Vibration at high throttle - 1977 Evinrude 4hp Weedless

[99cyclone]

Hi All,
I recently picked up a 1977 Evinrude 4hp with the weedless lower unit for my duck boat. Being a 1977, it has the electronic ignition. The engine starts and runs nicely, but seems to vibrate excessively when the throttle set higher than the start position.

If I hold the throttle at its absolute max, the engine is smooth, but even a minute decrease in throttle from full causes the vibration to return. No funny noises are present, but the control handle does vibrate/shake uncomfortably. Power delivery does not seem interrupted while in this range.

I have compared it to a friend's weedless 4hp (also a 1977) and his runs much more smoothly throughout the full throttle range.

Just as background info regarding maintenance: The outboard was checked out by a marina earlier in the summer for the previous owner. He hadn't used it much since, but had run it some. I ran fresh gas through it along with some Sea Foam. I pulled the spark plugs and cleaned them until I get around to ordering some new ones. Compression measurements were 95psi on one cylinder and 100psi on the other. It appears to be running on both cylinders (both were at full temp while running in a barrel, as measured with an infrared temp sensor).

What could be causing my vibration? Has anyone seen anything like this before?

The plastic prop shows some wear, but no damage. I have ordered a new one, as I'm sure some thrust has been lost due to the normal wear. Could this be a lower unit vibration rather than engine related?

 

[Rick.]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

If it were the transmission, I would expect you would find the vibration would be present through the entire speed range. I was going to suggest swapping props with your friend but if you have a new one coming, wait and see if that helps. One thing it might be is a bent prop blade but not likely if it is made of plastic.
These are fantastic little motors and will run all day on a gallon of fuel. Best of luck. Rick.

 

[crxess]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

Agree with above. Also, the 77 weedless uses a pin drive prop. After pulling the prop inspect the pin for twist.(bent) A bent pin will cause the prop to oscillate and vibrate.

Just replaced a bent pin on my weedless trolling motor. Vibrated horribly at times and fairly smooth at others. Now running Smoothly again.

 

[99cyclone]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

Thanks for the input. I will test it out again once the new prop shows up. I will post back here with the results.

 

[Daviet]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

By your name I am guessing you are a cyclone fan or grad. Sorry about the Iowa/iowa state game outcome, maybe next year.

 

[F_R]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

Thanks for the input. I will test it out again once the new prop shows up. I will post back here with the results.

The flywheel could be bent. Especially if somebody has removed it without the proper kind of puller. That would be consistant with the harmonics vibration such as you report.

 

[99cyclone]

Re: Vibration at high throttle - 1977 Evinrude 4hp Weedless

I received the new prop yesterday and installed it today. I also found quite a wad of junk (nylon string, fishing line, vegetation) wrapped around the prop shaft underneath the plastic splined coupling.

After cleaning all of that out and installing the new prop, I started it in a barrel. The vibration I was seeing is reduced drastically. I will need to get the boat out on the water for a true test, but it seems much better.

Comparing my old prop to the new, there was significant wear on the old blades.