1979 70hp Evinrude: Loss of Power

[dallie]

1979 70 hp evinrude has loss of power. "mechanic" has rebuilt carbs, set timing, run compression(125+/- all three). He thought that would solve problem but did not. He now thinks exhaust leak from gasket where Powerhead mounts lower unit is the culprit. The motor runs great on muffs and in water on low speed, but when throttled up to full speed the engine begins to bog down and never makes it past half throttle. I can remove motor cover and throttle up and can get a little more power and barely plane out the 15' fiberglass runabout.
I do see some exhuast smoke coming from that area. Could that gasket be the problem? If so how difficult is that to replace. I have moderate mechanical skills and think I can remove Powerhead and do the job myself.

 

[eavega]

Re: 1979 70hp Evinrude: Loss of Power

Before you go tearing into the engine looking for an exhaust leak, make sure you are properly propped and trimmed for your setup. Check the motor height, trim angle, etc etc. Make sure that you know the weight on your boat and what its max HP rating is. I have seen 15' fiberglass boats that have a max HP of 90, and some with as little as 65. You may be inadvertently overweight due to equipment or waterlogged foam. While this presents bigger problems, at least it is not the motor. You may need to pitch down your prop to bring up your RPMs.
Oh, and an exhaust leak would tend to strangle the engine a bit. If you are seeing exhaust in the vicinity of the motor and its not simply blowing in from the exhaust relief ports to make it look like its coming from the motor you need to track that down.

Rgds

 

[dallie]

Re: 1979 70hp Evinrude: Loss of Power

Thanks for the reply. The boat is rated for 85 hp. I purchased this used motor to replace the old blown up 70 that was on it for years. It has been running well with this motor for several days but with a "muffled" sound. Then the lost of power occured. I found this mechanics card at a conveinence store and had him do the troubleshooting. He did the carbs and other adjustments mentioned. The motor still has the loss of power and he suggested removing the powerhead and replacing the gasket, etc.

 

[outlaw241]

Re: 1979 70hp Evinrude: Loss of Power

did you replace the nozzle gasket's inside the carb bowls[small gasket about 1/2" diameter and has 3 tabs on the id of it] if these are buggered up it will bog at about half throttle.

 

[dallie]

Re: 1979 70hp Evinrude: Loss of Power

Yes those were in the rebuild kit and were replaced

 

[J79R]

Re: 1979 70hp Evinrude: Loss of Power

I am having the same issues. I have not had any work done to it yet though. It will reach full rpm's when out of gear, but not while in gear. definately bogs down at half throttle. I will keep you posted if I can figure anything out.

Thanks!