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Not turning enough rpmbs

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baba_bluey

Regular Contributor
"I have a 22'2 aquasport c

"I have a 22'2 aquasport cuddy, with a 470 mercruiser motor. Boat turns 4200rpm's, but is only doing 20mphs. I have a 15" pitch prop on it right now, but boat did about the same with a 19" prop.(only turned 3200-3400rpms)and 21mph. I keep reading that this size boat should do well over 30mph. Any ideas on what could be restricting it?"
 
If you have already swapped pr

If you have already swapped props I dought it would be a hub slipping so sounds like a spun coupler check the back of the motor and look for fragments of rubber or the smell of burnt rubber.
 
"Thats alot boat for that engi

"Thats alot boat for that engine. Check the ratio of the drive. Should be 1.84 I believe. How is the compression on the engine? Is the floor and foam underneath holding water? Other than mabey a fouled up hull, you may need to repower to a V8 to get the speed you want.
Chris"
 
"Bill, i checked behind the mo

"Bill, i checked behind the motor and don't smell or see anything. Chris how can i tell what ratio i got? Can i tell by the serial# on the outdrive itself?"
 
If it was a spun coupler it wo

If it was a spun coupler it would let go and not move thereafter. A spun coupler would break free like a slipping clutch and his rev's would go way high. Sounds like a prop ratio issue. You should be able to turn 4800 and perhaps get a little more speed.... but thats a big for a 470. Be nice to that motor... not the most durable plant.
 
"Ayuh,....

Or it could be j

"Ayuh,....

Or it could be just plain Down on power...

A Compression,+ Leak-down tests will give you an idea on the motor's overall Health...."
 
Thanks for the info guys. Gonn

Thanks for the info guys. Gonna check compression and do a leakdown. Also found a guy with a 4bbl. intake. Gotta be worth a shot.
 
"I got to ask????? What would

"I got to ask????? What would you expect out of a 4 cylinder motor ~ 140-160 hp, in a 22 foot boat??????????????????????????????????????????

GEEEEEZ Get a big block in there and be done with it and have fun.

Why would anyone under power a boat so severly???

Try putting that same motor in an old coupe deville from the '70's and see how fast you go.............DUH!"
 
"I hear ya Kghost. Boat came w

"I hear ya Kghost. Boat came with that motor. I have a v-8, but the weight of the motor is what is holding me up from dropping it in."
 
"Kghost, the 470 produces the

"Kghost, the 470 produces the same power as a V6, about 170-190hp. Its a torqey 4 banger with big pistons from a FORD 460! It does best in a boat 18-20' Ed, your boat needs that V8! Your boat will absorb the weight NO problem.
If you have a v8, do not spend one more dime on the 470. Sell the 470 on Craigs list and use the money to finance the swap! The outdrive ratio may be an issue- The number should be stamped on the drive, on the side when the Mercruiser sticker is.
Chris"
 
"Thanks for the info Chris. It

"Thanks for the info Chris. It's gonna be hard to pull the 470 out. Alot of blood, sweat and tears with that motor.(mainly blood) I will say, when these motors are running right, they are a good little motor."
 
"I hate to sound stupid, but,

"I hate to sound stupid, but, do you have the drive in the in position (all the way in/ down) before you throttle the 470 up. If the drive is not all the way down you will not bite water and the engine will sound as if you spun the hub on the prop.You have the correct size prop if you are turning 4200 rpm. Try a 15x17P or 15x19P prop, and keep the drive in the in position on the take off, then trim it out. That 470 is a strong engine and gives great mpg."
 
"Joe, i always put the outdriv

"Joe, i always put the outdrive all the way down until i plane off. Then i lift it up alittle until it porposes."
 
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