Bad knock and then not...Hydrolock?

[jonny rotten]

1990 Mercruiser 4.3
Boat has been running flawless and never had any issues internally.
After taking out the boat without issue I came home and put it on the muffs as usual. When cranking it over the boat made loud knocking sound. I immediately shut it down. I fired it up again and it happened again. Shut it down and went inside and told the wife I think the motor blew. I went back about an hour later and it fired up out without a knock. I ran it for about 5 minutes and revved it up to 2000 RMP and let it sit like that for about 30 seconds. I sounded perfect, purring away.

I'm trying to figure out what could have caused it. The only thing I can think of is the water pump, but that seems like it would be more of a grinding noise and continuous so I'm ruling that out.....or Hydrolock from bad risers. The risers are old and have some rust between the manifold. I'll be changing them ASAP. Is it possible some water got in there caused a bad knock, then seeped out of the cylinder and it ran great? Is that even possible? I've never experienced hydro locking. Oil was changed last fall and is at the proper level and looks new on the dipstick with no milkyness
The knock sounded like it came from the top of the motor but is hard to tell being it shut it down quick.
Could it be a coil issue with timing that went out and then came back somehow?
I know it a tough diagnosis but I'm nervous there is internal damage and if not I want to correct whatever it is so it never happens again

 

[Rick Stephens]

Takes a whole lot of water in the oil, and it being totally emulsified into the oil, for it to show at all on the dipstick. If you have an oil suction tool, pull a little off the bottom of the oil pan and look at it.

 

[dubs283]

Could be simple as a flooded engine upon initial startup, considering you ran it on muffs after a day of the water. A leaking carb would flood the engine, especially after being used.

What is your maintenance history? Tune up, carb inspection, etc?

 

[Scott06]

1990 Mercruiser 4.3
Boat has been running flawless and never had any issues internally.
After taking out the boat without issue I came home and put it on the muffs as usual. When cranking it over the boat made loud knocking sound. I immediately shut it down. I fired it up again and it happened again. Shut it down and went inside and told the wife I think the motor blew. I went back about an hour later and it fired up out without a knock. I ran it for about 5 minutes and revved it up to 2000 RMP and let it sit like that for about 30 seconds. I sounded perfect, purring away.

I'm trying to figure out what could have caused it. The only thing I can think of is the water pump, but that seems like it would be more of a grinding noise and continuous so I'm ruling that out.....or Hydrolock from bad risers. The risers are old and have some rust between the manifold. I'll be changing them ASAP. Is it possible some water got in there caused a bad knock, then seeped out of the cylinder and it ran great? Is that even possible? I've never experienced hydro locking. Oil was changed last fall and is at the proper level and looks new on the dipstick with no milkyness
The knock sounded like it came from the top of the motor but is hard to tell being it shut it down quick.
Could it be a coil issue with timing that went out and then came back somehow?
I know it a tough diagnosis but I'm nervous there is internal damage and if not I want to correct whatever it is so it never happens again

Pull plugs out and crank it over hydro lock would push water out of plug holes

 

[jonny rotten]

Could be simple as a flooded engine upon initial startup, considering you ran it on muffs after a day of the water. A leaking carb would flood the engine, especially after being used.

What is your maintenance history? Tune up, carb inspection, etc?

How do I test if the carb is leaking? I really haven't done much in maintenance besides oil changes. I rebuilt the carb a couple times, the last time being a couple years ago. New fuel pump last year. It has been a little rough starting, sort of sputtering bogging. the last few times with a cold engine. When it warms up it runs and idles fine. I thought there may be something up with the butterfly or linkage? While is running rough it seems like I have more blueish smoke on start up than normal with more exhaust smell. Actually I think normally there is no smoke and minimal smell, So there must be something up with the carb.
When this knocking happened the motor was still warm as it was on muffs 20 minutes after running on the water

 

[dubs283]

It's great you've paid attention to the carburetor, but have you done any tune up maintenance? Plugs, cap/rotor, wires, fuel filters, check/set timing?