# Engine Noise



## tomahawk (Mar 9, 2012)

I talked about the issue in my bragging thread, but I figured I would put a thread here as well.
2009 20 HP Nissan 4 Stroke.
The motor ran good, got up on plane well, no porpoising, topped out around 24-25 at 3/4 throttle. The only issue is at full throttle (while running, not out of the hole) it sounds kind of like its hitting a rev limiter (I don't know if this motor has one or not). If I back off the throttle it stops. It sounds kind of like a miss with a slight backfire maybe. Its hard to describe. It runs well at all other throttle positions. The prop is a little beat up. I have it set on the third hole up from the bottom. Ideas are welcome.
In the videos below you can kind of hear it, but not very good.
Ignore my wifes comments :



https://www.youtube.com/watch?v=QlqM9SR1WL0

https://www.youtube.com/watch?v=364EPuf4P3E


----------



## topnative2 (Feb 22, 2009)

I looked at the videos and pics...........I am a little confused regarding the mounting height of the engine---some pics look ok some look like it is to high.

Rule of thumb cavitation plate 2" above bottom depending on the loading

I know that on larger outboards ,when the limiter kicks in, the engine stumbles w/o a backfire.
Check a manual.

Next choices are: 1.timing to advanced
                         2. fuel starvation 

I would drop the engine down first and run it.
$$$$ pick


----------



## tomahawk (Mar 9, 2012)

This is a new motor for me. It didn't run when I bought it. The old guy that I bought it from said it had 10 hours on it and it looks it. 

It had set for a couple of years. I rebuilt the carb. 
From the tank to the motor it has new fuel lines. It did have a cracked line inside the cowling. I did temp fix it with a slightly larger line until I can either get one from boats.net or find one local, which so far no one carries one that small.

I took my file to the prop to smooth it out some, nothing extreme though. I'm going to get a tiny tach so I can check rpm's.

Here is a video of it running. (More of wifey's commnets to  :... Does the motor position look good? The rooster tail looks pretty big to me. There is a Doelfin on it and the AV plate is 1" above the bottom. It is set on the 3rd hole from the bottom.

https://www.youtube.com/watch?v=RsWl4P1aK5A

I've always had PTT's in the past that I bought set up and this is the first time I've ever rigged a skiff from scratch.

Thanks for the reply.

Edit: We were posting at the same time. I have a manual and its pretty vague on some things. It was a disappointment for me. If its timing I guess I'll have to take it somewhere.
As far as the backfire sound I heard that once and it was faint as it was windy and I was running about 25 when it happened. I tried to keep it at 3/4 throttle for the 2 hours we had it out, but it was hard  

Also to reiterate, it only does it while already running on plane and you twist it to full throttle. Out of the hole it seems fine.


----------



## topnative2 (Feb 22, 2009)

Rooster tail could be caused by the trim angle of the motor--to much tilt 

Do you have any blow out in a turn????

Blow out is caused by the motor mounted to high on the transom or to much trim.

However , looking at the attached vid I think the motor may be to low especially if u have a dolefin.
The boat has a lot of bow rise. 
The dolefin should be knocking water down.

Running the boat , sit down and look at the lower unit and see where the water is----u should see the dolefin above the water, If it is covered the motor is to low.

Do you have any weight in the front?

Stay off the full throttle till you get the engine height correct.....

Cutrunner(moderator here) should be more helpful since he is a mec-i-nic. ;D


----------



## cutrunner (Jun 8, 2010)

Sounds like the Rev limiter. Tint tach will tell for sure


----------



## Brett (Jul 16, 2008)

Sounds like it's topping out to me also.
Since the old prop is dinged up, use it as a backup
and invest in a new prop with more cup and/or pitch.


----------



## tomahawk (Mar 9, 2012)

Thanks guys. I'm hoping its the limiter and not a problem. As far as weight goes, the fuel is up front and I have a Pelican cooler in the front as well. The design of the boat has a strong bow curvature so it even sits a little bow high at rest also.
No blowout, she does slide a little with her flat bottom though 
The motor is as high as it goes. I built a jackplate for it but was advised by the designer not to use it because of the COG change, which makes the bow ride even higher. Not sure how to proceed. 
I will check rpm's first.
Thanks again


----------



## tomahawk (Mar 9, 2012)

It was the limiter. The tach confirmed 6200 at WOT...

I'll start a new thread in the prop section.

Thanks guys.


----------

