5.7 mercruiser low top speed not reaching optimum rpm

[thagan03]

I have a 1996 Mariah shabah 202 5.7 mercruiser, I bought this boat last year, my top speed is only around 33-35 mph, after talking to some people that seems very slow, I had a 19p prop on it and turned 3400 rpm with it, I put a 13p prop on and am only turning 3800 rpm, the previous owner put a se116 lower unit on it, I'm wondering what you all thoughts are on if the new lower has the wrong gears and is causing me to not tuen the rpm needed, from information I've gathered the 19p should have been the right size prop and I should be seeing low 50mph speeds, fresh tune up and new water separator, also just rebuilt the carb, what is you all thoughts on this, thanks in advanced

 

[thumpar]

Can't be the wrong ratio or you would be over revving since you need the tallest ratio at 1.5.

 

[Whateft]

Check shift linkage and shifter throw - something may have slipped so throttle not fully engaged, resulting in WOT is not actually wide open...

 

[thagan03]

Linkage is good

 

[Profittakr]

I have very same problem. I will be watching this post for resolution.
my thread: https://forums.iboats.com/forum/engi...rbolt-ignition

i assume (like mine) that the engine runs fine at the lower rpm range, just will not get past 3800rpm. Meaning, it is not sputtering, coughing, surging or otherwise running poorly; just won't go any faster.

 

[thumpar]

Check shift linkage and shifter throw - something may have slipped so throttle not fully engaged, resulting in WOT is not actually wide open...

With an Alpha it is either in gear or not. The clutch dogs hold it together. They only way for it to kind of slip would be a heck of a noise because the clutch dogs would be grinding against each other. If it was a Bravo the clutch could slip but a Bravo behind a 5.7l is rare.

 

[Scott Danforth]

see link 15. https://forums.iboats.com/forum/eng...ion-for-diyers-an-work-safe-search-version-of

your motor is not reaching full power.

 

[thagan03]

Not spitting or sputtering it runs great just won' turn full rpm

 

[thumpar]

You will just have to do diagnostics. Look at the list that Scott pointed you too. Some will be easy to rule out. Work on ruling out the others through testing until you pin point the problem.

 

[Scott Danforth]

I had posted the same link in your other thread you had, however it disappeared.

the first two go-to culprits are dropped flappers and clogged anti-siphon valve.

 

[KLFrank]

Check the adjustment on the secondaries. If out of adjustment, or the diaphragm is worn/torn then you might not reach above 3500rpm as the secondaries might not be opening. Just a thought

 

[QBhoy]

Must be an engine related problem. As mentioned the highest geared drive is a 1.47/1.5.
It likelt should be a 1.5 and a 19” would be about eight for that set up. It probably won’t ever see into the 50’s in gps, but should come close to or touch 50 trimmed up and light.
Id be looking at fuel system and carb first. Water in fuel or bad fuel etc. Check the fuel filter. If there has been signs of water then you will likely need to get the carb off and cleaned as well as getting rid of all he old fuel.

 

[Whateft]

With an Alpha it is either in gear or not. The clutch dogs hold it together. They only way for it to kind of slip would be a heck of a noise because the clutch dogs would be grinding against each other. If it was a Bravo the clutch could slip but a Bravo behind a 5.7l is rare.

I hear ya. I was talking about the throttle cable, not the shifter cable. If the cable going from shifter controls to throttle was replaced and not properly set, the drive would engage but conceivaby the shifter could bottom out, and still not fully "open up" the throtle. Granted, hard to do on a gen 2, but not impossible.