Rebuilt Alpha Leg no power 1973 165

Firstly Iam a newbie
I have a 1973 28'fiberform boat with two 250 merc inline 6 cyl. 165 hp motors.Alpha one legs
I recently had both legs worked on because of water in the oil. After the boat was put back in the water one side works fine. will run the boat at 13 knots @4000 rpm by its self however the other motor/leg will rev to 4000 but will only push the boat maybe 3 knots. The mechanic says maybe the props not big enough or the exhaust may be plugged on that side. Both legs have the same props. This boat is new to me and as far as the survey says she should go 28 knots top speed and 20 cruising.
Is it possible the exhaust is the problem?
Identical motors and legs and one side barely moves the boat.
Any Ideas?

"I have the same engines on an

"I have the same engines on an older drive. the second engine should perform the same as the first. sounds like the working one is propped right. with both running WOT or full throttle at the same time, you should have 3800-4200 rpms on each .

Sounds like either the Engine Coupler ( where the drive shaft of the upper part of the outdrive goes into the back of the engine ) is bad. or the prop on the slow one has a spun hub. ( easy test if both are RH rotation(looking at the prop), just switch the props.

However at the moment I dont remember if the Alpha 1 has counter-rotation, so make sure both props are identical before switching them.

ps.. if you're doing 4000 rpms on the 3knot engine, the exhaust isn't plugged.. you wouldn't GET 4000rpms..

First.. check that both props are the same. and NO damage to either (especially the 3knot one)

If same, switch them and retest if the problem changes engines.. proplem found !!! If it doens't, I'd check the coupler, removing the drive, make sure the engine is aligned ( and the front engine mount is OK..) I have would under mine and it was sagging (35 yrs old)"

I'll try switching the pro

I'll try switching the props
I'll let you know what happens.

"run a line with nail polish o

"run a line with nail polish or other marker thru the prop hub and if the line doesn't match up with a quick run on the water then you know if it's the prop, if it does line up look to the coupler next.

I won't ask if you ever did a compression test on the motor, cause that could be a reason also.

He didn't put in the wrong gears did he?"

well.. both engines do 4000 rp

well.. both engines do 4000 rpms so I wouldn't suspect any engine trouble.. just one only does 3 knots.. the other 13..

when I was using a 1.98r 4cyl ratio on my inline 6 ( normally uses 1.65r for my year ) I used a 23p instead of a 19p prop which worked out about the same spd-rpm.

one side works fine. will run the boat at 13 knots @4000 rpm by its self however the other motor/leg will rev to 4000 but will only push the boat maybe 3 knots.

Compression is good.
You migh

Compression is good.
You might be right about the mechanic putting the wrong gears in he had parts from three legs on his bench at the time of repair.If he did put the wrong gears in will a different prop fix this problem. How do I find out the gear ratios.

"[b]"If he did put the wro

""If he did put the wrong gears in will a different prop fix this problem."

Nope...

Diagnosis is a methodical process...
Change the props around,+ Try it 1st..."

it's different when you ha

it's different when you have 4 cylinder gears in a drive that has a inline six then running gears for a inline 6 and have gears for a 5.7 or 5.0l motor.

Line thru prop or change props is you have both turning engines going the same way.

Then you post the result if it doesn't fix it then i can post a link on how to figure out what gears you have.