New owner - Questions - 1968 Larson 120 Merc I/O

[TripleJackInGA]

Hi all -

I'm new, and admittedly know little about boats. I know plenty about cars, but boats are another story.

I recently bought a 1968 Larson All American with a 153 Mercruiser I/O, and I have a few questions. I'm sorry if these get asked all the time, but I didn't see these, so please forgive me.
The boat has a new floor, new transom, new gas tank, new steering, and runs fine so far.

- The boat has a 153 Mercruiser I/O. Right now it's running a 14.4x19P prop, which has a little damage to it. At 4000 RPM I'm running about 30 with 2 people onboard. How much more can i expect to get out of it with a nice new prop, and should I change to a different size?
- The only problem that i can detect is that when i put it in Drive, it seems to 'clunk' a bit. When i put it in Reverse, it seems to growl and then pop into gear......kind of like if you try and put a moving car in Park, if that makes any sense. Not quite that loud, but sounds like it's skipping over gears or splines until it finally pops in.

- The engine runs fine, and uses no oil, makes no smoke, and runs nice and cool. It seems to have proper power for it's size. If I want to swap in a newer, more powerful engine in the future, can a 4.3 drop in? I;m sure it's not that simple, but what's required? How can I tell if the hull/outdrive will handle it? I know you can probably beef up/upgrade the outdrive, but what about reinforcing the hull to handle it?

- What's the best way to keep water out of the gas? It doesn't seem to be a problem, and the boat has a new tank, but it seems inevitable to some extent. The engine currently has two simple inline fuel filters.

That's about it, and thanks for your opinions.

 

[tic]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

.. Make sure your idle isn't set too high, that will grind the gears when it's put in forward or reverse.. Idle should probably be around 600rpm max... If it's 1000rpm or higher, you will do damage putting it into gear. A little ratcheting noise is common as it snicks into gear but try and minimize it by going directly into gear, don't nurse it...

 

[tic]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

I run a bottle of dry gas in the tank every so often and haven't had any water in the fuel problems. There are also water/fuel separating filters you can buy.

 

[SofA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Jack, my advice to you is to always keep a couple cans of seafoam motor treatment laying around. I swear that stuff is magic!

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Jack, my advice to you is to always keep a couple cans of seafoam motor treatment laying around. I swear that stuff is magic!

And Seafoam is.....?

Ah, OK, it looks like some kind of 'cure-all'.

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

What about the Prop deal, and the possibillity of an engine swap down the line ?

Do you think I'll get any noticeable gains out of a new prop, and is an engine swap feasible ?

How do I tell what kind of Outdrive i have?

 

[firehog6305]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

jack, you have a merc outdrive, your best bet to find out which yours is, look for the serial number on the outdrive, and contact mercrusier online, I have contacted them before to see if I had an alpha1 or alpha2, they are more then happy to help

 

[thumpar]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

tic is right on the gear change. You need a positive motion. Don't be scared. When I first got my boat (2.5l 120 horse) I was moving it slowly thinking that was best. Well it was grinding. Now I just use a good forward or reverse movement and all is well.

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

- The boat has a 153 Mercruiser I/O. Right now it's running a 14.4x19P prop, which has a little damage to it. At 4000 RPM I'm running about 30 with 2 people onboard. How much more can i expect to get out of it with a nice new prop, and should I change to a different size?

One more thing I thought of, how can i tell exactly what size and pitch the boat came with originally? I would think that they put a lot of time and energy into deciding on the original prop when the boat was new.

 

[tic]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

I would also think 120 HP would be able to move a 16' boat faster than 30mph.. Should be closer to 40mph

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

I would also think 120 HP would be able to move a 16' boat faster than 30mph.. Should be closer to 40mph

If it was an outboard, I would agree, but an I/O I believe is heavier.

My friend has a 19' aluminium fish-and-ski with a 115 outboard, and it runs 42-43 with 4 people in some good choppy water. Much lighter boat though, especially aluminum.

