1965 100 hp Merc lower unit convert to jet drive questions

kuts

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I have a 1965 6cyl 100 hp Mercury outboard. I also have a 1970 6cyl 115 hp ?Thunderbolt? Mercury outboard.

Q 1) Can I swap the lower units on these two motors with no extra parts?

Each of these works but the question is more related to wanting to know more about lower unit compatibility.

Q 2 and 2a) Where can I find info telling me what year and horsepower lower units can be swapped with other years that have the same horsepower? Or perhaps lower unit swapping that is not the same horsepower?

I have been looking for a jet drive for these and apparently Specialty Manufacturing Co. lists jet drives for just about everything made after 1984 or so. I cannot find information on Mercury jet drive for pre-1980s.

Q 3) Is there a bolt on jet drive that will fit either of my 1965 and/or 1970 6 cylinders?

Thanks
 

Texasmark

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

From my repair manual for later model, 3 cylinder 75-90 and 4 cyl 100-125 engines:

Selecting a boat:

1. Should be as light as possible.
2. Should have a hull and transom that is designed for use with one.
3. Boat should be at least 13'.
4. Add approx 7" of transom height from regular 20" transom for outboard jets. The 20" is based upon your engines current mid section being the "long shaft". If a short shaft just subtract 5" from the numbers above.
5. for v bottom boats, the water intake is to be such that the outer edges of the intake plate are even with the bottom of the boat via a straight edge "line" from the hull. If too high cavitation/ventilation will be a problem, if too low too much drag. On flat bottom hulls, the leading edge of the water intake is right at the height of the bottom via straight edge measurement with engine vertical.
6. When running, engine is either vertical to boat or tucked in slightly, no trimming out beyond vertical. Cavitation/ventilation reduction is your optimum setting although is sharp turns some may occur normally. Book says to minimize it if possible or just don't execute the sharp turn.

The 4 cyl engines mentioned above are rated 80 hp with a jet drive!!!!!

I would assume that the jet would require a pretty flat hull or one with a slight dead rise at the transom. I saw a picture of one in a Merc sales brochure and it was a conventional alum boat similar to the Starcraft fishing series alum boats.

No mention of engine changes are made from one to the other LU except for the shift linkage obviously.

HTH,
Mark
 

kuts

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

Mark,

Thanks for helping me along on this. My mom gave me the motors now that my dad has passed and she said 6 cyl. I was wondering if they were actually 4 cyl.

The boat I have is a 1969 Glastron Aqualift V156 (15ft tri-hull that is almost a V at the transom). I used to get on the water a lot in the 70s and 80s but it has been a long time for me so I am new-old man to this stuff. I had a Jet Ski for 20 years and that is all I know about jet pumps. But the pump is on the hull of the jet ski. Much different set up. What you say about position of the pump sounds like good info. It does not sound like a jet drive will work very good with V156 Aqualift.

My plan is to run the Glastron with the 100hp motor and buy a flat bottom john boat for the 115hp and put the jet drive unit on the john boat if I can find a jet drive for the old 115hp motor. 80 hp is plenty for fishing around.

So, 4 cyl lower units all bolt up to each other? Should I look for a jet drive for 4cly. from 1984? (that as far back as the jet drive info is listed) I think this is series R from Specialty Manufacturing Co.

Thanks for your help. I have no clue about these motors.
 

matt167

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

4 and 6cyl lower units are the same, at least into the 70's. I think, you could use the lower unit for a Classic 50hp 4cyl. They made the 44CI 4cyl ( Merc 500/ Classic 50 ) into the late 80's. It would most likely be listed as a 45hp " Classic 50 ".. Best checking with the jet pump manufacture. They will have the answer.
 

Texasmark

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

4 and 6cyl lower units are the same, at least into the 70's. I think, you could use the lower unit for a Classic 50hp 4cyl. They made the 44CI 4cyl ( Merc 500/ Classic 50 ) into the late 80's. It would most likely be listed as a 45hp " Classic 50 ".. Best checking with the jet pump manufacture. They will have the answer.

