Mercruiser 5.0 wont start without help getting gas to carb. But runs fine once started...

KRU78

Recruit
Joined
Jun 1, 2021
Messages
4
My 5.0 will not start. Unless I run the fuel pump by throwing power to it separate. But once I do get it started I can let it run on its own and it runs fine. I am not a carb guy any help or ideas much appreciated!!
 

Kola16

Petty Officer 1st Class
Joined
Jun 23, 2019
Messages
203
It would help to know what the specs of your engine are. For example, year, serial number, checking the fuel pressure between the carb and your fuel pump, is this a new problem and it ran fine beforehand or did you just buy this engine, etc....
 

KRU78

Recruit
Joined
Jun 1, 2021
Messages
4
My parents used it all last year and said no problems at all until they went to pull it out of the lake to store. So it ran fine all last year and runs fine once you get it started that is why I am so confused. I will get the serial number off... Fuel pump is new and so is filter.
 

KRU78

Recruit
Joined
Jun 1, 2021
Messages
4
My parents used it all last year and said no problems at all until they went to pull it out of the lake to store. So it ran fine all last year and runs fine once you get it started that is why I am so confused. I will get the serial number off... Fuel pump is new and so is filter.
It has the J1223 on it
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
7,138
I assume it is a newer (1996 and up) vortec with electric fuel pump. The fuel pump gets powered I think off a lug on the starter for cranking and then once key goes to run it gets 12 v off the ignition circut through an oil pressure switch (closes at 4 psi- engine not running = no oil pressure= no power to fuel pump).

One of the smaller wires Yellow/purple) coming off the starter soleniod is where the power comes from- likely either the starter contact is no longer good or you have a loose or corrdoded wire. Once you prime the carb with fuel by jumpering it and the oil prssure switch supplied 12 v takes over it runs.

If your carb bowl is empty from sitting no fuel is availbe and it wont start if fuel pump isnt running during cranking...Either trace out wire off the starter or you can wire a momentary switch to bump the fuel pump for starting and fill the bowl...basically what you are doing now but splice it into the harness...
 
Last edited:

KRU78

Recruit
Joined
Jun 1, 2021
Messages
4
I assume it is a newer (1996 and up) vortec with electric fuel pump. The fuel pump gets powered I think off a lug on the starter for cranking and then once key goes to run it gets 12 v off the ignition circut through an oil pressure switch (closes at 4 psi- engine not running = no oil pressure= no power to fuel pump).

One of the smaller wires Yellow/purple) coming off the starter soleniod is where the power comes from- likely either the starter contact is no longer good or you have a loose or corrdoded wire. Once you prime the carb with fuel by jumpering it and the oil prssure switch supplied 12 v takes over it runs.

If your carb bowl is empty from sitting no fuel is availbe and it wont start if fuel pump isnt running during cranking...Either trace out wire off the starter or you can wire a momentary switch to bump the fuel pump for starting and fill the bowl...basically what you are doing now but splice it into the harness...
Makes sense. Will take a look tomorrow evening. I appreciate it in advance!
 
Top