Mercruiser 3.7 MCM 165 not getting to operating RPM

[Ray1888]

I recently purchased a 1987 Four Winns 190 Horizon with a Mercruise 3.7 MCM 165 ( 4 Cylinder) and an Alpha One stern drive. While taking the boat out for the second time this season we were able to cruise around 4500-4800 RPM. However, while bringing the boat down from speed something in the stern drive seized and tore up the gears in the upper and caused the engine to stall. I have replaced the stern drive with a remanufactured one and put a new OEM quicksilver black diamond prop on and now the boat is sitting around 3700-4200 RPM and seems sluggish. We recently replaced the spark plugs, and checked the spark arrestor which were good. We have new spark plug wires and distributor cap on order and will replace when they come in. The oil level is good. Is there anything else we could check or would the new stern drive and prop just cause more resistance on the motor so we wont see the same RPM anymore?

 

[Alumarine]

Welcome.
Is the the new drive the same gear ratio as the old drive?
Is the new prop the same spec's as the old one?
The gear ratio should be 1.84 approximately.

A change in either will impact rev's.

 

[Ray1888]

Welcome.
Is the the new drive the same gear ratio as the old drive?
Is the new prop the same spec's as the old one?
The gear ratio should be 1.84 approximately.

A change in either will impact rev's.

Thanks for the reply. I should have mentioned that yes the ratio is the same. we took off a 1.84 ratio, 17/19 tooth gears and put on a 1.84. The boat is currently in the water so we haven't 100% confirmed the ratio yet but the person we bought from is very reputable and swears that its a 1.84 gear set. The prop is the same pitch, would need to double check about size though.

 

[Scott Danforth]

start with the basics

check your compression
check your engine timing and dwell.

 

[nola mike]

would the new stern drive and prop just cause more resistance on the motor so we wont see the same RPM anymore?

Only if the drive ratio/prop pitch is different. Could have some fluctuation in RPM with a different prop of the same pitch, but not 800 rpm.

while bringing the boat down from speed something in the stern drive seized and tore up the gears in the upper and caused the engine to stall.

Could have broken something in the engine if the drive seized suddenly. Check compression like @Scott Danforth said.

 

[Ray1888]

Only if the drive ratio/prop pitch is different. Could have some fluctuation in RPM with a different prop of the same pitch, but not 800 rpm.

Could have broken something in the engine if the drive seized suddenly. Check compression like @Scott Danforth said.

ok. What reading am I looking for when checking the compression?

 

[Scott Danforth]

ok. What reading am I looking for when checking the compression?

150 psi is a new motor, so 130-150 psi on all 4 cylinders. if you have a dead cylinder, it will have low compression