As I recall, having had a 4 cyl 85 hp of the late '70's/early 80's vintage, and a 88 tower of power 115 6 cyl, yes these two LU's are the same size swinging a 13"+ prop....gear ratio's are probably different...like 2.33 for the 85 and 2 for the 115. The smaller engines, like the classic 4 cyl cross flow which originated as the 40 hp Merc. Mark 55, used a smaller gearcase and mid section and swung a 10" dia prop.

When I talk about the 4 cyl, in the '89 production year, Merc re-engineered their engines of that hp range and dropped the smaller engines (in the 75-125 hp range) from 4 to 3 cyl and the larger 6 cyl towers down to 4 cyls. The gearbox size was the same dia, but the configuration was much different...get the two side by side and you can see what I mean. I have/have had both.

I like your idea for 2 separate boats. I think that will work well for you. But, as the manual says, get a boat that is setup for a jet drive....flat bottom preferred and transom height at the clamp bracket 7" higher than you would have for a regular prop driven engine. On converting a jet pump from a Sea Doo or something of the sort, I have no idea as to what that would involve.

Mark
 

kuts

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

Thanks everyone! I love reading the information and I am glad many are willing to share their time and experience. I have a lot to learn and I bet with these older motors I will be needing more information. The 115 hp won't start. Mom said it runs great but is very hard to start. I cannot figure out how to get the motor cover off to look at the carbs. I know carborator is another topic. My guess is the carbs need to be cleaned out. I will search the forum for more information on this and maybe start a new thread if I cannot get to the carbs.
 

Texasmark

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

Thanks everyone! I cannot figure out how to get the motor cover off to look at the carbs.

At the top front of the escutcheon (the pretty cover at the front of the powerhead with the name, brand badge, and hp on it) is a knob that you turn 90 degrees and pull forward. Once the top is away from the engine lift and it will come off.

Beneath are 3 strap locks that hold the cowl in place. The come off by pulling up on the U shaped part of the latch....the other end has the hook in it. If properly adjusted, it will take some effort to get the lever to move toward you and break loose. Then just take the cowl off.

If you want the top off, look up under it and there are 4 studs with nuts holding it onto the engine block. I don't remember on the choke lever. Either you can turn the top flared "handle" ccw and unscrew it, or you have to unhook it from the carbs. I think the handle unscrews. Hold onto the shaft while you remove it.

That's it.

Mark
 

kuts

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

Thanks Mark!

I pulled the cover on the 100 hp. It is six cylinder. I have a stand for this motor. Theother one is mounted on the boat. On the 100 hp I think two of the top cover mounts are missing and my dad homemade a hold down bracket that works somewhat different than you describe but it works great. Dad was and experimental Engineer from McDonald Douglas. He did all the wiring for the Mercury and Gemini rockets during the space race. He was a master mind.

Anyway there are three carbs, two coils, a distributor, and everything has new wiring. I can tell that my dad did the wiring by his tie downs. No signs of fire or gas leak. No signs of critters or mice. No muddobber or wasp either. Dad has notes written on the inside across the smooth surface areas. He mapped out the hook up wiring color code chart. He has marked a spot on the flywheel. Actualy he has two marks on the flywheel, a big one and a little one about 1 inch apart. He has written the compression of each cylinder on the head. Most of them are 96 or 98 but the top #1 is 92.

I bought a service manual and even they don't tell how to get to cover off. I was confused with the straps but I finally got them to open. The service manual should help me a lot because it has both motors listed in it. I might even be able to help answer some questions on this forum at some point once I get into this and actually get these two motors running. I don't want to run the flusher this winter so I will wait until spring before trying to start the motor. I am sure I will open up some more topics on these motors this spring. I want to restore each one and make them look new again. I guess I should take some photos.
 

Texasmark

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Re: 1965 100 hp Merc lower unit convert to jet drive questions

Looks like you are moving in the right direction. Great.
 
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