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Mcm 120 power loss

Z

zootschief

Member
I have a recently aquired MCM120.Boat starts and runs very good.Problem is when opened up on the water It comes close to Plane but RPM only gets up to 3000 and boat is not getting the speed it should have.I spoke with prop guy and said prop is the right prop(after he did the numbers)

Here is the question where can I start IE;Timing,Carb,points,Distributor,wires.

Outdrive is a 98:1 mercruiser /120 h.p.
 
you mean 1:98 ?

Compression test first.

What size boat?

Dirty bottom ?

Water in the hull? Wet stringers ?

If everything is good with the timing, not backfiring, plugs good . Full tune up done when ?

Then the prop size is what ?
 
if very low compression won't have the mojo to move the boat.

First compression test ! And you can read the plugs. Do the test on a warm motor throttle fully down and ALL the plugs out. Keep track of what plug goes in what hole.
Spark Plugs
 
wow.... yeah, well it should plane under the right conditions... but not with a whole load of people, and not with a waterlogged hull...and not if it's not running absolutely perfect

I had a 21.5 footer with a 3.0/140 and it had to be tuned right up and running great to plane with a load of family and gear....
Once on plane I could get WOT and 30 mph.
34 mph with just me.

underpowered boats are weight sensitive for sure
 
You had 20 more hp. I had a 120 in my 19' boat, now with the 140 it's like a different boat.

I have to say it again 120hp is way too little power for a 21' boat.
 
yes you can drop in a 3.0 / 140 hp motor and the drive stays the same. It will be a direct change out, also is you can lay your hands on a inline 6 the front and rear motor mounts will stay the same you would need a 6 extra inchs in front of the motor to fit the extra length. In your case i wouls think the inline six is the best bet.

The easiest 3.0 / 140 hp any motor before 90'. Doesnt matter the make merc, omc, volvo all the same chevy block.
 
chiefalen said:
yes you can drop in a 3.0 / 140 hp motor and the drive stays the same. It will be a direct change out, also is you can lay your hands on a inline 6 the front and rear motor mounts will stay the same you would need a 6 extra inchs in front of the motor to fit the extra length. In your case i wouls think the inline six is the best bet.

The easiest 3.0 / 140 hp any motor before 90'. Doesnt matter the make merc, omc, volvo all the same chevy block.
Click to expand...

Any idea,s on a good aftermarket engine distributer/And also does the gear ratio change??
 
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