Low WOT on 1975 Evinrude 70 hp

[jdaghir]

I recently acquired a 1975 24' Riveria Cruiser pontoon. It's powered by a 1975 Evinrude 70 hp outboard, long shaft, mechanical shift, model 70573E. After doing some work on it, it starts easily, idles smoothly and accelerates well. However, it tops out at 3200 rpm and around 12 MPH per GPS at WOT, which is well below the 5000 to 5500 rpm it is supposed to do.

It is currently propped with a 3 blade aluminum prop, 13" diameter x 19" pitch, in moderate but not perfect condition. The motor is mounted in the middle of its height adjustment and the anti-ventilation plate rides about 2-3" under the surface of the water at WOT. I occasionally get a small amount of ventilation when I hit chop or waves.

The pontoon has an aluminum hard top on it and it looks like it has been re-floored with 3/4 pressure treated plywood, along with having homemade benches/seats from 3/4" treated. It also has a house battery, a start battery and a trolling battery, a trolling motor along with both a 6 gallon and a 20 gallon fuel tank, so I suspect it weighs more than it did from the factory. After a day in the water the logs do have a small amount of water in them, maybe a pint each (they have bungs in them allowing them to be checked and drained).

I've checked the compression and all three cylinders were within a few pounds of each other and in the low 100's (don't recall the exact numbers).

I had one weak ignition coil so I replaced it and all three cylinder now jump a 7/16" gap with a spark tester.

I ran a can of Seafoam mixed with a gallon of gas through it in an attempt to decarb it. Both tanks have fresh gas with some Seafoam in them.

I rebuilt the fuel pump, however it is actually a fuel pump from a 2004-2009 40-90 hp. I assumed that since it is for the same horsepower range it should supply sufficient fuel. Pumping the fuel bulb at WOT does not change the speed. The fuel bulb and hoses appear to be in good shape.

I cleaned and rebuilt all three carbs including replacing the high speed orifices as all three had been stripped by the previous owner (had to use a screw extractor to get them out - luckily the threads in the float body weren't damaged).

I did a link & sync following the factory manual. I found it a little confusing at first as the procedures were somewhat spread throughout the manual. Basically I did the following:

1. Removed the throttle cable from the throttle lever.
2. Loosened the cam roller screw and moved the cam roller away from the cam.
3. Loosened the carb throttle link fastener screws, allowing the throttle valves to completely close via spring pressure.
4. Tightened the throttle link fastener screws to synchronize the throttle valves.
5. Adjusted the throttle stop screw so that the throttle valves were horizontal and fully open at WOT.
6. Confirmed that the timing pointer aligned with the 0 degree mark on the flywheel with #1 cylinder at TDC.
7. Set the throttle cam yoke to an initial position of 4-31/32" on the throttle rod.
8. Attached a timing light to the #1 cylinder and adjusted the idle speed adjustment screw so that the timing was at 0 degrees while in neutral and running in the water.
9. Moved the cam roller into contact with the cam, and adjusted the throttle cam yoke position so that the lower mark on the cam lined up with the center of the roller, and then tightened the cam roller screw.
10. Readjusted the idle speed adjustment screw so that the motor ran at about 780 rpm while in gear, idling in the water.
11. Adjusted the throttle cable so that it applied light pressure to the throttle lever to ensure that it fully returns to the idle speed adjustment screw and reattached the cable.
12. Adjusted the warm up lever stop screw in the control unit so that the cam roller aligned with the start mark on the cam when the warm up lever was raised.

I set the WOT timing at 17 degrees as specified in the manual by using the Joe Reeves method and verified it by using a timing light while a friend piloted the boat at WOT. It was set higher, around 22 degrees or so. The timing base seems to move smoothly without sticking both when moved by hand and by the throttle.

I'm at somewhat of a loss as to what to do from here. I'm guessing it needs a different prop but 2000 rpm seems to be a lot to pick up with just a lower pitched prop and I would like to be certain the motor is running as it should be before throwing money at new props. Should I raise the motor to the top set of holes even though I already ventilate occasionally? Should I replace the fuel pump with the correct one for this year? Should I pull the intake off and examine the reed valves? Did I do the link & sync properly? Is there something else I'm missing?

Sorry for the long post but I wanted to provide as much detail as possible, hoping some of you with more experience would be able to make some suggestions.

Thanks in advance for sharing your knowledge and wisdom!

-
Jeff

 

[phipps]

Re: Low WOT on 1975 Evinrude 70 hp

You need a lower pitch propeller. We just got a 50hp Evinrude for our 24' pontoon and it has a 17 pitch prop and is only turning about 3800rpm. I have been suggested to go to a 13x11 prop to increase the RPMs. Sounds like that is your issue as well.

