1990 90 hp Johnson V4 cylinder burn down

[jaylo600]

Re: 1990 90 hp Johnson V4 cylinder burn down

Thermos on these are absolutely NUTS! Absolutely my lease favorite job. It is almost easier to pull the powerhead to get to the thermos than fight with that cramped, ridiculous engineering design.

I can see what you mean, fortunately I have my powerhead off and the bottom shroud off also, nice and easy now ! lol

 

[toddschubert]

Re: 1990 90 hp Johnson V4 cylinder burn down

I can see what you mean, fortunately I have my powerhead off and the bottom shroud off also, nice and easy now ! lol

I found that loosening the lower cover helps a little but that's no fun either. Small fingers and tiny incriments

 

[BrianCrosley]

Re: 1990 90 hp Johnson V4 cylinder burn down

I just picked up a winter project yesterday and tonight I dove into it. Like the title says I have a melted top of one of the pistons but the other 3 look great. I knew the engine had problems but I bought the boat on a gamble. I was just wondering if anyone could tell me why only one piston went bad and not the one underneith it also ? I apologize as I don't know wich # cylinder it is. I have read the good and bad about the VRO systems but there is no way that it was to blame in this incident. Is it possible there is a dirty carb and it caused a lean condition in one cylinder only ? I will become more educated on this project as I go but I'm trying to pick some brains out there so I know what common things to look for on this motor. Thanks for reading !

ANSWER=Your #1 top Piston melted because your timing is advanced.

 

[emdsapmgr]

Re: 1990 90 hp Johnson V4 cylinder burn down

I really doubt the timing is the issue. If it were advanced, other 3 cyls should also show distress and they look fine. Likely the bad cyl ran very hot, which basically means it ran lean-due to the misplaced water diverter. That mis-shaped diverter has completely closed off water circulation around that cylinder.

 

[BrianCrosley]

Re: 1990 90 hp Johnson V4 cylinder burn down

ANSWER=Your #1 top Piston melted because your timing is advanced.

I have been working on these motors and rebuilding them for 25 years, and this is what I have found, it may be beneficial to you to get your timing checked.