1975 Johnson 70hp 7800rpm?

[LouReed]

I have a '68 Starcraft 16 Supersport with the above mentioned 1975 Johnson 70. I just got a tach for it because I thought I might be revving a bit high. I've currently got the stock 17p prop on it, although it's a little dinged up. The tac show me revving at 7800rpm going 32 mph, which indicates 38% prop slip. Something tells me my tach is off. That being said, is it even possible to rev my engine that high? Do these motors have a limiter on them? I also got a brand new 19p stainless prop from iboats, and I still go about 33, but it says I'm at 5900 rpm, which gives a 25% slip. Considering i've read reviews on this prop and everyone else calculates about 11% slip, I'm thinking my tach is for sure wrong. Any thoughts?

 

[BonairII]

Re: 1975 Johnson 70hp 7800rpm?

My 16' SS w/ 1977 75hp Evinrude does 35mph @5800 rpm.....so your in the right neighborhood. Your tach is wrong.

 

[boobie]

Re: 1975 Johnson 70hp 7800rpm?

There's a dial on the back of the tach. It should be on position # 6.

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

My 16' SS w/ 1977 75hp Evinrude does 35mph @5800 rpm.....so your in the right neighborhood. Your tach is wrong.

What prop do you have on there?

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

There's a dial on the back of the tach. It should be on position # 6.

Oddly enough, it actually doesn't have a dial on the back, it's supposed to be a 6p only tach, it was real cheap. I think it's a Teleflex knock off. It has a sticker over a hole that looks like it has markings for adjustment, 2, 5, and 6.

 

[boobie]

Re: 1975 Johnson 70hp 7800rpm?

Take the sticker off of the hole and try to adjust it. If not, get another tach.

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

Yup, got it adjusted! I hooked up a dwell tach and just played with that screw until they read the same. With the accurate tach, I took the boat out and I'm at 32mph at about 5800 or 5900 rpm. In real smooth water I can bump 6k at 33 mph. My prop has a couple of chunks out of it, so I'm sure with a good prop I could hit 34 or 35.

That brings up the new prop I got, the Michigan Wheel Apollo 19p. I'm running about 4400rpm at WOT. I can't believe going up 2" would drop me by that much. Does anyone have any experience with this prop? It doesn't advertise as being a cupped prop, but it looks to me like the trailing edges are cupped a bit, at least more than the stock Johnson prop. If I'm at 4400 with a 19p, that suggests that even a 15p Apollo wont get me to where I want to be (5500 - 5700).

 

[BonairII]

Re: 1975 Johnson 70hp 7800rpm?

What prop were you using when you hit 32mph @ 5800rpm?

Losing 1400 rpm by changing to a 19p doesn't make sense at all


On a side note.....I hit 37mph(briefly) today with my 17p prop @5700rpm (just me in the boat). I just raised the motor to it's highest setting on the transom bracket. Flat calm water.

 

[boobie]

Re: 1975 Johnson 70hp 7800rpm?

How did you get a dwell tach to work on that mtr ??

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

I'm using the stock OMC aluminum prop, although it's got a few chunks out of it, not a good prop at all.

It turns out, the Apollo prop for my motor is cupped, which wasn't advertised. So, in effect, it's a 21p. MI Wheels is being really cool, so I think they're going to send me a 15p Apollo, we'll see how that works. If it doesn't, anyone wanna buy a MI Wheel Apollo;-)

37? Really? That's awesome! My motor is 1 hole from the top, maybe I'll move it up 1 more. What trim position are you set at? Which prop are you using, stock OMC? Thanks for all the info!!

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

How did you get a dwell tach to work on that mtr ??

I hooked the green lead up to the gray pulse wire, and the black to ground. It had a 6 cyl scale so I just divided that in half. Of course, the scale only went up to 2500 rpm so I pretty much had to almost idle my motor to get it to match up. When I first hooked it up, the dwell was reading 2500 and my tach was 1750. Now it read 1250, should be right on.

 

[BonairII]

Re: 1975 Johnson 70hp 7800rpm?

37? Really? That's awesome! My motor is 1 hole from the top, maybe I'll move it up 1 more. What trim position are you set at? Which prop are you using, stock OMC? Thanks for all the info!!

I hit 37mph for about 5 secs and then it settled down to 36. I have the trim set on the second lowest hole, but I didn't have a chance to test any other trim positions. I'm using a standard aluminum OMC prop.

I'm sure I could get another 2mph with a 19p prop, but not worth it IMO. A 19p will most likely make it a dog coming out the hole.

 

[1kruzer1]

Re: 1975 Johnson 70hp 7800rpm?

I have the same boat and motor, but all 1976. It's amazing how much difference there can be between different models of props of supossedly the same pitch. Anyway, I run a new Solas Rubex 3 blade aluminum 15 inch pitch, and with a moderately heavy load on board my gps speed is 33 MPH at exactly 5500 RPM. The Solas is an efficient prop as aluminum ones go, and I suspect you'll find a stainless 15p Apollo to be a pretty good set-up. Just make sure you are getting at least 5500 RPM at WOT, as these motors need to be set up for the highest side of their RPM range.
Good luck.

 

[LouReed]

Re: 1975 Johnson 70hp 7800rpm?

I hit 37mph for about 5 secs and then it settled down to 36. I have the trim set on the second lowest hole, but I didn't have a chance to test any other trim positions. I'm using a standard aluminum OMC prop.

I'm sure I could get another 2mph with a 19p prop, but not worth it IMO. A 19p will most likely make it a dog coming out the hole.

Do you know what gear ratio your motor is? Something tells me the 75hp has a bit higher ratio than my 70. We'll see how the 15 Apollo works, and I might have a line on a 19p stainless OMC. It looks like I'm just going to have to do some playing around.

 

[BonairII]

Re: 1975 Johnson 70hp 7800rpm?

Do you know what gear ratio your motor is?

2.41