1975 70 hp Evinrude WOT/IDLE rpm

[Nandy]

I left my book at one of my friends house and I dont feel like doing the 1 hour drive to get the book. If someone is nice enough and can reply back with the WOT RPM and the IDLE RPM I will appreciate it much. I installed a loaned tach and want to check how I am set up with the 2 props I have.

Thanks!

 

[HighTrim]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Set the idle to 650-750 RPM under load in gear. If you are setting it in the driveway on muffs, that is about 1100-1150 roughly, then fine tune on the water.

Your WOT RPM should be in the range of 5500-5900 RPM. I like mine on the high side of that spectrum, but that is my own humble opinion.

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Thanks HighTrim! I will be just checking them in the lake. I will do the link/sync later on. My major thing is making sure my wot is not over the max recomended. I have kept myself away from wot until I find out.

 

[HighTrim]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Equally as bad to be lugging the motor, or having a low WOT RPM.

Let us know what you find.

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Sure will!!!!

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Idle RPM under load 650
Idle RPM off load 1050
WOT RPM (30 MPH) 5450
"Cruising" RPM (20 MPH) 3950

Sounds by the book. The prop I used was a 13 1/4 x 17. I tried the other prop I have that has no markins and it acted the same so I am guessing it is the same pitch. With the prices of gas I havent try to take it to the prop shop. It is quite a ways, but they will check it for free.

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

I've got a (new to me) 77 70 hp motor so I'm curious what your results were.

At WOT, you've got about 17% slip. That's assuming your speed is accurate. Did you use a GPS?

And I'm sure others brighter than I will suggest you need to increase the WOT rpm by adjusting you motor or prop.

But this gives me a ballpark for when I take mine out the first time, hopefully in a couple of hours.

 

[HighTrim]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

The only thing that I would mention is that I would want a bit higher WOT RPM for maximum life out of your outboard. If you do invest in a new prop, I would decrease the pitch by about 2", to give you roughly a 400 RPM gain at WOT, since for every 1" of pitch size, the effect will be approximately 200 RPM. Decreasing pitch raises the WOT RPM and increasing the pitch lowers the WOT RPM.

sorry, tacklewasher beat me to it!

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Yup! used my Garmin for the speed. I will post my findings at the prop forum so if you guys want to give your advise there too you are welcome.

Tacke, is that 17% slip a good/bad or typical reading? How did you got to that number? Come back and let me know how your motor did please.

High, So I should adjust my WOT screw to get maybe to 5700? How do that maximize the life of my motor? Please explain if you have time.

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

[nelson voice] Ha ha [/nelson voice]

Do you have a picture of your boat? Be interested to see how high the motor is on the boat before suggesting a prop swap. From what I've read, most have their motor too low and can improve WOT rpm and speed by lifting the motor, not trimming it but actually moving it up a bolt hole on the transom.

The reason why I'm suggesting that is your calculated slip is a little high and it may be more of a setup issue than a prop issue. You want your A/V plate to be about even with, or a little higher than the bottom of the hull.

Just my thoughts and someone brighter may explain it better.

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Yup! used my Garmin for the speed. I will post my findings at the prop forum so if you guys want to give your advise there too you are welcome.

Tacke, is that 17% slip a good/bad or typical reading? How did you got to that number?

Oh the power of google.

http://www.mercuryracing.com/propellers/propslipcalculator.php

Our gear ratio (from searching around here) is 2.42:1 (and someone correct me if I'm wrong). Punching in the rest of what you said gave me 17%. I seem to recall Dhadely (or someone else in excess of 10,000 posts) wanting to see less than 15% and around 10 is where he would stop playing.

Come back and let me know how your motor did please.

Will do.

High, So I should adjust my WOT screw to get maybe to 5700? How do that maximize the life of my motor? Please explain if you have time.

No WOT screw that I know of.

Again, from reading here, the general consensus of the experts is to set up these motors close to 6000 at WOT. Prevent them from carboning up, which has killed many a motor early.

It's all in the set up and prop pitch. But if I were you, I'd tune the setup and maybe do a decarb (search here for instructions) before buying a prop. It may not help, but it is a lot cheaper and will not hurt.

 

[HighTrim]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Tacklewasher has explained it, but to add, when the RPM's are lugging, it is kind of like trying to climb a hill in your car in 5th gear. Not good. Your WOT RPM's aren't horrible, but I would want them higher. Dhadley has stated many times that they should be in the higher spectrum of the reccomended range, I take his word as gospel.

I should have mentioned the setup as well. Have a look towards the stern of your boat, at eye level, or take a pic and post it on here. If the setup is fine, the prop will need attention. There is no screw that will adjust the WOT RPM.

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Oh the power of google.

http://www.mercuryracing.com/propellers/propslipcalculator.php

Went to that website, I plug all the numbers but the prop slip and all I get is Nan when I click calculate.... How did you got it to work? I used IE and Firefox...

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

BTW, here is the post I placed in the prop forum.
http://forums.iboats.com/showthread.php?p=1756977#post1756977

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Went to that website, I plug all the numbers but the prop slip and all I get is Nan when I click calculate.... How did you got it to work? I used IE and Firefox...

For gear ratio, just put in 2.42

Don't put in 2.42:1.

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

That worked!!! ty!

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Come back and let me know how your motor did please.

Hmmm. Not so good if my tach is correct.

Could not get past 3000 rpm but hit 28 mph at that so I'm thinking something is wrong with the tach but as I don't even know the pitch of my prop, it's kinda hard to tell.

Guess I need to clean it off in the morning.

Other than cleaning out the engine some more with seafoam, I'm not going to put any money into this and will just work on getting my 115 working well.

 

[Nandy]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Sounds like you are having a speed similar to mine so maybe your tach is not set up properly. Hopefully you will find what the problem is. Thanks for your help.

 

[HighTrim]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Check the back of the tach. Make sure the rotary switch is set to 6 or 6p.

 

[Tacklewasher]

Re: 1975 70 hp Evinrude WOT/IDLE rpm

Hmmm.

You mean a -40% slip isn't right?

Silly thing has a 17p prop on it. But I've obviously got to track down the tach issue. If I plug in a 12% slip, I should be running around 4800. But I think it is too much prop for this setup. My goal is to get it working well and then sell it so I'm not putting money into a prop that may not work for someone else.

Back to the tach, it is set on 6 on the back, everything on the motor checks out so I really think I've got the wiring buggered. I pulled it off my 81 with the 3 plug tach and tried to splice it in to the 2 wires on this older control.

So any clue as to the proper colors to wire up? It's an old OMC tach that I picked up cheap and someone else had re-wired for their use prior. It worked fine plugged into the controls for my 81 115, but doesn't seem to like being spliced in to this thing.