4.3 mercruiser doesnt gain speed past 3800 rpm

Hi i am new to this, i took out my boat for the first time this weekend its a 1989 20' century 2500 sport lx with 4.3 merc alpha one outdrive 1.84 ratio and a 19 pitch prop everything runs fine i get up to 35 mph at 3800 rpm and goes to 4500 rpm with no speed change i do have a bit of prop torque the boat leans a bit to port side when accelerating it has one of those fins on the drive ( hydrofoil i believe) any input would help and ill try to give as much info as possible thank you

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

Hi. How familiar are you with trimming the boat? Have you tried trimming the drive up a little once she's on plane? Different trim settings can make a big difference to max speed.

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

Im not familiar with trimming at all but i did bring it up a bit while on plane didnt seem to do much but maybe i didnt trim enough

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

Trimming the drive starts with the drive all the way down when you go to take off. Once the boat starts coming up on plane you start bringing the trim up slowly. When fully up on plane, lest say 3800 rpm your cruising. Bring the trim up some more until you start gaining rpm but NOT gaining any more MPH. If the trim comes up to much you will start what is called porposing, where the bow starts bobbing up and down. If it starts to porposing bring the trim back down a bit until it stops. This is your optimum trim setting for the rpm you are turning. If you speed up or slow down the trim is adjusted accordingly. If you're not bringing the trim up much you will be burning more fuel and gain little speed.

There could be an issue with leaning to one side, post a pic of the outdrive and maybe something will show. It could also be because you are to heavy loaded on that side.

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

Thanks for the reply i will try that next time out and see what happens and ill try to get some pictures of the outdrive thanks again

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

Ok so i took out the boat today i trimmed out the boat and i noticed a difference on the rpm and load on the engine but didnt gain any speed would still rev up no problem but not gaining any speed from about 4000-4800 rpm always around 35mph. Sorry but i didnt get any pics of the drive i do know that the anode is not in the greatest shape, do you think i need more of a pitch on the prop currently its a 19p thank you

Re: 4.3 mercruiser doesnt gain speed past 3800 rpm

high_boost said:

Ok so i took out the boat today i trimmed out the boat and i noticed a difference on the rpm and load on the engine but didnt gain any speed would still rev up no problem but not gaining any speed from about 4000-4800 rpm always around 35mph. Sorry but i didnt get any pics of the drive i do know that the anode is not in the greatest shape, do you think i need more of a pitch on the prop currently its a 19p thank you

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Your WOT should be between 4400-4800 rpm. With being 1000 rpm's off that is a bunch to make up with a prop. Rule of thumb is 200 rpm for evey inch of prop increase/decrease. This would bring you to a 9 or 10 degree pitch prop which is not made nor should you do it. It's starting to look like the drive has been chaged and an incorrect ratio was installed. Only other issue is the drive is not setting in correct location to the boat.