 

[phillnjack2]

Re: Low WOT on 1975 Evinrude 70 hp

you need a 13x13 pitch prop, or a 12x 15 pitch.
A 24ft boat is a bit big for a 70hp engine, and the revs you are getting is about right as no way will that size prop plane a boat that big.
with a much smaller prop the revs will climb up and the power will develop.
personaly I would go 13x13 and vent the prop to let it spin up and get revving, it could then get to the plane and into the 20's.


phill

 

[jdaghir]

Re: Low WOT on 1975 Evinrude 70 hp

Thanks for the advice Phipps and Phill. I'm actually glad to hear it is most likely a prop issue. I like to tinker and fix things, but I've had my fill on this particular motor for a while. I'll get either an 11" or 13" pitch prop on order and report back with the results after I try it out.

Thanks!

-Jeff

 

[jdaghir]

Re: Low WOT on 1975 Evinrude 70 hp

Huge improvement. I went with a Solas Amita 13.25" x 13" x 4 blade aluminum prop. I chose the 4 blade in the hopes that it would have less slip than a 3 blade and better mid-throttle economy at the expense of top speed and acceleration. The boat now hits 4800 rpm and 20 mph at wot. Idle speed is slower also which makes docking much less "exciting". And I was able to raise the motor to the top notch without ventilation issues. A prop with a 9" pitch would probably be dead perfect, but I couldn't find a 4 blade prop with that low of a pitch. Next season I'll pick up a 3 blade - 9" pitch and see how it compares.

Thanks again for the help!

 

[fireman57]

Re: Low WOT on 1975 Evinrude 70 hp

I have to give you kudos for probably one of the most informative posts. Your original post is a thing of beauty. A long read but didn't ramble just all the facts. It's nice to read something like this rather than the usual "I have a 35hp that doesn't want to run. Help please". Nice job Jeff.

 

[phillnjack2]

Re: Low WOT on 1975 Evinrude 70 hp

no 9 inch is way too small.it would rev like crazy and give about 12mph flat out, that's only for very slow river running.

next prop to try is a 12 diameter x 13 or maybe 12x15 pitch 3 blade.
get a cheap one off fleabay and try it, you don't really need 4 blades, but that's up to you.
you really need a couple of hundred more revs, ideal 5200 and you'l get that by smaller diameter without losing the top end so much.
you should eventually get to about 25mph no problems, and save a fortune on juice when its all perfect.

once you have found the ideal size then you can upgrade to nice stainless prop and get even better results.
props are a strange thing to play around with, and it can be a very expensive trial and error thing, so get used or borrow
if you can.
any of the 3 cylinder omc will fit and even the early v4 and the later 75 and 90 etecs, so youl be able to get someone with
a prop to let you see if it works for free or a crate of beer !!!!!

crate of beer can work wonders for trying a friends prop ha ha ha and if no takers, then you have the beer,

its a win win situation ha ha


phill

 

[Chinewalker]

Re: Low WOT on 1975 Evinrude 70 hp

If you can drop another size in pitch your RPM will be right in the sweet spot. Do NOT go up in pitch. Your motor will be happiest and last longest with the RPM capable of peaking around 5500. 4800 revs is still borderline lugging (akin to driving uphill while towing a trailer, in high gear).

 

[phillnjack2]

Re: Low WOT on 1975 Evinrude 70 hp

I still think the diameter is the problem.
he is trying to push a big boat with the engine, and by dropping the diameter will give the engine a chance to build up the revs.

By dropping to 12 diameter he could go up in pitch easily,
the 13 1/4 pitch is way too big a prop diameter for pushing a big heavy boat around.
its a 24ft long pontoon that has planning ability, its not a slow cruiser that needs slow torque type prop !!!!!!!
so he needs to come down in diameter to let the engine rev, and then see if raising the pitch will give him more top speed.
by using large diameter the engine is not getting a chance to develop and get up into its power band.

if this was a 15ft speedboat it could handle the 13 1/4 diameter, but its a big 24ft pontoon, so pitch is only half the problem.
20mph is terrible top speed from a 70hp engine, if he drops down pitch its going to still worse and be too low max speed.
by dropping the diameter and going for something like a 12 x 15 the revs will come up and so will top speed.

its all going to be trial and error, but no way on earth to go down anymore in pitch unless you don't want to plane.

9 inch pitch will get you a fast walking pace and engine screaming its nuts off.

wish you were close to me, I would let you try any of my 7 different props for this engine.

your gear ratio is 2.42-1, with anything less than the 13 pitch its going to be rubbish and waste huge amounts of fuel.
as it is you have not got a cruising speed to stay on plane, with more pitch and less diameter you should be hoping to
cruise at 20mph with at least a 24-25mph top end easily.
the boat should be on plane at around 3750 to 4250 rpm, and planning speed is around 18 to 20 mph I would think for your boat.

try and see what other pontoons with same size engines are running, it will give you a good starting point.

and do not seek 4 blade props, just look for 3 bladers until the max speed prop is found, then you can see if you really would
benefit from a 4 blade prop.
personally would not go with 4 blades for a pontoon with a 70hp.

must admit the 4 blades do look nice, but not always efficient for every application !!!

phill