I recently rebuild my 1980 Mercruiser 470. It has a 2-brl Rochester carb. When starting it runs just fine and takes throttle for 2-5 minutes but then it stalls out and won’t start for some time.

I don’t think it’s the ignition. Everything is new accept of the coil. Points gap is adjusted and timing ist right.
The carb just got rebuild. Fuel lines aren’t clogged up and the carb still has fuel in it after it stalls. Chokes fully opened when it’s warmed up.

My guess is the carb. Just before stalling I hear hissing from the carb area . especially when I try to apply throttle to keep it running. Is it some kind of vacuum leak? But why is it running just fine at first?

I got a spare mercarb but it needs a rebuild too.

Has anybody encountered something like this before? I am clueless

Is there fuel being squirted in by the accelerator pump when you move the throttle?

Thanks for the reply!
Yes the accelerator pump works. Even after it stalled it squirts fuel when I move the throttle.

Some electrical ign parts can fail when heated-up, but don't think that's your issue.

If fuel was left inside the float bowl/s for an extended period, when it evaporates, it leaves behind residue. This residue can plug-up small orifices. The idle circuit is the first thing to get plugged since the holes are much smaller. When eng starts cold, the choke is closed and as eng warms-up the choke starts opening. If the choke opening coincides with the eng wanting to stall, you can try closing the choke partially and see if it recovers. If so, it's likely the idle circuit is plugged or partially plugged and will need to be cleaned-out. However, if there's a vacuum leak, the symptoms can be very similar to plugged idle circuit in the carb (not enough fuel for the amount of air entering the intake).

The choke being closed will block a lot of the hissing sound you are hearing from the butterfly valves in the carb.

I'm assuming the idle mixture screws haven't been tampered with. If they are screwed in too far, that can make it run lean also.

This is what I would do:
Verify the idle mixture screws are set according to spec. Try adjusting choke so eng stays running after warmed-up. Check for vacuum leaks, If none found removed and rebuild carb and ensure idle circuit passages are clear. When removing carb, double check the base gasket under the carb is installed correctly. Sometimes they can be flipped over and look OK at a glance but passages won't line-up right and can cause a vac leak..