1974 Evinrude prop giving low RPM

[Urbina]

Hey guys just wondering if I could get some feed back on what prop size I should be using.
its a 70 Hp Evinrude 1974 as of now it came with a 12 3/4 19 Pitch prop.
I took it and all I got was 4800 - 4900 RPM and a bout 30 MPH ( according to the speedometer )
the operating range calls for 5500 - 6000 RPM
How can i get it with in those operating ranges I'm thinking about replacing the prop with a lower pitch say a 17 P.
I also played with the engine angle and this is the best I got.
thank you in advance for your feed back.

 

[boobie]

Re: 1974 Evinrude prop giving low RPM

The first thing to do is make sure your tach is correct and then start switching props.

 

[Bifflefan]

Re: 1974 Evinrude prop giving low RPM

After you check the tach, read this.

 

[glenn property of pam]

Re: 1974 Evinrude prop giving low RPM

After you check the tach, read this.

+1 unless we know what the problem is then theres no ans. "the operating range calls for 5500 - 6000 RPM" thats the max rpm range. reving the engine higher doesnt always mean extra top end speed it has more of a affect on how fast it gets to that speed and the amount of fuel used.

 

[Texasmark]

Re: 1974 Evinrude prop giving low RPM

Unless you have a light, easily planing 15-16' boat your 19 is probably too much prop. I had a '75 70 on a Kingfisher brand pretty much flat bottom bass boat and was very light for a bass boat....built before the advent of the current $40k cruisers. I ran a 19P on it but had it jacked up one notch also which made a big difference in how the 19 performed and I usually fished alone in reasonably calm water.

Mark

 

[Urbina]

Re: 1974 Evinrude prop giving low RPM

After you check the tach, read this.

Its a 70 Hp Evinrude 1974
it has a 12 3/4 19 Pitch prop aluminum
the boat is a 1974 glastron 17 ft Tri-hull
the problem that I have is that I'm not within optimal operating range.
my tachometer is brand new its a tele-flex and just installed it last week.

 

[steelespike]

Re: 1974 Evinrude prop giving low RPM

I think a 15" prop may help out. It seems that a 19 is the biggest size available here at Iboats.
A 15 could produce about 30.1 at 5700
a 17" could produce about 31.7 at about 5300.
My info indicates rpm range is 5000-6000 rpm.
The 15" prop would be peppier.

 

[jimmbo]

Re: 1974 Evinrude prop giving low RPM

Before you spend time swapping props, make sure the engine is in good shape as in, good compression, timing properly set, timing is fully advancing and carbs are opening fully, Might require throttle cable adjustment. Also is the engine set at the proper trim angle? That too will affect WOT

 

[Urbina]

Re: 1974 Evinrude prop giving low RPM

Before you spend time swapping props, make sure the engine is in good shape as in, good compression, timing properly set, timing is fully advancing and carbs are opening fully, Might require throttle cable adjustment. Also is the engine set at the proper trim angle? That too will affect WOT

Yes Sir you are correct I just rebuilt my carburetors and now I am sure my engine is working 110%
I took the boat on a spin and changed the prop to 13 1/2 inch 15 pitch and I'm still getting 5100 rpm
do you guys suggest i try to reduce the diameter or go to a 13 pitch ????

 

[Georgesalmon]

Re: 1974 Evinrude prop giving low RPM

The 13 should get you up to 5500ish. That would be good.

 

[steelespike]

Re: 1974 Evinrude prop giving low RPM

You dropped 4" in pitch and only gained 200 rpm.Should be closer to a 700 or 800 rpm.
It would help to have your gps speed.Some cells have a gps app.
Perhaps it is a poor setup. Is the anti vent plate(just above the prop) about even with the bottom.Pictures would be good.
Does the boat respond to up trim?
Does the boat seem to lay down like its glued to the water?
Boat should get up and plane on the rear 1/3 to 1/4 of the bottom.
Are you sure the link and sync is set right?

 

[jestor68]

Re: 1974 Evinrude prop giving low RPM

Only changing 200 rpm with a 4 inch pitch change is virtually impossible. One inch of pitch will always result in 150-200 rpm change; "with any normally set up" boat/motor combination.

Either you're reading the prop sizes wrong or your tachometer is wrong. Are you certain that you set the tach for a 2 stroke motor?

Your 17 ft Glastron is supposed to have a dry weight of about 1200 lbs ( boat&motor ). Any prop selector will come up with a 17/19 pitch prop recommendation for that combination with a 2.42:1 gear ratio.

Even if your motor is low on compression(power), it will still respond normally to a pitch change.

 

[glenn property of pam]

Re: 1974 Evinrude prop giving low RPM

does the boat have the power trim option or is it set by a pin. it sounds like the trim is either pushing the front down causing extra drag or is set to far up causing the prop to break free. when on plan the steering should fell light and responsive if its heavy then try trimming the front up. if the front is bouncy then trim it down. if its a manual pin then clean the pin so it fits easy then test run. after a run tilt and lock the motor then you should be able to lean over and pull the pin and change position and test again. from experiance on that type of hull and motor i would expect about 5800-6000 rpm with the 15' pitch with 2 people on board.

 

[Texasmark]

Re: 1974 Evinrude prop giving low RPM

Only changing 200 rpm with a 4 inch pitch change is virtually IMPOSSIBLE! One inch of pitch will always result in 150-200 rpm change; no matter what the boat/motor combination.

Really! I just dropped my prop from a 24P to a 21P and my rpms went from 5600-5800 to right at 7000 as long as I left the throttle at firewall which I didn't. That's 3 inches and 1200-1400 rpm.....dahhhhh!!!!!

Nothing sir is always. Everything is condition dependent!!!

Have a nice day, sir.

Mark

 

[jestor68]

Re: 1974 Evinrude prop giving low RPM

Really! I just dropped my prop from a 24P to a 21P and my rpms went from 5600-5800 to right at 7000 as long as I left the throttle at firewall which I didn't. That's 3 inches and 1200-1400 rpm.....dahhhhh!!!!!

Nothing sir is always. Everything is condition dependent!!!

Have a nice day, sir.

Mark

The point of your post is?

That we should forsake the time honored expectation of 150-200 rpm per inch of pitch and go with 433 rpm per inch of pitch(based on your median 1300 rpm change) with your set up?

I totally accept the fact that some folks are capable of totally missing their set up, and get results that defy logic.

In spite of your unusual experiences, I will continue to suggest 150-200 rpm per inch of pitch for those folks who intend to keep their prop in the water(normal set up).

Please note that I edited my previous post to take into account the "dahhhh" factor.