1990 140Hp Johnson grinding noise

[mstahl1171]

Motor ran fine all day. One my back in to the dock...at idle speed..the motor suddendly made a grinding noise for a quick second then it went away...kinda like someone running their finger nails down a chalk board. The motor ran fine for a few more minutes till I got back to the dock. Afterwards I decided to check compression to make sure i have not scored cylinder walls...etc. My reads where 120 psi on 1,2,&3 #4 was 95 psi. I look to see if the wall was scored...no luck. Is this reading fine or should I tear the head down? Motor seems to run fine...plugs look good. VRO pumps oil. Maybe I am worring too much??

Thanks

 

[racerone]

Pull the cylinder head off and inspect # 4 ---This is not going to fix itself and will only get worse and more expensive !

 

[mstahl1171]

Will do....if the walls look good, what else should I check for? What are common causes for the compression to drop?

 

[M Strawn]

Stuck or broken piston rings could do that.

 

[mstahl1171]

before I tear the head down...I decided to borrow a bore scope from our mech shop and look inside the cylinder for scoring and piston head damage..I also put oil in the cylinder and let it soak overnight just incase it is the piston rings stuck. I'll check and scope this evening and keep you posted. Maybe I'll get lucky...who knows...anything is possible...and since its a free tool use and quick check...why not.

 

[mstahl1171]

before I tear the head down...I decided to borrow a bore scope from our mech shop and look inside the cylinder for scoring and piston head damage..I also put oil in the cylinder and let it soak overnight just incase it is the piston rings stuck. I'll check and scope this evening and keep you posted. Maybe I'll get lucky...who knows...anything is possible...and since its a free tool use and quick check...why not.

 

[mstahl1171]

Update: Bore scope revealed no cylinder wall or cylinder damage. Check the carbs again to make sure they were not restricted..all good. Verified that the VRO and oil lines are free and clear....all good. Ran motor again on seperate tank with a 24:1 mix...compression came up to about 105 psi...but still heard the grinding noise for a brief moment..im pulling the head tomorrow and checking to see if there is something obvious..like stuck or worn rings. Maybe a bearing is going bad or something else. Hopefully I know after tomorrow.

 

[Faztbullet]

Lower bearing may be failing as seen several that lost inside retainer.

 

[mstahl1171]

So i was getting ready to tear the head off when a neighbor friend stopped by to see what I was working on. Anyways...he had me warm a pot full of marvels mystery oil and pour into the cylinder with it just below TDC. I let it sit for several hours, cleared the cylinder, ran motor for a few to warm up the engine...wow...118 psi. I repeated and also ran on a seperate tank with SeaFoam. Now the cylinder is up to 125 psi and the others are now at 122 psi. Maybe the piston rings where stuck and carbon build up on the reeds for # 4 cylinder?