4.3 mercruiser rpms

[pacman78]

i been trying different props on my boat i installed a 17p today the boat hit 4300 rpm and i heard a beep and then the rpm went down and as it hit 4300 again the beep and knocked the rpm back down the boat went 37 mph at 4300 and would not go over that because of the beep looked like a warming came on my screen but goes away to fast. i tried a 19p i got the boat to 40 mph 4200 rpm full throttle i trim up a bit more and the beep hit again fast knocking my rpm down again. i am trying to get some speed out if it but not sure if something else is wrong. i read the 4.3 wot is 4400 to 4800 and ideas

 

[Bondo]

Ayuh,..... Welcome Aboard,.... Dependin' on how yer 4.3l is set up, I'm guessin' yer tach is way off, 'n yer hittin' the rev-limiter at 5 grand or so,....

 

[pacman78]

but if i am hitting the rev limiter and my boat is only doing 37 mph with a 17p and 39 to 40 top with a 19p that don't sound right either. the boat should hit top speed at least 45 mph some days i am lucky if i am doing 34 mph at full throttle

 

[Scott Danforth]

Verify the tach with a known diagnostic tach.

 

[CV16]

Also, don't trust the boat speedo, use a gps.

 

[achris]

Dash tachos are notoriously inaccurate. The only accurate tacho is a proper shop calibrated tacho, or one in an advanced timing light.

What (exactly) is the engine? 4.3 was used from 1985, and was seen 5 major changes, and dozens of minor ones. Best if you post the engine serial number. That way we can tell if it is hitting a rev limiter (which will be WAY more accurate than the dash taco)...

Chris......

 

[Scott06]

What boat is this in? Depending on exact engine (2bbl?) those speeds at various prop pitches are quite possible depending on the set up.

 

[pacman78]

the 4.3 is fuel injected the serial number is 0W065522 the boat is a 05 searay 200 sundeck with 260 hours i am also curious what should the boat top out at from the research i been doing on theses boat and motors 46 to 48 mph should top out i can barely hit 40 mph and i am using the gps also

 

[achris]

MPI in roughly the same size boat as mine. I make 42.3 knots with a 19" prop. I normally run a 17" as it makes the boat drive a lot better, but I do sacrifice top speed, it drops to just on 40 knots. But where I boat you get days you can do 40 knots about twice a year. I have a 1.62:1 drive ratio. Do you know your drive ratio?

Based on the information in your first post it sounds like you need to get an accurate tacho on the engine (or the Diacom software) and a 21" prop.

The ECM555 you have has tacho simulator output. Hook up Diacom software and you can simulate a tacho output. You can check the accuracy of your tacho. I've done mine and it's spot on to about 4000 after that it reads high.

 

[pacman78]

i did try a 21p my rpm is lower but don't get good speed out of it either i think is was 36 mph at 3600 rpm full throttle

 

[achris]

Sounds like you need to get the Diacom software on the engine and check for codes. Plenty of things on these engine will limit it to 90% power. And you'll get beeps when you try to exceed that.