1968 6hp maxing out at 5mph

disclaimer - I'm new to outboards.

I just picked up a Mercury 60 1968 6hp. I cleaned out the lower unit, replace the impeller, replaced the spark plugs, rebuilt the carburetor & fuel pump, and replaced the fuel hoses.

I can get it running now, and finally took it out on the water. I have it on a 10ft inflatable zodiac type boat that weight about 50lbs.

I expected 6 horses would make my little raft fly. It was disappointing though. With 2 adults (~300lbs) we maxed out just over 5 miles per hour.

I got home and ordered the only 6 spline prop I could find on the internet, hoping it has a steeper pitch than the banged up one I'm using.

Any ideas what else I can do to move a little quicker?

Thanks!

.."hoping it has a steeper pitch than the banged up one I'm using."|

That's assbackwards. You need a lot less pitch.

Jeff

Less pitch should accelerate faster but max out at a lower speed, as I understand it. But like I said, new to this world.

Compression test? Is it a "Super Six". Should be able to water ski with it.

I just ordered a gauge, and I'm afraid of what I'll find.

I'm guessing a compression test should have come first. Live and learn!

Give it a fair chance. Both plugs out, nice swift strong pull. Just finished a 1975 15 Johnson. Great power slow smooth idle. Starts first pull Everytime when warm. Compression with new head gasket is 90psi. Put a drill to spin it and can get over 100, but that's "cheating".

Thanks! I needed a little optimism!
I'll let you know how it shakes out. From reading, it sounds as if both cylinders come in low, a head gasket is likely the problem. If just one comes in low, I might have a piston or ring issue. Is that more or less in the right direction?

I think your motor is without head gasket.....that's just a water jacket cover there.

That would explain why I couldn't find one on the parts digram.
Can you point me toward what I might need to replace?

Unless you see water leaking/squirting out from the engine block proper, nothing.

I don't share Timguys optimism....on this, and no I don't do what he does for whatever reason, nor where he does it. Just think he is overly optimistic in his estimate of what your boat should be doing with that engine.

Having had a Johnson 5 ½ back in the mid '60's when the engine was a 58-59 yearmodel. Grand dad had a new Evinrude 7 ½ that would plane a light 14' aluminum rental boat with us and tackle....plane speed on that boat was probably just over single digit MPH and with me on the front seat to help it get up....i was in early double digits of age and we were of light composition.... thin. I had a new 10' Lowes Lake John, flat bottom, light aluminum boat, easily planed, new 7 ½ Merc, in the early 70's. With just me it planed out and ran pretty good....doubt it got to 20 mph. Would have put a 9.8 on it had I bought that rather than the 7 ½.

I'd still check compression, as recommended, just to know what it is. I never knew of an inflatable to be fast....with my limited experience. RIBs are pretty quick, but the hull is fiberglass and the hull is what contacts the water.....mostly, at speed. Check with "Sea Rider" from Peru, posts on another forum and runs RIBS with low HP engines. He might give you a better idea as to what to expect.

"Any ideas what else I can do to move a little quicker?" Yeah.....more ponies.

If compression numbers are down we have to assume it's cylinder wear, or perhaps stuck rings, or scorn piston/pistons. Could have been overheated once or somehow got rust going in cylinders, then upset the rings ability to seal. Maybe somebody had run it with little or no oil. Can you post some pictures of significant parts that normally show life history.....like lower unit, pull rope, side of motor block....close up of bolt heads....stuff like that helps a person learn about their motors history. Virgins from 1960 are quite rare, but the less "work" that has been done on a motor, it's normally a better motor.
If it's rings or very light damage to pistons, then it might be okay to dismantle the motor and refresh it with a light home job and new piston rings, after carefully cleaning up the pistons. If the damage is more severe then we are to consider the overall condition of the motor and is it really worth boring oversize the cylinders and replacing the pistons and rings. For me, if I like a motor....I will do it, but it has to be done carefully and piston clearance must be precise when measuring difference between cylinders and your new pistons. It takes a skilled machinist, especially when working on small bore "blind" cylinders ( cylinders only opening to the bottom). Ports have to be carefully and lightly radiused, meaning sharp edges cleaned off with medium/fine emery paper. Sounds like a lot, but I completely rebuilt my first outboard at 10 years old in 1966. A 2.5 hp Johnson twin cylinder Sea Horse. We have tremendous help available now on forums like these and videos from great mechanics.....right on our cell phones.

I decided to do some investigating. Did a WWW for a Soft Inflatable Boat....SIB with a 6 hp engine. Found Utube Sea Pro 340 inflatable boat with 6 hp Johnson...can't copy url for some reason. Gives you an idea as to what to expect with 1 person. Note it is planing.....barely....with just one person, but I doubt it is up to 15 mph. Put that second person in there, you are wetting a lot more hull, more drag, and with only 6 ponies it WILL make a difference.[h=1][/h]
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