Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

[galveston]

I have a Johnson 15 hp outboard on a 14ft welded v-hull wide body. At 90% throttle and under it runs excellent, but on full throttle it makes a noise as if its hitting a rev limiter. I ran 93 octane through it the last time on the lake and the motor kicked at full throttle. Thats the best I can explain it. Here is an older video where you can hear what I am talking about.

Listen at 00:18 you can hear an wah wah wah wah wah oscillation.
http://www.youtube.com/watch?v=b-2e3DW7g6Q

Is the motor revving too high, is this normal, can i fix this?

 

[HighTrim]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

You likely have a ventilation/cavitation problem.

Ventilation is simple. It is when surface air or exhaust gas from your through hub is drawn into the spinning prop blades. Since the prop is now pushing mainly air, the load on the outboard is reduced, causing the outboard to over rev and the prop to spin fast enough to cause cavitation.

Your motor could be mounted too high, you may be trimmed too far out, or your prop or cavitation plate could be damaged.

 

[galveston]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

You likely have a ventilation/cavitation problem.

Ventilation is simple. It is when surface air or exhaust gas from your through hub is drawn into the spinning prop blades. Since the prop is now pushing mainly air, the load on the outboard is reduced, causing the outboard to over rev and the prop to spin fast enough to cause cavitation.

Your motor could be mounted too high, you may be trimmed too far out, or your prop or cavitation plate could be damaged.

Interesting...the motor was trimmed all the way in. I noticed when I got up to speed to it probably needed to be trimmed out but it was too late as you have to pull and reinsert a pin to do so and I didn't have time to re-trailer it. Also, the prop is not exactly pristine, its has some scratches on it.

 

[HighTrim]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

Scratches wouldnt cause that. Take a pic of your setup from the rear, eye level with the bottom of the boat if you can. Has is always done this since you have bought the boat, or is this new?

BTW, no need to waste your money with 93 octane

 

[galveston]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

Scratches wouldnt cause that. Take a pic of your setup from the rear, eye level with the bottom of the boat if you can. Has is always done this since you have bought the boat, or is this new?

BTW, no need to waste your money with 93 octane

Its always done this. Yea I will take a picture tomorrow. I figured $0.40 extra for 2 gallons of premium gas wasn't that bad.

 

[kbait]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

Spend the extra on non-ethanol fuel, if available. Search for 'ethanol' in this forum to see why.

And your issue sounds like a set-up issue rather than engine itself. If it's a '74-'76, check the armature plate for lateral play, if there's any, it'll change the point gap and potentially cause mis-fires, but I don't believe that's what I heard on your vid. Good luck!

 

[galveston]

Re: Johnson 15 hp outboard makes noise/kicks at WOT (video) Help me diagnose.

Spend the extra on non-ethanol fuel, if available. Search for 'ethanol' in this forum to see why.

And your issue sounds like a set-up issue rather than engine itself. If it's a '74-'76, check the armature plate for lateral play, if there's any, it'll change the point gap and potentially cause mis-fires, but I don't believe that's what I heard on your vid. Good luck!

I had no idea that E10 was bad. Thanks for the warning.