Motor Rpm low

[fishchaser]

Heres my question looking for advice
I have a 250 ccp with twin 150 hp counter rotating yamaha outboards they have 17 pitch stainless props on them
at wot i only getting 4500 rpm i know i can gain some rpm if i change the pitch in the props but i dont think i can gain enough ?
do you think its a fuel issue?
it cruises very nice at like 30knts but when i open it up doesnt get to the rpms i need looking for some help guys ?
its not that i want to go faster but im afraid it will damage the motors if the low rpms continue

[Capt Matt]

Did the boat ever turn higher rpm or is this a new rig
Capt Matt

[Blue Agave]

Did the boat ever turn higher rpm or is this a new rig
Capt Matt

X2

Also are the motors mounted too low?  Have you checked the throttle linkage at the motor to see is the motors are wide open when the throttle is pegged on the helm?

[fishchaser]

it is a older aquasport that i rebuilt i never pushed it because of the transom (shakey at best)that i put a new transom on in the spring
the motors are at the height that was suggested bye proffesionals and sit up on the plate very nicely when running

i didnt check the linkage since i took the motors off that is a great idea sometimes when you have problems you think worst case first and miss checking the obvious ! cant wait to check it

[Blue Agave]

the motors are at the height that was suggested bye proffesionals and sit up on the plate very nicely when running

Are the cavitation plates above or below the surface when you are on plane?  They should be riding on top of the surface.  Hope things work out for you.  

[CLM65]

What year and model motors?  2 or 4 stroke?  Fuel injected or carbed?  There are many variables that could affect top end rpms, including height of the cavitation plate as mentioned above.  Depending on which motor you have, you probably should be in the mid 5k area.  I'm dealing with a loss of wot rpm on my 2002 Yamaha OX66.  I did alot of maintenance on mine and gained back a few hundred rpm, but I'm stil only getting about 5000 rpm.  I would suggest going through the maintenance before looking at a prop change - better to fix any problems rather than compensate for them.

[jaycal3]

I am willing to bet your motors are mounted too low, your cav plates are submerged creating drag (times 2) and you may also be underpropped...  At WOT your cav plates should be at or very slightly above the surface of the water... if this is not the case I would start there.

[fishchaser]

the motors are 1986 150hp 2 stroke carb motors the cavatation plates do run on top of the water at full throttle i have 17 pitch props

[CLM65]

It sounds like both motors are only hitting 4500 rpms?  It's been a while since I've had a carbed engine, so take my advice with a grain of salt...  How is your fuel line configured?  Is it a single line from the tank and a tee where it splits to each engine?  If so, do you have a fuel filter in the common line?  Have you changed that filter recently?  My theory is that a restriction in the fuel supply may allow enough fuel for operation at lower speeds, but  may not allow enough fuel to achieve the expected rpm at wot.  One way to rule out a fuel supply issue is to hook up a portable fuel tank to one of the engines and see if performance changes.