Help with OMC 120

[M0bi0us0ne]

I just swapped my OMC 140 for a 120. I needed an engine ASAP and didn’t have the time or the money for a new engine, sterndrive, and transom, so I swapped in the engine. I’m now having issues with the “new” engine. After about 30 minutes of flowless running, it stutters and shuts off. It usually starts with small stutters and progressively gets worse. After a few minutes off, it kind of restarts. After a day, it started up fine for about 30 minutes. When installing the engine, we replaced:

• ⁠Spark plugs
• ⁠distributor cap
• ⁠points
• ⁠condenser
• ⁠Ignition coil
• ⁠IC resistance
• ⁠plugs wires
• ⁠alternator (from 140)
• ⁠starter (from 140)
• ⁠fuel pump (from 140)
• Wiring harness (from 140)

The thermostat is stuck open, I have already order a replacement, this makes the engine run really cool 110-120 F.
We also cleaned and rebuilt the carburetor.
The sterndrive is a Stringer 400.
I'm out of ideas, and I need the engine to work as fast as possible. All the mechanics I called are either “busy” or don't work on such old engines, or they simply don't show up. Any idea what it could be? Thanks

 

[alldodge]

Carb is most likely needs cleaning

 

[M0bi0us0ne]

Carb is most likely needs cleaning

We cleaned it and rebuilt it with new gaskets, float, needle, and accelerator pump

 

[alldodge]

It ran great for 30 minutes than it started having problems and it keeps getting worst. Could be a sucked up something and clogged up. Could be other things but this is my first thought

Does it run better ant higher rpm than lower (below 1800) RPM?

 

[M0bi0us0ne]

It ran great for 30 minutes, then it started having problems, and it keeps getting worse. Could be a sucked up something and clogged up. Could be other things but this is my first thought

Does it run better ant higher rpm than lower (below 1800) RPM?

It runs for 30 minutes every time; if there was something stuck in the carb, it would constantly run badly. I didn't have time to see how long it needed to sit before it ran again. But every day I go to the boat, try something new, do a test run, where for about 30 min it runs fine, and then I have to limp back to port. I assume it's a heat built up, but nothing feels excessively hot. It feels like an issue I had with the 140 when I was using the wrong coil, but this is the correct one so I'm completely lost

 

[alldodge]

It runs for 30 minutes every time; if there was something stuck in the carb, it would constantly run badly. I didn't have time to see how long it needed to sit before it ran again. But every day I go to the boat, try something new, do a test run, where for about 30 min it runs fine, and then I have to limp back to port. I assume it's a heat built up, but nothing feels excessively hot. It feels like an issue I had with the 140 when I was using the wrong coil, but this is the correct one so I'm completely lost

ok, pump the throttle does it squirt gas good? If so it's not fuel it's electrical

 

[M0bi0us0ne]

ok, pump the throttle does it squirt gas good? If so it's not fuel it's electrical

It does! Is the fuel pump the same for the 120 and the 140? I put the pump of my 140 since it was much newer.
Today it run for 15 min…