1977 Mercury 1500 lower unit swap

[delta07d]

Ok have it running good except at WOT only get 4600, last year was getting 5800. Only difference is the lower unit is from a 1972 1150, which may have a different gear ratio (maybe)??? Pretty sure lower unit I was running last year had a 2.1 and I think this one may have 1.78. Would that be the cause, or not? Running the same 17P prop.

If the above is true

so what options do I have, different prop pitch, have the gears swapped? The lower unit that was on when I bought it was a short shaft with a 5 inch extension kit, which was getting to be a pain to reinstall so I found the one from the '72 1150, thought the gear ratio was the same...

Thanks​
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[Chris1956]

The gear ratios from both those lower units should be 2::1. It is easy to check. More likely you have lost a cylinder since last year. Check for good compression, water intrusion and spark plug wire arcing.

 

[mr 88]

^ or throw the original lower unit back on and see what happens

 

[Faztbullet]

The 71-72 1115hp was 1:78 ratios as was some of the 1135's. You got the wrong ratio..

 

[delta07d]

Thanks guys, I have a lower unit from a 1500 that I am going to use, verified that it is 2:1 ratio, hope it makes a difference.

 

[delta07d]

Hmm, well it seems I have other issue, when I removed the lower unit I saw a milky white substance in the prop exhaust hub, and in the exhaust area of the lower unit. Pulled the plugs and mainly 5 and 6 have the same milky white substance. So either the seals in the end cap or a water leak in the exhaust baffle area?. Going to pull the powerhead this weekend and remove the manifold cover, what should I look for?

 

[Chris1956]

The manifold cover is not your problem. Just loosen some of the crankcase bolts to allow the end cap to be removed. Replace the seals in that. Since the likelihood of removing the exhaust cover and inner exhaust cover without breaking some of the bolts is low, I would retest the motor after replacing the lower crank seals. If she still leaks water, you will need to try to remove the exhaust covers and replace the gaskets and perhaps the exhaust baffle. Hopefully, it will not come to that.

 

[delta07d]

Do I need just these seals
SEAL | 26-41953

Should I get the "O" ring just in case...


Thanks for your time.

 

[delta07d]

Decided to take a look under the exhaust cover, this is what I found: