1975 MERC 165 (6) ENGINE VIBRATION AT 1700+ RPM

[sprintin]

This season, I went to start the 'ol girl up and she fired up fine (after having to replace the fuel pump). She starts and idles without problems. There is no misfire or rough idle.

But when I rev to about 1700 RPM, an immediate noise comes from the engine. It sounds and feels like a vibration. It does not increase in speed or intensity if I were to keep accelerating, but once I drop below 1700 it goes away. This is all while parked with the muffs on. I didn't want to pull the prop and put her in gear to see if it would also happen. I instantly remembered this happening when I was on the water last year for a split second under load, but then it went away. Between that day and last week, I've run it at high speed without incident.

I pulled the drive belt off to see if maybe the water pump or alternator pullies/bearings were causing the vibration, but she still did it.

The best I can describe the sound is if you went to a playground and climbed the big toy, there is usually a big metal wheel that the kids pretend is a steering wheel. If you were to spin that fast enough, it would begin vibrating. That's what this sounds like - like a moaning vibration.

It's not a click, clunk, rattle, clank, ping, knock, hum, or hiss.

It's whatever sound a vibration makes. It sounds ridiculous, but I can't figure out what this might be.

Any help would be appreciated! Thank you!

 

[Alumarine]

It might be the gimbal bearing or ujoints.
You should pull the drive and inspect them.

If that's not conclusive you can run the motor without the drive attached if you hook up a water hose.

 

[CamaroMan]

check the front damper - might be wobbling.. engine mounts? sure firing on all? one cyl missing makes it run VERY rough. Removing belts doesnt eliminate the damper. Get a timing light and check each wire is firing perfect.

Possible electronic break down at higher rpms.. bad/fouled plug?

alot of side movement on damper indicates its gone - get it replaced, those things can sink ur boat if they come loose!

 

[sprintin]

Thanks for the suggestions. I'll take a look at it Friday after work.

Thanks, fellas!

 

[sprintin]

Alrighty. I checked the damper - there is zero movement. Pulled the plugs, they all looked good but I replaced them anyway. Wires, cap, and rotor were new 2 years ago.

I bought a timing light but won't be able to get to use it until tomorrow. Looks like this will be a pull 'er apart and take a look. I've watched a ton of vids on changing/inspecting the gimbal bearing and u-joints. I'm pretty solid when it comes to mechanic work, but I've never done it before. Seems straight forward enough. It has to be in gear when I disassemble the out drive, correct...?

Thanks again.

 

[sprintin]

After a little more investigation...

The vibration only occurs when the out drive is up (in both neutral and in gear). When I lower it as low as I can, there is no evidence of the vibration. Probably a rookie mistake on running the motor with the out drive up, but...

Thoughts...? What could this vibration be?

Thanks for all the input.

 

[Scott06]

U joints and or gimbal bearing based on drive position changing it . Both are easy fix, as suggested pull the drive and inspect, turn the gimbal bearing by hand, should be smooth if not replace, ujoints can be tougher to check by feel but look for binding or roughness, or just replace they are cheap. There is a u joint cross reference in the stickies above. Also check condition of the bellows, likely water intrusion will cause this.

 

[sprintin]

I think I'll end up pulling it apart to take a look. When i ran it this weekend, the vibration was gone, but there was a constant "Knight Rider" sound coming from the out drive. It sounded like the sound "Knight Rider" makes when his red light goes back and forth, only faster. Just an observation, but I'll end up pulling it apart anyway.

Thanks everyone.

 

[harleyman1975]

Your boat makes some interesting noises!

 

[sprintin]

Alrighty. For anyone remotely interested, I pulled the upper and lower yesterday and pulled the gimbal bearing. Before I removed it, I reached in and spun it and could hear it rubbing. Not terrible, but loud enough to indicate that it was likely my problem. Luckily for me, removing the bearing didn't require drilling or a 13-pound slide hammer (a 5-pounder did the job). Probably a dozen whacks and she popped out.

I looked over the u-joints and, although I ordered replacements, I decided against it. They looked just fine. Greased 'em up and put her back together. Replaced the impeller while I was at it.

Thanks for all the help.