Purchasing a mark twain 120 mercruiser

[1drummer]

New to the forum so here it goes.
I have the opportunity to get a late 70s model mark twain boat with a mercruiser 120 horse. I'm not going to give much for it but wanted some tips to make sure it was worth my time. The floor needs to be replaced which for me is not a big deal. What has me wondering is how to make sure the outboard section of the motor is ok and what to check for before deciding to take on this project. The oil looks good in the motor no water but other than that don't know much about it. It's a late 70s model mark twain I be leave its a 160t not sure on that but it has the 1/0 horse mercruiser motor and outboard unit. Thanks in advance.

 

[81 Checkmate]

Re: Purchasing a mark twain 120 mercruiser

Welcome to iboats........go to the top of Merc I/O forum and click on the stickes by Don S and read up on what you are getting into.

1. Be sure to check for a cracked block?
2. Lake test if posible.

It might sound like a good deal..but if you have to spend a grand on the motor to get it running...Well you know!

Good Luck!

 

[haulnazz15]

Re: Purchasing a mark twain 120 mercruiser

I'd be checking the transom for rot more than anything. If the floor is soft, there's a good chance that the transom could have soft spots, too. Floors are relatively easy to replace, but the transom repair involves pulling everything off of the back wall of the boat, including the engine, drive, and all associated hardware.

 

[Bondo]

Re: Purchasing a mark twain 120 mercruiser

I'd be checking the transom for rot more than anything. If the floor is soft, there's a good chance that the transom could have soft spots, too. Floors are relatively easy to replace, but the transom repair involves pulling everything off of the back wall of the boat, including the engine, drive, and all associated hardware.

Ayuh,.... I'd Count on the transom bein' Rotten,...

The motor/ drive is known as a solid reliable unit, if it checks out OK,...

Pressure test the block, 'n a compression test will tell ya the Motor's general health,...
The drives are a roll of the dice,...
Look for metal in the oil, 'n sun rotten Rubber parts,...

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

Thanks for your responses I appreciate it for sure. I will check the transom. I went ahead and got the boat literally gave nothing for it so figured it would be worth checking out. So far the motor cranks and the oil looks good. I'm not getting any fire though. Traced it back to the cylinoid so I will pick up a new one tomorrow. Here is what I think is a dumb question. The guy I got it from said that it ran on mixed gas but unless I am mistaken it does not. Am I right? Once again thanks in advance. It's a four cylinder mercruiser 120 horse

 

[haulnazz15]

Re: Purchasing a mark twain 120 mercruiser

No, it doesn't run on mixed gas. Regular unleaded will be just fine, preferably zero ethanol if you can get it.

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

Just thought I'd let you all know I got the old boat running this eve. Purrs like a kitten. The transom appears to be good and I've drained and replaced the oil and impeller in the lower unit. I've got a slight problem with the power trim however it goes up but not down unless I put pressure on it but for sure I am making progress. If you have any info on the trim situation once again your input would be much appreciated thanks.

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

So I've got a new problem after the initial start up where everything seemed to be ok the motor now dies shifting into neutral. From what I understand with the boat not being in the water and running on muffs in the driveway the interrupter switch would not be engaged. I've seen lost of questions about it running great in the driveway and dieing on the water but not the other way around. Please help.
Here's what I've done to this point.
New points, plugs,cap, rotor,coil,gas tank,fuel lines,I have cleaned every connection I can find. Adjusted carb. I'm used to working with carberators but this all has me stumped. The motor will start right back up but if I run it up a little then back to neutral it dies thanks.

 

[achris]

Re: Purchasing a mark twain 120 mercruiser

Just thought I'd let you all know I got the old boat running this eve. Purrs like a kitten. The transom appears to be good and I've drained and replaced the oil and impeller in the lower unit. I've got a slight problem with the power trim however it goes up but not down unless I put pressure on it but for sure I am making progress. If you have any info on the trim situation once again your input would be much appreciated thanks.

Good indication that the end caps of the trim rams are corroded.

So I've got a new problem after the initial start up where everything seemed to be ok the motor now dies shifting into neutral. From what I understand with the boat not being in the water and running on muffs in the driveway the interrupter switch would not be engaged. I've seen lost of questions about it running great in the driveway and dieing on the water but not the other way around. Please help.
Here's what I've done to this point.
New points, plugs,cap, rotor,coil,gas tank,fuel lines,I have cleaned every connection I can find. Adjusted carb. I'm used to working with carberators but this all has me stumped. The motor will start right back up but if I run it up a little then back to neutral it dies thanks.

Are you talking about putting it into gear and it dieing when you pull back into neutral, or just stalling our after just giving the engine a rev in neutral?

Chris.....

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

When you put it into gear and back to neutral. Thanks for your response.

 

[achris]

Re: Purchasing a mark twain 120 mercruiser

Sounds like the shift interrupt switch is being activated, cause is usually a sticky lower shift cable....

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

That's what I thought as well but the switch does not appear to be getting activated. That's what has got me stumped. I think the switch may even be way out of position. The cable does not even touch the switch while shifting. Could that cause my problem? Thanks again.

 

[achris]

Re: Purchasing a mark twain 120 mercruiser

The switch is only tripped for a very short time, then it moves back to the 'static' position. Even when watching it, it very difficult to see it operate... Disconnect the shift cable from the shift plate and move it by hand. It it has any more than the slightest resistance, it need to be replaced.

Chris.....

 

[1drummer]

Re: Purchasing a mark twain 120 mercruiser

I was able to fix the dieing in neutral issue turned out to be a gap issue on the points Im surprised it was running at all. I took it out this eve and it ran great. I ran it for an hour or so stopping and starting it was great until the fuel pump quit about 200 yards from the boat ramp. But it's getting there for sure. I did learn something I thought I might share. I took it out yesterday and thought that the engine coupler was going out I didn't smell rubber but it seemed to be slipping when throttleing up. I took it by a boat guy and he noticed that the prop was for an outboard he let me borrow a prop to try it and all is good throttled up and ran really nice. I didn't know there was a difference or that it would make that big of a difference just a little food for thought.