1990 OMC 4.3 process of elimination

[Hvac00]

It’s my father-in-laws boat. 1990 four Winns with the omc 4.3 cobra. Was out on the lake last weekend when his boat just made a clunk and locked up. Got towed him to the cove and pulled the plugs and there was no water in the cyclinders. Put plugs back in and boat would run for about 30 seconds. Now that I’ve got it to my house. Pulled plugs checked for water again and there was nothing. Did a compression test. All cyclinders but one we’re 160-170. Cyclinder 4 was 145. Checked flappers to they weren’t stuck and good. Did find that the oils velocity was very runny but not milky. Also found that motor isn’t pumping water. Could that possibly be a reason for motor to shut off like it is?

Any help is much appreciated. Know my way around a motor but I’m stuck on this one.

 

[alldodge]

Did find that the oils velocity was very runny but not milky

With the oil being thin, my first thought is your fuel pump diaphragm has ruptured and is leaking gas into the oil. Oil pressure goes down and shuts the motor off

 

[Scott Danforth]

would start with draining the oil. does it smell like gas?

AD has you on the right path with fuel pump if your oil smells like gas

 

[Hvac00]

Oil did have a little gas smell to it. I will check the fuel puml

 

[Hvac00]

Ok so I’m finally getting to work on it again. Noticed it wasnt pumping water as well. Checked impeller and it’s good. Does the pump have to be primed if I drained all the water out?

 

[Wkd179]

no you don't have to reprime pump.
How is the gearoil?