 

[Haut Medoc]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

You have an MC-1 drive, which is a pre Alpha.....
It is considered Gen. 1 for the most part, (the innards differ slightly)......
I think 14.5 x 19 is too big for that motor......
More like 14 x 19 or 14.5 x 17 would do better.....
Your WOT should be 3900-4300, & you should try to be at the upper end of that range........
Has a full tune up been done?......
What is the length of the boat?.....​

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

You have an MC-1 drive, which is a pre Alpha.....
It is considered Gen. 1 for the most part, (the innards differ slightly)......
I think 14.5 x 19 is too big for that motor......
More like 14 x 19 or 14.5 x 17 would do better.....
Your WOT should be 3900-4300, & you should try to be at the upper end of that range........
Has a full tune up been done?......
What is the length of the boat?.....​

Thanks for the info.

The boat is a 16.5 foot, closed bow (if that matters).

Would the 14x19 or 14.5x17 be about the same, kind of like plus-sizing tires, or what? I;m still learning the dynamics of prop sizing.

I usually run it across the lake at 4000, or maybe slightly higher, but the throttle will go higher if I push it all the way over. I just don't want to rev it that high.

I just bough tthe boat, and haven't done anything to it myself. I'll do plugs, cap, wires, oil-change, etc. in the next week or so.

Speaking of which, the engine is missing the air filter. I looked online, and for boats they always call it a Spark Arrestor, and insist it's not for filtration. What's up with that?

 

[Haut Medoc]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Thanks for the info.

The boat is a 16.5 foot, closed bow (if that matters).

Would the 14x19 or 14.5x17 be about the same, kind of like plus-sizing tires, or what? I;m still learning the dynamics of prop sizing.

I usually run it across the lake at 4000, or maybe slightly higher, but the throttle will go higher if I push it all the way over. I just don't want to rev it that high.

I just bough tthe boat, and haven't done anything to it myself. I'll do plugs, cap, wires, oil-change, etc. in the next week or so.

Speaking of which, the engine is missing the air filter. I looked online, and for boats they always call it a Spark Arrestor, and insist it's not for filtration. What's up with that?

I thought you were only getting 4000 out of it....
You need to see what it will do at WOT.....
Damage, (even slight) can have a real effect on performance....
The spark arrester is just that-....
It prevents a backfire from igniting any fumes that might be in the bilge....
You must have one.....
Not an auto one.....

 

[ziggy]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Speaking of which, the engine is missing the air filter. I looked online, and for boats they always call it a Spark Arrestor, and insist it's not for filtration. What's up with that?

The spark arrester is just that-....
It prevents a backfire from igniting any fumes that might be in the bilge....
You must have one.....
Not an auto one.....

+

there's not much dirt to filter out in a boat. yer on water, not a dusty road....

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

No, I said I try and keep it at 4000, maybe a little higher, and at 4000, I get around 30-32MPH, but I've never run it at WOT. I'm afraid of over revving it. I mean, an old 60s era engine, even a 4-cylinder, probably wasn't designed to see 5000+/- RPM.

Do you honestly think I should go out and see how high it will rev? I'm kinda leary of that.

I'll get a spark arrestor then before i go out again.

Very good point on the filter deal......I didn't think of the obvious.....that I;m on the water.

 

[tic]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Our 1988 16' Four Winns freedom will do 37mph on an old OMC 130hp I/O.... Not bragging, just think you can get a few mph more out of yours

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

Our 1988 16' Four Winns freedom will do 37mph on an old OMC 130hp I/O.... Not bragging, just think you can get a few mph more out of yours

What kind of prop are you running, and at what RPM, and with how many people ?

 

[tic]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

13 and 1/4 X 17" prop, two adults plus gear. approx 5000 rpm..... In your first post you said you were running "about 30mph".. Now you say 32mph....MOST people when they say "about 30mph" mean less than 30mph... 32mph @ 4000 rpm sounds GREAT!.... I don't think you have a problem

 

[TripleJackInGA]

Re: New owner - Questions - 1968 Larson 120 Merc I/O

13 and 1/4 X 17" prop, two adults plus gear. approx 5000 rpm..... In your first post you said you were running "about 30mph".. Now you say 32mph....MOST people when they say "about 30mph" mean less than 30mph... 32mph @ 4000 rpm sounds GREAT!.... I don't think you have a problem

Thanks for clarifying how to correctly use vague terms for me, hehehe.

Yeah, I didn't really think it was going to get much better than what it is now, and that's fine. It does what it needs to do, and I enjoy it. Besides, to go faster takes a lot more money, even slightly faster, considering I only paid $1500 for the whole thing.