Shuts down after 1 minute

[Marty Levenson]

Wondering where to start troubleshooting. Engine ran great in June, but now, after recent exhaust rust out and rebuild, starts fine and then dies after one minute or less....then won't restart. When I come back a few days later, same behavior.
Shut down is sudden. I see plenty of oil pressure when I'm cranking.(yes, closed the water intake). Turns over strongly.
Engine is fresh water cooled, electronic ignition, electric oil pump.
OPSS? Seems too quick to be an overheating coil. Seems like an electric problem, but not certain.
1- tempted to jump the OPSS
2 - if that doesn't fix it, replace the coil
3 - check the spark at the main coil lead.
Any suggestions are gratefully received!

Thanks.

[tenders]

Are you sure the fuel valve is open?

[edwardc]

One minute is about the time (depending on throttle setting) that the engine will run on the gas that is in the carb float bowl. Check that your fuel pump has sufficient pressure AND flow.

[ndutton]

Marty, it seems to me the first thing to do is determine if it is fuel or spark related. Either are easy to determine at the time of shutdown. Only then can you take a logical approach to troubleshooting rather than guessing.

[Al Schober]

Agree with Neil - Give it a shot of starting ether when it quits, then try it. If it stays dead, it's spark. If it roars for a bit, it's fuel.

[joe_db]

The ether trick works.
Also I have to throw in the 534th recommendation for a fuel pressure gauge. Mine has paid for itself over and over again.

[ndutton]

Agreed, the fuel pressure gauge is an effective diagnostic tool but its effectiveness stops at the carburetor inlet. You could have good fuel pressure with a stuck float valve (or other internal carburetor malfunction) and still have a fuel related shutdown.

I'm rebuilding a 5 bolt carb for another forum member and the new needle valve body is sticking in the new seat. I did not install the small wire clip that attaches the needle to the float lever because none of my other carbs needed it but it's going back on so the dropping float pulls the valve open.

[Marty Levenson]

Fixed

Thank you for your replies...always helps me get into a diagnostic frame of mind rather than "try this", "try that".

Pulled a plug and saw there was a spark when cranking, so assumed a fuel issue. Thought about the fact that there was no problem before messing around with replacing the heat riser. Next checked the choke and throttle cables as they were disturbed in my wrestling with the exhaust manifold, but both looked okay and moved normally when the controls moved. Was tempted to pull the fuel hose and check for flow to see if the fuel pump or filter were faulty....which would point to the carb next if not. Decided it was easier to check the opss wires first, and ta-da....one was off its lug....no doubt it got yanked when I was messing with installing the exhaust riser. Easy fix once I deployed some logic.

Thanks again for the suggestions.