Engine rattleing noise at high RPM

[TrophyFisher]

I have a 1996 Force 120 Hp outboard with low hours. It runs great but yesterday I noticed a noise at high rpm. It sounds like it is a rotational type noise- a type of rattle. I only notice it at high rpm near top speed. any ideas what this could be?

 

[Jiggz]

Re: Engine rattleing noise at high RPM

Symptoms like you mentioned are just too vague to even make a conjecture. Try running the engine with the cowl off and then try localizing the source of the "rattle". As soon as you have an idea where it is coming from or at least a general idea, then stop the engine and start checking for loose items. It is the only way to find out. Rotational type noise will start mostly (if the source is in the powerhead area) loose flywheel key, starter pinion loose and bouncing up and down hitting the flywheel, loose stator, loose trigger plate, etc.

 

[San Diego Ed]

Re: Engine rattleing noise at high RPM

I also have a '96 Force 120 that is making the noise you describe. There are only two things in the outboard I am aware of that can make a sound like this: worn engine bearings or worn outdrive gears. If worn engine bearings it would probably have a bad rod knock at lower rpm. My current guess is the lower end may have been abused earlier in its life and is showing signs of wear, but still trying to complete the diagnosis.
Ed

 

[labawg]

Re: Engine rattleing noise at high RPM

If this sound is slightly metallic and momentary becomes louder when you back-off from WOT, then try increasing your fuel mixture around 1/8 turn and test if there is a decrease in sound. Could be slightly lean.

 

[San Diego Ed]

Re: Engine rattleing noise at high RPM

OK I figured it out. My motor was under-propped (13" dia x 15" pitch). Because the rattling sound occured below the max 5250 rpm I thought it was something else. I put on a 13" x 19" prop and the noise went away, and the motor now performs better throughout the rpm range. I may have even been able to go to a higher pitch but was afraid of losing bottom end power, which right now I can't even notice. In hindsight, a prop incorrectly matched to the engine power curve puts irregular stresses on the drivetrain, thus the rattling noise.

 

[jerryjerry05]

Re: Engine rattleing noise at high RPM

Called lugging the motor.
Pre ignition?
Throwing the timing off?
Make sure this prop is the right one.
You should be able to run about 53-5500rpm's at wot.
Too much of a load and it will affect the results.
2 adults a cooler maybe a kid and 1/2 tank of fuel.
Test and report the results.
If your running a 19 and need a 17 your gas consumption can go up as much as 20-25%(I did this on my boat)

 

[San Diego Ed]

Re: Engine rattleing noise at high RPM

What I didn't mention earlier is that the motor would easily spin to 6000 rpm and beyond! The rattling sound would come and go at different rpms. But cruising with a full boat at 30-35mph made the nasty rattling sound at 5200-5600 rpm. It now cruises about the same speed at 4800-5200 rpm.

 

[jerryjerry05]

Re: Engine rattleing noise at high RPM

If your motor goes to beyond 5500 at full speed and load the prop is wrong or the tach is reading wrong.
Hey Ed maybe start your own thread.They get upset if you hijack someone elses.