Prop advice 1996 21ft marada

[Rufdod]

Like title says, I have mercruiser 5.7l with alpha one, and it's open now. Mx three edition

So at wot with 2 adults including myself 1/2 tank of fuel , fairly calm water and negligible amount of gear, my best wot is 3500 rpm @ 39-41 mph with a 14x21 prop

motor runs strong and I've replaced a lot of things this season, plugs, cap and rotor, rebuikt carb, new oil, coil pack, alternator , and battery , based on all of this and what the local boat shop said, I should be seeing more Rpms and speed

The shop gave me a 15x17 prop to try , not it seems like I'm almost over revving and I'm not getting the same speed as before , not I'm somewhere around 5000 rpm @ 35 mph but I come out of the hole like crazy

Any recommendations on what to try ? Or with my original prop , is that the best in gonna get? Which I won't be terribly disappointed with..

 

[Maclin]

The engine should be able to turn the 21p to the 4400-4800 rpm range in my estimation.

Has it always behaved this way?

How did it come out of the hole with the 21p?

A 17p did about what I would have expected, way over-revving and stupid-good hole shot at expense of top speed. And it is not right for the boat. A 19p would be interesting to try, but I really think your engine is in some state of tune that is not allowing it to rev on up past the 3500 rpm with load.

If it comes out of the hole good with the 21p then that is in indication that it should be able to rev on up to top of recommended RPM range. If a 21p was really too much to spin then climbing out of the hole would be really slow.

 

[Rufdod]

It comes out of the hole.. Maybe slightly sluggish, but this is really the only boat I've spent much time on and the first one I've owned so I don't have much to compare other than the 17. Do you think it's worth trying a 19 or do you think there's some sort of fuel delivery issue? Really the only things I have left to check and adjust or adjust are fuel pump, timing, and compression... Bt since I am able to rev all the way with the 17 I wasn't sure if those would really be the issue..Nita still under load of some sort just not as much I guess

 

[jimmbo]

Is the timing set correctly, and is it advancing correctly as the engine speeds up? Is the throttle linkage opening the carb fully? Are the secondaries opening?

 

[Rufdod]

I will have to get back on that, I assume the carb is opening fully but not sure how to check? I just tried a 14.25x19 and got good out of the hole, and Rpms are around 4200 at wot, but still no better than 40mph

 

[Maclin]

Do not go back to the 17p. Leave the 19 on and work through some diagnostics on the state of tune on the engine. Hopefully you will find something(s) and get it(them) fixed and then "have" to put the 21 back on

I suspect there is not just one thing out of tolerance.

 

[Rufdod]

I'll give it a shot .. My only other question would be... The new props I have are significantly lighter than the original 21 I had, would a lighter 21 be the way to go or would they not be any lighter..