Slight popping sound when going up on plane

[~kev~]

30 hp Evinrude, around 1996 model.
15 foot alumaweld boat

Last summer I noticed a slight popping sound, which was barely noticeable when the boat goes up on plane. If I go slow and ease the boat up on plane there is no popping sound. The pop does not sound like metal on metal, it sound more like plastic. At first I thought it was my fuel tanks pulling a vacuum, then popping when the air pressure stabilized. But I am doubting that theory.

I recently did a carb tear down and clean as there was lots of trash in the carb. Fuel lines from the carb all the way to the fuel tank were replaced.

There is very little slack in the prop back and forth movement, no more than usual. I would say the prop moves back and forth maybe an 1/8 inch.

Oil in the lower unit is changed twice a year or so. Boat is used in fresh water, mainly on rivers.

Any suggestions?

Pic attached of motor on local river.

 

[JL8Jeff]

I get that once in a while with my Mercury. I think it's either water in the gas or air getting into the fuel line. Try disconnecting and reseating the fuel line at each end and look closely at the o-ring to see if there is any sign of air bubbles. I do run a water separator and I still get it from time to time. I've heard other boats out on the river doing the same thing so it does make me think it is fuel related since it happens to all brands of motors.

 

[Pappy]

Since the rate of acceleration makes a difference it may be "oil canning" of the aluminum hull somewhere?
Try stepping on and off the cavitation plate while in the driveway and see if you can duplicate the issue/sound.
I would not chase down the above fuel suggestion. The carb bowl just needs to be full and the fuel pump on the engine makes sure it is or a simple squeeze of the primer bulb.

 

[airshot]

Being an aluminum boat perhaps air bubbles getting trapped under the hull upon acceleration???? Being a diehard aluminum boat fan, I have heard weurd noises on ocassion from the hull at various speeds but especially faster speeds

 

[Still Afloat]

It could be the beer cans rolling around in the cooler. Try using more ice.

 

[JL8Jeff]

I've had the popping noise on my fiberglass 15' with a 40 hp prop motor, my aluminum 16' with jet outboard and heard it from other boats with fiberglass inboard jets. That's mainly why I think it was fuel related. I could feel the "miss" or loss of power while it was popping as well so maybe it could be spark related as well.

 

[RaisedByWolves]

Since the rate of acceleration makes a difference it may be "oil canning" of the aluminum hull somewhere?
Try stepping on and off the cavitation plate while in the driveway and see if you can duplicate the issue/sound.
I would not chase down the above fuel suggestion. The carb bowl just needs to be full and the fuel pump on the engine makes sure it is or a simple squeeze of the primer bulb.

I was thinking transom related.

OP what year is the hull? If you can see the transom wood does it look discolored?

 

[Pappy]

JL8jeff - In here we try not to send folks who are asking for help down the proverbial rabbit hole money and time wise.
As far as your theory goes about the fuel line it holds zero merit. You can fill the fuel bowl by pumping the bulb and then disconnect the hose completely and accelerate the boat/engine on plane. As long as the fuel bowl has fuel it doesn't care if there is a fuel line attached. Yes it will run out of fuel but it will run for wayyy longer than just getting on plane. If you doubt me then read my profile and I will read yours.....fair enough?

RaisedByWolves......fine minds think alike! Or at least it is a common sense and experienced approach.

 

[thill]

Is it a power trim motor? Does the motor make popping sounds and/or jumps a bit when you raise and lower it? Like it's sticking?

I had a motor that made those sounds, and the nuts on the ends of the steering tube, that the motor pivots on when going up and down, were crazy tight, and it needed lubrication.

I loosened it, greased it up and tightened it back up to a reasonable degree, and the popping sounds went away.

I've also had boats pop when the transom was not good, and it was coming apart. I recommend that you try to find it, maybe by listening closely while someone else is driving.