Things to check: So we need three things for the engine to run, 1) Air 2) Fuel 3) Spark Timing of these is essential and the assumption is that the timing is correct.
Sounds like you have two of three, air and spark however the fuel seems to be restricted. From the description it sounds like the engine runs until the fuel in the carburetors is gone then a trickle of fuel eventually fills then up over time. Check that the fuel line flows enough fuel to keep up with the needed.
Answers & Comments
Things to check:
So we need three things for the engine to run,
1) Air
2) Fuel
3) Spark
Timing of these is essential and the assumption is that the timing is correct.
Sounds like you have two of three, air and spark however the fuel seems to be restricted. From the description it sounds like the engine runs until the fuel in the carburetors is gone then a trickle of fuel eventually fills then up over time.
Check that the fuel line flows enough fuel to keep up with the needed.