5.0 Stalling at cruise

jotisbrown

Seaman Apprentice
Joined
May 9, 2016
Messages
39
Mid 80's 305 GM crate motor, not sure of the year still looking for the serial on the motor. Carb is off the original mercruiser 260 that cracked the block, 305 was installed at that point. Boat is a 78 Cobalt 19CD, bought in June, ran 5 times for short periods never burning more than an 8th of a tank. Outdrive is an alpha I think? The serial numbers are gone off the sticker in the back, but I can read the gear ratio of 1.5 R. Pics below.

When purchased had 50 gallons of at least year old fuel... dropped in a couple cans of seafoam to compensate. Has ran great until yesterday with only minor sputtering at times when under hard acceleration which I attributed to the old fuel. Fuel sending unit was replaced, at which point I was able to see inside the tank a little, very clean and shiny aluminum in there. Temp stays around 165-170, oil pressure stays right in the middle, though the gauge has acted up a couple times the last time we went out. Well either the gauge is messing up or Im really running no oil pressure while cruising? Ill be testing the gauge today.

Yesterday was easily the most we have run the boat, ran around half a tank of fuel out of it with no issues. Around sunset we were getting ready to anchor and I noticed at cruise the boat was running rough, I thought I had trash in the prop but a quick inspection and it was clean and spun free in neutral. Got back it, cranked right up and cruised easily. A minute later it stumbled and died at cruise, I tried to easy back to neutral but the motor was already dead. Cranking was difficult, (Thats a whole different story but when the motor is hot that starter DOES NOT want to turn over the motor. The battery is no spring chicken so I will be replacing it as well, but it could just as easily be the starter... or the old battery cables in the pic below. Bought new cable but it wasnt long enough... Ordering a new zero gauge cable today)

After repeating this procedure of cranking, cruising, and dying, we tried slow... The motor ran rough but it held 5mph without dying and that got us back to the marina.

So where do I start? Im pretty handy, and have rebuilt a few jet skis from the ground up so I think I can figure this out with some help. What manual do you recommend? Im looking to order one today, but as it isnt technically a "mercruiser 305" I didnt know if that would make a big enough difference???

Im thinking today I will replace the fuel water separator as thats cheap and wont hurt. Ive got around 20 gallons of boom boom juice left in the tank, thinking about transferring (some) that to my wifes car and let her toyota corolla deal with it.

As for spark stuff, the distributor I will give a once over but I am not sure what to look for. Hints and tips on that are welcome. I can already see that the connections on some of the electrical is shotty as hellllllll and need to be redo and/or cleaned up in places.

OH and my dang shift boot is tore. Gonna have a lot of boat projects this fall....
 

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jotisbrown

Seaman Apprentice
Joined
May 9, 2016
Messages
39
It also appears it could be a bad coil, as it runs great, gets hot, and starts acting up.
 

alldodge

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Staff member
Joined
Mar 8, 2009
Messages
40,581
Pull the filter off and dump contents into a clear container, see what is in the filter. It running along and falling on its face it could be the pump is failing. Using the link you posted to see what the fuel pressure is will be a good thing to do.

With the motor going from cranking OK, getting overheated and now has trouble cranking, there could be some issues.

Did notice you have corrosion and cleaning/replacing items is another good point
 

jotisbrown

Seaman Apprentice
Joined
May 9, 2016
Messages
39
No overheating issues, just read that coils that are going bad tend to work okay when cool and get worse as they warm up. Ive pulled my coil, going to get it warmed up and see if the ohms read the same. I hate to try more than one thing at a time, the lake it only 15 min away so I will try the new coil most likely and pour out whatever is in that fuel/water separator. Ill send pics
 

jobrown

Seaman
Joined
Jun 16, 2011
Messages
53
No Title

Will be testing this tomorrow after work!
 

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