starts right up and sounds good. No skipping or anything. Just doesn't seem to have the get up and go it used to have. Don't know if the carb has ever been rebuilt either. Was planning on doing that over the winter anyway and would like to not do now if I can avoid it since there's only a couple of months left in the season. I was pondering a new engine, but really don't want to yank a perfectly fine engine if I can avoid it, especially if it was good at the beginning of the season. Which is leading me to just give her a good tuneup and see what that does. Boat is an 01 Maxum 2400SCR Alpha 1 Gen 2 - 5.0 with a carb. Any thoughts very much welcomed!

Thanks!!!

 

[CaptnKingfisher]

Could be a number of things causing lack of top end performance. I'd start by warming the engine, then shutting it down and pulling the plugs out and reading their condition to get insight on what's happening in each cylinder. You can usually spot a dead cylinder by a wet oil fouled plug. You can identify other issues as well, bad mix, water in cylinder, etc. While you've got the plugs out, do a compression test and post the numbers. Go ahead and verify plug gaps before reinstalling and put a little dielectric grease on the spark plug leads when you put em back on.

 

[viper1216]

I was planning on taking a look at the plugs. Though if I have em out, I'm just gonna replace them at the same time.

 

[alldodge]

New to you boat should have had some maintenance done. Fuel filter, impeller, check the plugs and cap. Drive alignment check, etc, etc

Remove the fuel filter and dump the contents into a clear jar to see if there is only clean gas. Agree, pull some plugs and check that the wires are attached securely. Look at distributor cap and rotor

 

[Scott Danforth]

do a compression check
check fuel filter as indicated above

how old was the fuel that you had in the boat prior?

 

[viper1216]

Tank was pretty much empty, but had fuel from the year before, so 9/2020 I would gather. He had it professionally winterized by his marina so I would gather it had all the appropriate additives. I filled it fully before launching, so what very little gas was in there got mixed with about 70 gallons or so.
Impeller was done in 2019, so that was good and he had service records for tuneups and oil changes, but not sure if that included fuel filters. I'm going to start there.