Mercruiser 120 Wont stay running

Froggy44

Cadet
Joined
Jun 2, 2010
Messages
6
OK just started to get boat ready for summer At first there was no spark to plugs replaced points, condenser, distributor, rotor and now has spark engine fires but will not stay running. Has gas and new filters not sure what else could be wrong. will run if i stay on the starterbut shuts down as soon as i let off. Thanks for the help guys
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,363
Re: Mercruiser 120 Wont stay running

ballast resistor or the resistance wire is bad ,not allowing the coil to see power in the "run" position
 

Froggy44

Cadet
Joined
Jun 2, 2010
Messages
6
Re: Mercruiser 120 Wont stay running

How do I go about fixing that? Is this the one wire that goes to condenser under cap? Thanks Again for the help.
 

Froggy44

Cadet
Joined
Jun 2, 2010
Messages
6
Re: Mercruiser 120 Wont stay running

I dont see a ballast resistor before the coil Thanks Again
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,363
Re: Mercruiser 120 Wont stay running

1st, go to the Adults Only section and download the proper manual . Depending in the year , the ballast resistor is a porcelan looking item mounted on the rear of the engine and has 2 wires ,usually purple or purple/yellow. Later models used resistance wire built into the harness.
 

13adluck

Cadet
Joined
May 20, 2010
Messages
8
Re: Mercruiser 120 Wont stay running

I had the same problome with my 120 last week. the prob with mine was that i had replaced my starter with new. The new starter lookd exactly the same except the new starter had only 2 wire terminals. The old had 3. I was told an auto mechanic that i could put both the yellow and the brown wire on the small terminal. i tried this and found that moter would start only while hitting the key. would not stay running. i ended up disconnecting the small brown wire and left the yellow on. some how leaving the smaller brown wire disconnected fixed the issue and motor now runs great. Maybe this will help you out. maybe not. ?
 

Froggy44

Cadet
Joined
Jun 2, 2010
Messages
6
Re: Mercruiser 120 Wont stay running

I want to check and see if the coil is getting juice when the key is in the run position first I know it does in the start position because the engine fires but if it doesnt have juice in the run position it wont stay running right.
 

Froggy44

Cadet
Joined
Jun 2, 2010
Messages
6
Re: Mercruiser 120 Wont stay running

Dont see any porceline ballast so how do I test the resistance wire to see if it is that the boat is a 85
 

turbinedoctor

Petty Officer 1st Class
Joined
Sep 7, 2009
Messages
225
Re: Mercruiser 120 Wont stay running

Switch could be bad. You can check both the resistance wire and switch at the same time. Check voltage to the positive side of the coil with the switch in the run position. If you have voltage both are good. If not one is bad. You will have to follow the wiring diagram in the manual you downloaded earlier and test each wire to tell which one is bad.
 
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