2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

[MonkeyBird747]

Hi everyone,

I'm running this engine on a 18' Starcraft Mariner center console, circa 1974. I tried numerous props before settling on a Turning Point 13x23 stainless 4-blade to get me in the right RPM range at WOT.

I'm getting a knocking noise in the lower unit around the prop area only at idle forward and idle reverse. With the engine off and in gear you can rotate the prop slightly back and forth and duplicate this sound.

I don't know the various gear terminology very well, but it almost seems as if, while in idle forwarad/reverse, the prop slows down enough that there is plat in the gears, and then the engine slams the gears together to get it going again. This happens over and over and sound like a loud knock and causes vibrations.

Bumping up to 1000 rpm everything smooths out. It is definitely not coming from the powerhead, which is buttery smooth at all power settings. Could this prop just be a bad match/too heave? Or could something be wrong with the prop install or internal working of the lower unit?

Any help is appreciated before I have to drag it to the local shop.

Tyrel

 

[racerone]

Is this while running on a hose ?----If yes that it may well be the prop causing the clutch dog to make the noise.----Tested in the water ?

 

[MonkeyBird747]

Thanks for the quick reply.

On a hose and in the water. In gear and engine off I would say there is about 10-15 degrees of rotational play if you turn the prop back and forth by hand. It's as if a constant forward or reverse pressure is not being maintained on the gears to keep them meshed. Instead, this slack is being taken up over and over while in idle gear.

 

[racerone]

Some will argue.-----It sounds normal to me.----Have a look on the parts books for Suzuki and Johnson to see what the clutch dog looks like.-----The pinion gear / forward / reverse are always meshed.----It is the clutch dog that moves between forward and reverse that has that 10 / 15 degree motion.-----And you are aware that this motor was built by Suzuki I hope.

 

[MonkeyBird747]

Ok, looking at the exploded diagram in my Seloc manual I see the clutch dog and see what you mean about always being in mesh. A bad description on my part. I should have said something like "keeping the gear teeth in continuous contact in one direction". The play seems normal to me too, but for some reason I think this play isn't being removed consistently in one direction, and is being taken up over and over in idle gear. I could be way off on this. I'm basing on the fact that the sound made by moving the prop back and forth in gear is the same sound that is being made in gear at idle. That and the fact that the sound is isolated to the gear case area.

As to the Suzuki pedigree, I recall reading something about that when I bought the motor, but had forgotten until you mentioned it.

Hoping to rule out something more sinister before buying a different prop to try.

 

[racerone]

Good luck with your motor.

 

[MonkeyBird747]

Thanks for the replies.

 

[jakedaawg]

I'd start by draining fluid and looking for metal shavings. Almost sounds like you are describing a worn clutch dog except for the works at higher rpm aspect...

 

[racerone]

Take the prop off and test run in forward and reverse.

 

[MonkeyBird747]

Both of these are on my to do list. I’ll report back.

 

[racerone]

What you are hearing may be normal for a heavy prop , and running on a hose !-----No point in wasting good gear oil.

 

[MonkeyBird747]

What you are hearing may be normal for a heavy prop , and running on a hose !-----No point in wasting good gear oil.

Running without the prop first this morning.

 

[MonkeyBird747]

Ok so confirmed a few things. Reverse is worse sounding than forward, and the whole issue isn’t present with the prop removed. Tried a lighter weight aluminum prop and same sound. The sound is coming from right behind the prop, and not deep in the hub. Trying to include some shirt videos. Am I chasing ghosts here?

First clip is neutral idle, second forward idle, and third reverse idle.
View attachment IMG_3577.MOV

View attachment IMG_3578.MOV

View attachment IMG_3581.MOV

 

[MonkeyBird747]

Well, after spending the day boat tinkering I did some more reading on this subject. I see it often referred to as "prop knock". Some say it's pretty normal, some say it could be certain prop combos, some call it clutch dog rattle. One explanation was similar to what I described as the "lash" being taken out of the gears over and over until more load is placed on the prop. There were several dead-end threads that describe exactly what I'm experiencing. Didn't see much resolution other than try a different prop, or bump up the rpm and don't worry about it.

I wouldn't worry if it was just on the muffs, but this happens in the water in idle forward and reverse. It's very noticeable, and would not want to troll for any amount of time at that RPM. I have to get to 800-900 to smooth it out.

 

[Faztbullet]

Ya found the ghost....

 

[MonkeyBird747]

Well some learning occurred during this little troubleshooting adventure. One important thing to note is that I installed a tach when I purchased this motor not long ago. I thought I had the 10 pole/12 pole switch on the back set correctly for this motor but during this idle test I noticed the idle was around 700-750 instead of 600-650. I had chocked this up in the past to being insignificant and maybe within the error tolerance of the gauge. However the switch was not set correctly. The idle RPM difference between the two settings is small, but at WOT the difference is nearly 1000 rpms. And....I used this RPM gauge with incorrect setting to make my prop selections. So, I'm fairly over-propped with this 23 pitch 4-blade stainless, and is probably contributing to the prop knock. Have a lower pitch 3-blade aluminum on order to test. None of this hurt the fishing today! If the new prop makes a difference in the prop knock I'll follow up and close this out.

 

[MonkeyBird747]

For anyone that stumbles upon this, dropping to a lower pitch and lighter weight 3-blade Rubex prop completely solved this. I also gained 4mph.

 

[tphoyt]

Congrats!
And thanks for letting everyone know.

 

[racerone]

With a high speed camera you would observe what happens to that heavy prop.----The engine fires ~ 4 times for every revolution of the ---PROPSHAFT----And accelerates that heavy prop 4 times.----Not visible to the eyes at normal speed.