120 Mercruiser runs good for 20 min then starts missing

KRAKR

KRAKR

Recruit
Joined
Jun 21, 2010
Messages
2
I have a 1969 Mercruiser 120 HP inline 4 on my boat, and it runs very well while hooked to the hose. Once you put it in the water it runs awsome for about 20 min then it will start caughing out of the carb, and missing really bad. Acts like a car when it's out of gas, but if you shut it off and let it sit for 10 min (about one cigarette) it runs awsome again for about another 20 min.

I have changed the points, condenser, cap, rotor, plugs and wires. I don't think it's the carb because I have fiddled with the adjustments on it while it was acting up and it changed nothing.
 
EddiePetty

EddiePetty

Lieutenant Junior Grade
Joined
Aug 25, 2008
Messages
1,008
Re: 120 Mercruiser runs good for 20 min then starts missing

...measure your coil primary voltage when it is running well to establish a base line. When it begins to act up, measure the coil voltage again. I'll bet the coil is hot and the primary voltage is low/erratic.
 
N

newcdnboater

Seaman Apprentice
Joined
May 14, 2008
Messages
40
Re: 120 Mercruiser runs good for 20 min then starts missing

I had a very similar issue on my '87 3.0 merc. I had installed new points, condenser, distibutor and set everything to spec. Turned out the condenser was failing when the temp went up. Put the old condenser back in....problem solved. Good luck.
 
KRAKR

KRAKR

Recruit
Joined
Jun 21, 2010
Messages
2
Re: 120 Mercruiser runs good for 20 min then starts missing

Well here is the skinny. It was the accelerator pump in the carb. I bought a rebuild for it this morning, cleaned and rebuilt the carb, put it on and it runs like a 120 should. :D

Thanks for all the input.
 
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