1978 evinrude 70hp model 70873 problem

[benny1996]

Hello all I have a 1978 evinrude 70hp model 70873 on a 1990 bass tracker tx17. I added power trim and tilt as it did not come with it.
When I bought the boat it has/still has a 13x15p 4 blade prop and spins up to 5550 at WOT and does about 34mph with just me in the boat. It has a problem tho where some times when I punch it the boat spins up past 5550rpms (closer to 6k) and only goes about 18-20mph but if I get off the throttle then punch it again it gets up to speed just fine. Any ideas what this could be? It has a new prop with the same diameter and pitch on it.

 

[saltchuckmatt]

Hello all I have a 1978 evinrude 70hp model 70873 on a 1990 bass tracker tx17. I added power trim and tilt as it did not come with it.
When I bought the boat it has/still has a 13x15p 4 blade prop and spins up to 5550 at WOT and does about 34mph with just me in the boat. It has a problem tho where some times when I punch it the boat spins up past 5550rpms (closer to 6k) and only goes about 18-20mph but if I get off the throttle then punch it again it gets up to speed just fine. Any ideas what this could be? It has a new prop with the same diameter and pitch on it.

Either you have a spun prop hub or somehow your cavitating the prop.

When you start out and punch it are you trimmed all the way down?

 

[benny1996]

Either you have a spun prop hub or somehow your cavitating the prop.

When you start out and punch it are you trimmed all the way down?

Yes I am trimmed all the way down and the cavitation plate is even with the bottom of the hull. The prop is new(a few weeks old) How do I test for a spun prop?

 

[F_R]

Since it recovers, most likely cavitation.

 

[benny1996]

Since it recovers, most likely cavitation.

Any way to fix this?

 

[saltchuckmatt]

Any way to fix this?

Pictures I f the boat out of the water with motor down.

Picture of inside of prop from the back.

 

[flyingscott]

Is it a short shaft motor?

 

[benny1996]

Is it a short shaft motor?

No it's a long shaft (20in)

 

[benny1996]

Pictures I f the boat out of the water with motor down.

Picture of inside of prop from the back.

Here are the pics you asked for. I realized that when I put a straight edge on the cavitation plate it's about a inch lower than the bottom of the hull

 

[oldboat1]

Shaft length sounds right. If the plate is only an inch below the bottom of the boat, there should be no issues (no drag). You can always raise it up by putting a one inch piece of wood on top of the transom, and reseating the motor.

I would get compression readings if you haven't done that yet. Also, check for overheating when it is acting up (If too hot to touch on top of head, it's overheating.)

 

[saltchuckmatt]

Here are the pics you asked for. I realized that when I put a straight edge on the cavitation plate it's about a inch lower than the bottom of the hull

Everything looks ok....when it over rev'd did it sound like it was over revving?

Might have an electrical issue that made the tach read incorrectly and bog the motor down.

 

[benny1996]

Shaft length sounds right. If the plate is only an inch below the bottom of the boat, there should be no issues (no drag). You can always raise it up by putting a one inch piece of wood on top of the transom, and reseating the motor.

I would get compression readings if you haven't done that yet. Also, check for overheating when it is acting up (If too hot to touch on top of head, it's overheating.)

Compression is 105 on all 3 cylinders. And it has a brand new thermostat/waterpump and theres no blockages in waterpassages. The motor sits around 150-165 degrees after running a while.

 

[benny1996]

Everything looks ok....when it over rev'd did it sound like it was over revving?

Might have an electrical issue that made the tach read incorrectly and bog the motor down.

Yeah it sounds noticeably louder and not as smooth and slight vibration. When it bites and gets on plane and gets up to 30ish mph it has a specific Humm to it. It sounds alot smoother.

 

[oldboat1]

May have water under the flooring, perhaps sloshing back and forth.

 

[Scott06]

Yes I am trimmed all the way down and the cavitation plate is even with the bottom of the hull. The prop is new(a few weeks old) How do I test for a spun prop?

Mark hub and prop with a sharpie, take it out and make the prop slip, see if hub to prop marks move. Possible the hub is bad from factory, also looks like it is nicked up, is blade bent damaged?

also can you take a pic of cav plate but with a 4 ft straight edge coming off the hull ? Looks like hull and cav plate are not in same plane with each other

 

[saltchuckmatt]

Mark hub and prop with a sharpie, take it out and make the prop slip, see if hub to prop marks move. Possible the hub is bad from factory, also looks like it is nicked up, is blade bent damaged?

Yes, I missed that. Your prop is banged up already....in fact your whole gearcase doesn't look so good.

Inspect and correct.....then go test.

 

[benny1996]

Mark hub and prop with a sharpie, take it out and make the prop slip, see if hub to prop marks move. Possible the hub is bad from factory, also looks like it is nicked up, is blade bent damaged?

also can you take a pic of cav plate but with a 4 ft straight edge coming off the hull ? Looks like hull and cav plate are not in same plane with each other

 

[benny1996]

The hull is about a inch higher than the cav plate. And it has a few small dings but nothing major. Mostly from sand. I already marked the hub and prop and am gonna take it out later this week after it warms up a little.

 

[benny1996]

Yes, I missed that. Your prop is banged up already....in fact your whole gearcase doesn't look so good.

Inspect and correct.....then go test.

Yeah the guy before me didn't take too good care of it. I bought new gears for it and plan on replacing them this winter. Also I had the same problem when I put this prop on brand new. So unless it's ruined from factory its something else...

 

[oldboat1]

running a bit too hot (+/- 20 degrees F); good that compression is even, but it is low.