Small 4 stroke sound question
 

Hi all,

I just bought a 20hp Merc 4 stroke to replace my 15hp 2 stroke Yamaha. I have had small 2 strokes in the past and all would make a sound like marbles bouncing around in the upper rpm range. The 4 stroke also makes the marble sound. I was very surprised. Is this normal for 4 strokes as well? Is not detonation. Engine calls for 87 octane and I'm running 91 non ethanol fuel.

well i,ve herd a lot of descriptions but never marbles to describe a 4 stroke outboard! are there any rolling around in the boat?

Cant help you with your marbles but if the engine calls for 87 that's what you should be running. It will probably run better and use less fuel.

lol only marbles rolling around are in my head:bigbird:Maybe marbles wasn't a good description, more like a rattle. Went out again yesterday and didn't hear the sound.

Only reason I put the 91 octane in we were tubing and figured the engine would be loaded. I can't get ethanol fuel by my house and seen it at the pump and said what the heck. I knew it could hamper performance a touch but figured it would be safer with the engine loaded. Don't want to damage it the first few trips out. The fuel economy is great. This is a fuel injected engine. Definitely better then the 15hp 2 stroke It replaced. Probably burned 3 1/2 gallons of fuel with 3 trips out. Drove 50 plus miles with one day of tubing. Engine came with a 10 pitch prop. With 2 on board it runs 27mph over the max rpm. 4 adults runs 24 at a safe rpm. Hooked up a tack to verify. Have a 11pitch stainless prop on order. Should run 30ish mph. If it still over revving they sell a 12 pitch. With 4 adults on board the 10 is perfect. Might but a 9 for tubing. Still have more testing.

I know this is a performance boat web site, but I enjoy my inflatable for exploring. :silenced: Well enough of my rambling. Back to the lake now to enjoy my vacation.

That's for the help guys!!!
https://cimg8.ibsrv.net/gimg/www.off...64dd59c585.jpg