Mercruiser 120 low top speed

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ryanr623

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I have a 1982 imperial 18 foot boat, that according to iboats, weighs 1200 lbs. Top speed is 24mph (GPS), at WOT, which is 3900 rpm. I figured I was over propped (19p), but i doubt that a 17p and 300rpm would make much of a difference.

Im wondering if the imperial hull design is horrible. My buddy has a newer merc 120 boat, and his weighs 2000lbs and his top speed is 35mph.

Anyone else with a merc 120 with low speed numbers? Im hoping to rule out engine problems, as i am selling come spring.
 
Bondo

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Re: Mercruiser 120 low top speed

Im hoping to rule out engine problems,
Click to expand...

Ayuh,.... A complete tune up, 'n compression test should sort that out,....

Otherwise, this is a Boat question, 'n Not a Motor, I/O question....
 
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ryanr623

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Re: Mercruiser 120 low top speed

Bond-o said:
Ayuh,.... A complete tune up, 'n compression test should sort that out,....

Otherwise, this is a Boat question, 'n Not a Motor, I/O question....
Click to expand...

Suppose I should do that before i sell anyways, thanks bud! Doesn't that seem awful low to you?
 
Bob_VT

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Re: Mercruiser 120 low top speed

Try a 15 x15 Prop.
 
egleas84

egleas84

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Re: Mercruiser 120 low top speed

Bond-o said:
Ayuh,.... A complete tune up, 'n compression test should sort that out,....

Otherwise, this is a Boat question, 'n Not a Motor, I/O question....
Click to expand...

If the OP's compression is shot what can be done? New piston rings, valves? I just got the same motor. I had a compression test and it checked out; just wondering for future reference.
 
Bondo

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Re: Mercruiser 120 low top speed

egleas84 said:
If the OP's compression is shot what can be done? New piston rings, valves? I just got the same motor. I had a compression test and it checked out; just wondering for future reference.
Click to expand...

Ayuh,... At that point, the motor has to be Rebuilt....
 
egleas84

egleas84

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Re: Mercruiser 120 low top speed

Bond-o said:
Ayuh,... At that point, the motor has to be Rebuilt....
Click to expand...

is low top end the only definitive symptom of bad comp? I hope for the OP's sake that 15x15 prop works out :)
 
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ryanr623

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Re: Mercruiser 120 low top speed

egleas84 said:
is low top end the only definitive symptom of bad comp? I hope for the OP's sake that 15x15 prop works out :)
Click to expand...

The motor was rebuilt 3 years ago, i doubt it's that. Hard starting is another, dying at idle, poor running.... all signs of low compression, and mine starts immediately and runs great.
 
egleas84

egleas84

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Re: Mercruiser 120 low top speed

ryanr623 said:
The motor was rebuilt 3 years ago, i doubt it's that. Hard starting is another, dying at idle, poor running.... all signs of low compression, and mine starts immediately and runs great.
Click to expand...

Guess I don't need to worry about mine either. I had an Imperial tri hull 16'er that had a messed up hull (floor was rotted to heck and all the wood weight went to one side because i never dug it out...) and that thing still did 32 w/ a 85hp OB. Gotta be pitch then...

How much did that motor rebuild cost you? DIY?
 
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ryanr623

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Re: Mercruiser 120 low top speed

egleas84 said:
Guess I don't need to worry about mine either. I had an Imperial tri hull 16'er that had a messed up hull (floor was rotted to heck and all the wood weight went to one side because i never dug it out...) and that thing still did 32 w/ a 85hp OB. Gotta be pitch then...

How much did that motor rebuild cost you? DIY?
Click to expand...

The guy i bought it from last year had a shop do it. He showed me the receipt but i honestly cant remember what the cost was.
 
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steelespike

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Re: Mercruiser 120 low top speed

Low compression also will produce slower hole shot. Both rings and valves contribute to low compression.Bad valves will usually produce a rough idle.Other things causing poor pewrformance:
Throttle not opening all the way,poor tuneup,timing not advancing as it should.Poor hull condition,
waterlogged flotation foam.Typically a boat like that will produce speeds 35-40 mph and 4400 to 4800 rpm.
 
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