1979 Johnson 6hp Prop slip

[ugmold]

Oh brother, I bought this $50 Facebook motor (1979 Johnson 6hp) listed as low compression. New head gasket, fixed. Spark one cylinder, new coil fixed.
Runs great but the motor gets to a faster speed and the prop slips, rpms up. At first I thought it was just weeds from the pond, but apparently it seems to be getting worse.
Is this fixable by a backyard mechanic?
And what years might fit if I need to find a new lower unit.
Thanks

 

[flyingscott]

Does it bang out of gear or do the rpms suddenly shoot up. If the Rpms suddenly shoot up with no bang get a different prop.

 

[ugmold]

Stays in gear. It feels like if you suddenly shut down the engine. But the rpm's race, if you back off a little you can get it to catch, but more acceleration and it will do the same thing.
I'll try the prop of my 1971 Johnson 9.5.

 

[flyingscott]

I don't believe those props interchange.

 

[jimmbo]

Perhaps the engine is mounted too high and the Prop is ventilating
You can check to see if the Rubber Hub is slipping, by marking the Hub and the Prop, running it, and then checking if the two are still Aligned.

 

[ugmold]

I tried the 1971 9.5's 3 blade prop. Now I have 2 - 12ft aluminum boats. I use one for rowing, 1 for the motors. I currently have a 1968 6hp on the motor boat, it has a 2 blade prop.
The 1971 9.5 prop fits, at first it seemed too slow out of the gate, bow high (tilt is all the way forward) was like this for about 15 seconds, so I backed off, putted around and tried it again, same but when it finally planed out it was fast! Tried again and it was slow out of the gate, but planed off and fast again.
The third time was not the same, started to get a little speed and it sounded like the prop wasn't deep enough in the water (short shaft about 29 inch from engine to bottom of prop).
My 1968 6hp does not have this problem and it is short shaft (I measured, they seem the same).
The prop I took off the 1979 says 8x7.3 873 342. I didn't look for a number on the 9.5 prop, they do look different.

Jimmbo, ventilating would be a good descriptive sound. Almost seems like it makes a trough and the prop is half in the water half out. I can't watch it, I'd be man overboard if I tried.
what part is the rubber hub? Is it the piece that holds on the prop with the cotter pin?

 

[jimmbo]

The Hub would be Metal, with a Rubber Insert between it and the Prop.

Here are the props that would fit the 6hp. Also I included the Props for the 9.5
Some of the Props shown are Plastic. I can't recall if they had Rubber Hubs or were Sacrificial

 

[KJM]

Probably a spun prop. The older props had a pressed in round rubber insert and when these get old they loose their grip under load. I experienced the same thing one time. I could go slow but give it gas and the engine would rev up but the boat would slow down. Newer props have a different type rubber insert that isn't round. The old prop looked fine, you can't really tell by looking at it in my experience. You likely need a new prop and hub.

 

[ugmold]

I guess a spun prop would make sense. So if I mark the brass and I mark the prop (facing outward) would I be able to see it? I don't see any rubber visible. So I'm guessing this was a safety feature if you hit something it would be a little easier on the prop?

 

[jimmbo]

I guess a spun prop would make sense. So if I mark the brass and I mark the prop (facing outward) would I be able to see it? I don't see any rubber visible. So I'm guessing this was a safety feature if you hit something it would be a little easier on the prop?

The rubber hub is actually there to protect the expensive parts of the Drivetrain(Gears/Clutch Dog, etc) during Shifts, by providing a bit of Cushioning. I guess it offers a bit of protection, but an impact hard enough to cause it to slip, will likely be hard enough to do Damage to the Blade, again saving the Drivetrain.

 

[ugmold]

I watched a couple of videos on youtube, I think both props I tried have this issue. I also found a website that sells new "hubs" https://www.hmshubs.com
Thanks for all the input guys I really appreciate it. I need to also get a handle on prop pitch, but I guess that's another thread.
I'm going to put the old prop back on and mark it.

 

[jimmbo]

Be cheaper overall just to replace the Prop, than to replace the Insert. If it were a RAKER, that costs close to a G note, then yes, but I doubt a new Prop for 6hp is going to get near a C note.
The Dia and Pitch Info, and Part#, will be on the Prop, if it is an OMC Prop. If a Michigan, or other 3rd all Bets are off

 

[ugmold]

Well, I just came back from my test. Marked the prop with a chisel and a marker, I had the same symptoms on the water, but the marks are in the same place? Most props I see are around $100.

 

[jimmbo]

Okay. Are the Blades bent, or very uneven, chucks missing?
Where is the Anti-Ventilation Plate, in relation to the Bottom of the Hull?
Are there any items that could disrupt the flow of water to the Prop Blades?

 

[ugmold]

Blades look intact and even. The transom on both my boats is around 16 to 16.5 inch. The anti-ventilation plate seems to be around 18. Nothing else looks like it is bent.

 

[ugmold]

So I was out on my "motor boat" with the 1968 6hp, and it did it too! Did they put oil in the water or something? I'm starting to think that 16+" transom is too high for these motors. I read that 15 inches is the norm. This boat is so old and crappy it wouldn't bother me to cut it down.

 

[Doh]

First off, the 6hp and 9.5 are SHear Pin, not Slip Hub props, so they are either engaged or not spinning at all.

Second, if both motors are doing it, it is Venting or just plain running out of Pitch. Try running with other person in the boat and see if it is the same thing.

16" shouldn't be an issue, but if you run light you might want to cut it down 1"

 

[flyingscott]

First off, the 6hp and 9.5 are SHear Pin, not Slip Hub props, so they are either engaged or not spinning at all.

Second, if both motors are doing it, it is Venting or just plain running out of Pitch. Try running with other person in the boat and see if it is the same thing.

16" shouldn't be an issue, but if you run light you might want to cut it down 1"

That is wrong. Both those props have a rubber hub.

 

[racerone]

Correct ---they have rubber hubs !!----They left the factory with a steel drive pin.----A wise owner could use a shear pin on a 6 HP as it is cheaper than prop repair / new props.

 

[ugmold]

Development: I took out my "motor boat, 12 ft aluminum, old) with the 1968 6hp on it and set the trim one notch out and no spin! However, no real trim either, bow would stay relatively high. It was windy and choppy. I use to put my 9.5 with trim adjusted all the way in so that it would trim better, it is a heavier motor which I'm sure is a factor.
I want to try the 1979, (I'm soldering on some new pins on the coil connectors right now). It was really fast (top end), slow out of the hole with the 9.5hp prop on it, but not being the right prop can it damage it in some way?