96 Monterrey Boat Is Slow It Only Makes 3000 Rpm And No More Than 30 Mph

[brownyg1]

Hi i have a 96 monterrey montura v6 the motor seems fine i got a shop to do compretion check they said its fine it only reaches 30 mph and only 3000 rpm at full throttle is my first boat so im not sure where to start i tryed to adjust thrttle cable and did not work can some body please give some ideas thank you for your help

[J-Doc]

Paging Dr. Fishinwrench. :-)

[fishinwrench]

Mercruiser/Chevy 4.3 I/O (I assume ???)

Carb or throttle body injection ? (1996 could be either one).

Top RPM is only 4300-4500 so it could just be a wrong or damaged prop.

Need more info.

[brownyg1]

Hi thank you for replying yes it is a 4.3 chevy carbutator in the rpm reader it says it can reach up to 5000 or i think 6000 but im not sure if that is only the reader i dont know if this motor can reach that much rpm is also in full throttle when it reaches 3000 or a bit more thank for any help you can give me

[fishinwrench]

Are the carb throttle plates fully open when the controls are in the "wide open" position ?

Raise the outdrive and inspect the exhaust bellows. No Mouse nest/Bird nest or any other exhaust restriction?

How does the engine react to a quick throttle snap in neutral.... Does it rev up immediately or stumble/hesitate to respond ?

Can you safely do a "cylinder drop test" (remove and reattach each plug wire individually while the engine is idling) to compare the effects or loss of RPM when individually dropping each cylinder ?

That will let you know if all cylinders and banks are handling their fair share of the load, or if one or more cylinders are "being lazy'. (Do not attempt this if the engine compartment is cramped, or if you aren't confident in your mechanical ability....it's easy to get hurt....Bad !)

If all the above checks do not unearth the issue let me know and we'll take it a step deeper.

[mjk86]

Same thing happened to my boat a few years back.....when I took the cover off and ran at full throttle I noticed only 2 out of the 3 carb throttle plates were opening. The other was stuck....after a little cleanout it ran good as new.

[brownyg1]

Thank you both for the reply i will check the carb plates as soon as i get home as far as the cylinder check i can do that also i think ill be ok trying that but just to be sure and sorry if this a dumb question but whe u check the carb plates you do this with the engine off or on ? Thank you for the answer

[brownyg1]

As far as the reaction of the motor in a fast snap at the throttle i have not try this but i will and i will let you know i was using it all this week end and try it slowly but i will check on that and also how many rev are normal in a full tthrottle even if the rev reader can go up to 5000

[fishinwrench]

Thank you both for the reply i will check the carb plates as soon as i get home as far as the cylinder check i can do that also i think ill be ok trying that but just to be sure and sorry if this a dumb question but whe u check the carb plates you do this with the engine off or on ? Thank you for the answer

Engine OFF.

[fishinwrench]

As far as the reaction of the motor in a fast snap at the throttle i have not try this but i will and i will let you know i was using it all this week end and try it slowly but i will check on that and also how many rev are normal in a full tthrottle even if the rev reader can go up to 5000

4500-4600 is typical top end RPM for stock I/O's (Inboard/outboard).