First thread/post. Q about prop size 1973 Johnson 115

[12thManFalseStart]

Just bought a boat last week and had it out twice now with a few quirks. 1980 17 foot fiberform waikiki. There are a couple of dings on the prop I have now, which is a Michigan 13 1/4 with 17 pitch. I'm only running 34 tops at 5,300 rpm. Used to have a simular size glastron 78 with a 135 evinrude that did 45 easy. Only 20 more horses gives it 10mph more, and probably a heavier boat?

I have noticed I can't trim the motor up while running. It doesn't leak down, but I can't trim up at speed. If I stop, and put it in gear, it'll hold until I give it gas, and it will drop down with a thud.

Should I try a 13 1/4 19 pitch? Motor manual says it makes peak hp at 5,000, even though it's range is 4,500-5,500 in the manual. Thanks guys.

P.S. Gear ratio in manual says 13:26. I'm looking for the highest speed prop I can get since I don't really tube/ski with it all that much.

 

[steelespike]

Re: First thread/post. Q about prop size 1973 Johnson 115

Its possible you have a tilt only option or your t&t needs service.
You may be able to manually trim up for improved performance.
You won't ba able to get your true wot rpm and speed unless you set your trim or get the trim fixed.
Props in the 17" pitch are pretty typical of the 115.Don't make any prop changes until you get your true wot results.

 

[12thManFalseStart]

Re: First thread/post. Q about prop size 1973 Johnson 115

Its possible you have a tilt only option or your t&t needs service.
You may be able to manually trim up for improved performance.
You won't ba able to get your true wot rpm and speed unless you set your trim or get the trim fixed.
Props in the 17" pitch are pretty typical of the 115.Don't make any prop changes until you get your true wot results.

Your right. The guy said it doesn't have trim. Guess I thought it did since I can tilt the motor up out of the water. Hmm. There are 4 or 5 holes. I tried the farthest hole that trims it up. Would go straight fine, but when turning, would cavitate too easy, so moved down one notch, and doesn't cavitate in turns now. Guess I'm at max up trim with this one. I'll have to find someone that knows how to check compression, being a 40 year old engine that I have no Idea if it's been rebuilt over the years.

I see another boat for sale, about the same size and style with a "newer than mine" 115 mariner engine, and a prop that looks like a 5 blade SS. The guy clams 50 plus. Either he's lying, or exaggerating, right? lol.

This is supposed 50 plus boat.

Here's mine.

15 mph difference is quite a bit for similar style boat/weight, and same size engine.

 

[etracer68]

Re: First thread/post. Q about prop size 1973 Johnson 115

Are you using GPS speed, or the boats speedo. Boat speedo's can be off a lot.

 

[12thManFalseStart]

Re: First thread/post. Q about prop size 1973 Johnson 115

Both. Boats says 35 by myself trimmed up full tank up front. Gps hit 37.
I'm hopeing the small dings on the prop are affecting the top speed by 5 plus mph.

 

[12thManFalseStart]

Re: First thread/post. Q about prop size 1973 Johnson 115

What if I threw on a 5 blade prop like this guys similar boat and motor since he's getting 50+ he says?

 

[MARV1]

Re: First thread/post. Q about prop size 1973 Johnson 115

Try a 19p prop and go from there, might be able to spin a 21p. Johnson/Evinrude outboards were not known for speed...

 

[steelespike]

Re: First thread/post. Q about prop size 1973 Johnson 115

Your boat is only 850 lbs. so should at least be over 40.
Your 115 is rated at the crank so probably closer to 105 or 110 at the prop so may not measure up to the "newer"Mariner.
Most motors went to at the prop in 84/85.
I think his 50+ is unlikely.I don't think a 5 blade is the best choice for speed with a 115.
A good prop may let you trim higher. Solas Amita has a good reputation among aluminum props.
Your straight line rpm and speed with the problematic trim would be interesting.
You can buy or make some wedges to add or subtract a little trim.
The antivent plate(just above the prop) should be even with to about an inch above the bottom of the transom.
A good prop will help improve the height as well.
A compression check may be in order.
Be sure the throttle opens all the way.
A careful tuneup with attention to timing and timing advance.

 

[12thManFalseStart]

Re: First thread/post. Q about prop size 1973 Johnson 115

thanks for the info guys. I just signed up since everything I googled would send me to this site.

 

[12thManFalseStart]

Re: First thread/post. Q about prop size 1973 Johnson 115

I just noticed that the manual calls for premium, or no less than 94 octane, and I've been cheap "not really at almost $4 a gallon" getting the lowest grade at 87 I believe it is. Would this make much of a difference, or just a difference in my wallet?

lol, it also calls for "leaded". 1973 owners manual.

 

[etracer68]

Re: First thread/post. Q about prop size 1973 Johnson 115

I would follow Steels advise, and as far as octane, try the 94 and see, if theres a differance. I would think the compression has to tested, and the timming. Depending of compression readings, you may be able to get away with lower octane if you retard the timming. I would post in the Johnson section for more info, on fuel and timming. The higher octane=, slower burn rate.

 

[steelespike]

Re: First thread/post. Q about prop size 1973 Johnson 115

Check with the guys at the Evinrude/Johnson forum on octane requirements.
Typically a motor forced to run on too low an octane will suffer damage.
Don't worry about lead.

 

[ezmobee]

Re: First thread/post. Q about prop size 1973 Johnson 115

Interesting, I've never heard high octane being required for older V4's. Quite the opposite in fact. I ran my '73 and currently run my '81 on 87 and never had any issues.

Power trim makes a BIG difference in speed. Without it you really have to sacrifice to have a good hole shot and decent planing speed. My old boat was a 16' with a '73 Johnson 85 without power trim. Top speed was only like 34. Current boat is an 18' with an '81 90 with power trim and I can just touch 40MPH.