Constant Knock in 120 mercruiser

[scott killmon]

Hey guys, I have a 1981 120 Mercruiser that is running fine but after wot last weekend developed a knock. I pulled valve cover and without spark turned engine over, everything seems to be fine. I then ran the engine and started to remove one plug at a time, noise changed on cilinder #4 but did not go away completly. Like i say it starts on a dime and no lack of power just knocks. Any thoughts on my next step in troubleshooting this futher or sugestions on what could be wrong? Thanks

 

[thumpar]

Re: Constant Knock in 120 mercruiser

Rod bearing maybe. Try using a stethoscope to pin point the location. If it is in the lower end then rod bearing or pin most likely.

 

[Coors]

Re: Constant Knock in 120 mercruiser

Valve lash ok? check compression, that match other cylinders?
Then piston pin or rod.
A rod will stop knocking when letting off of pressure(throttle); pin will stop noise with no spark plug firing.
Main bearing knocks under a load.

 

[Silvertip]

Re: Constant Knock in 120 mercruiser

Hydraulic lifters -- hence no valve lash. Noise would be rather "ticky" and definitely not a knock if a collapsed lifter or loosened adjustment nut were the culprit. Is oil pressure reading ok?

 

[scott killmon]

Re: Constant Knock in 120 mercruiser

Thanks for the quick responce guys, and sorry for my delay. I am going to do a compression test,lash and all that good stuff but i think from what everyone said it is going to take some internal work. So i have decided along with friends to have a blow the boat party!!!! Going to be fun. I have a 140 engine that i have held on to and if im going to dump more money its going to be in that. Keep you posted on how long it takes.


scott

 

[fishmen111]

Re: Constant Knock in 120 mercruiser

My money's on the rod. Scott, I would not intentionally blow it. It takes all of 2 hours tops to pull that motor. Take nothing off...flip it over...pull the pan and for $80, put in new rod and main bearings, oil pump and slap it back in. If the motor is really worn out, you will blow it anyway and be out $80. If it is relatively in spec., I have had motors last for years with the "backyard rebuild". Just a thought.

 

[BAYLINER185]

Re: Constant Knock in 120 mercruiser

I agee dont blow up the motor.

Granted motor and drive are seperate parts BUT what if something goes horribly wrong and you wind up ruining the drive in the process? I woldnt want to take that chance. Drives cost way too much and with this crap economy its not worth risking more out of pocket expensive!
Put the 140 in and call it a day....The 4th of July will be here way to soon and you will want a good running boat for that day on the lake!

 

[thumpar]

Re: Constant Knock in 120 mercruiser

Or it could break apart and put a hole in the bottom of the boat.