5.0 litre alpha one mercruiiser prop question

[Stiknfish]

Goodday all, I have a 2452 Bayliner classic .I just had the motor rebuilt .I had the boat out on the weekend and could not get the boat up on plane .MaxRPM was 3500 and 11 knots with a 4 blade 15 pitch stainless steel prop. I tried a 3 blade 15 and had pretty much the same results . i also tried a 3 blade 17 but the RPMs were even less with that ,about 3350 .Any suggesstions on which way to go with this . Also the engine revs fine with no load on it I just cannot get the rpms up while in gear .
thank you
Stiknfish

 

[Maclin]

If this was the same engine as before with just a fresh rebuild, and this is the same prop and drive and it planed out before, then I would suspect the engine is not quite in tune yet. It could be valve lash, or ignition timing, things like that. Valve lash if too tight will in effect cause a loss of compression at 3500 and above and it seems limited to that rpm. Same thing with engine timing if it is not set to proper advance. Either of these conditions will allow the engine to rev freely with no load. Revving freely in neutral only shows basically that the engine runs... any engine can hit top rpm's with 2 sparkplug wires off in neutral .

 

[steelespike]

I agree with Maclin something is up with the motor.Being a fresh rebuild Might be as simple as throttle not opening all the way.
Timing not set right or advancing as it should. Plug wires crossed, one or more plug wires off. More to comre.

 

[Stiknfish]

Thank you for the replies . What you are saying makes sense . I will take it back to the mechanic ,