RPM issue.... Advice, please?

[SaltyDogg]

Hey guys,

I've been running my recently purchased 2000 Maxum 2800 for about the last month or so on the weekends. And I was out on the water yesterday and suddenly had trouble staying on plane which eventually became a bit of a struggle to keep about 10mph back to dock. I checked the props while out before returning and they appear to be spinning freely, couldn't find any obstructions. The major thing that I've noticed is that the engine while it seems to want to rev up will only make to about 4000rpm and will not go above. I can manage to hold about 10mph but it sounds like it's struggling to do so. Here and there it sounds like it wants to rev up briefly and then just dies back down to just about 4k. The engine is also drinking a ton of fuel all of the sudden, not sure if that's related or not. If I rev the engine freely it seems to climb in rpm without issue. I don't hear any stumbling, backfiring or wanting to hesitate or stall. Before this issue I was able to hit about 4600 to get on plane which put me at about 32mph and then I would drop back to between 3800-4k to cruise at about 22mph. I just thought I'd post to see if you guys might be able to give me any tips on what to check. This is my first boat so I'm kind of in the dark with some things. I have the single Merc 7.4 mpi w/ B3 dual prop. Thanks in advance for any input.

[seapuppy]

well...could be a few things....is it laid up in the water when your not there??..ie is it in the water 24/7....???with that thought..is the bottom clean??..even a little skum can cause tremendous loss of speed.
..when was the last time you had the engine tuned up.....???....check the flame arrestor for any debri......filters??..when was the last time you changed the fuel filter??....


did you spin a prop??..your going to need to pull the props and have new hubs put in....

these are some of the first things to check....if this still doesn't seem to help..then it's time to do some serious engine diagnostic's and get someone with a computer to come out and connect to the module on the boat...compression check......


SP irate:

[SaltyDogg]

Originally Posted by seapuppy

well...could be a few things....is it laid up in the water when your not there??..ie is it in the water 24/7....???with that thought..is the bottom clean??..even a little skum can cause tremendous loss of speed.
..when was the last time you had the engine tuned up.....???....check the flame arrestor for any debri......filters??..when was the last time you changed the fuel filter??....


did you spin a prop??..your going to need to pull the props and have new hubs put in....

these are some of the first things to check....if this still doesn't seem to help..then it's time to do some serious engine diagnostic's and get someone with a computer to come out and connect to the module on the boat...compression check......


SP irate:

Thanks for the fast response Seapuppy! The boat is in the water 24/7 and the bottom appears to have a little junk on it but not too bad. It was pulled from the water and cleaned less than a month ago. If I were to spin a prop would that stop the engine from revving as high as it used to? I'm not sure that there is a flame arrestor on this one as it's the MPI version. I was thinking to start checking the filters to see if something might be up with that. I'm just a little thrown by how healthy the engine seems when I rev it but performs so poorly when in gear. Again, thanks for your thoughts, I really appreciate it.

[seapuppy]

Thanks for the fast response Seapuppy! The boat is in the water 24/7 and the bottom appears to have a little junk on it but not too bad. It was pulled from the water and cleaned less than a month ago. If I were to spin a prop would that stop the engine from revving as high as it used to? I'm not sure that there is a flame arrestor on this one as it's the MPI version. I was thinking to start checking the filters to see if something might be up with that. I'm just a little thrown by how healthy the engine seems when I rev it but performs so poorly when in gear. Again, thanks for your thoughts, I really appreciate it.

well...after thinking of it some more...no..I don't think the engine would loose rpm with a spun prop...but just about any growth on the bottom will slow the boat down to a crawl really quick.....I'd clean the bottom first before spending anything else....the filters..yeah..that can do it...also with an mpi engine...the fuel pump could be the problem..but that will show an error on the fault detection program......so....first ..re clean the bottom...then check the filters...then start doing the diagnostic stuff....and yes ..you should have a flame arrestor on your mpi engine..I have twin 350 mpi's....they both have flame arrestors on....aka air filter...

SP irate:

[SaltyDogg]

Turns out the bottom and the tabs are fouled, gonna get try and get that cleaned up before the weekend comes in. Thanks so much for your thoughts!