Saving Old Seagulls

Hi,
I have posted my engine problem under "Unruly Featherweight" in the General Topics section. Unfortunately, I have not been able to diagnose the problem with this engine and am out of ideas. I have twice disassembled this engine completely trying to find the problem and have been unable to. I have checked the carb 4-5 times including the main jet, float, needle, fuel line, filter, and vent on tank. The breaker points and plugs are new and the coil has been tested twice. The spark has always been bright and blue. I have run the engine on 10:1 and 25:1 after changing needles and there is no difference. The water galleries are clean and there are no visible leaks in the back or head gaskets. The crankcase has no visible leaks at the interface between the upper and lower halves. There is no oil leakage on the top crankcase bushing but perhaps some on the lower.
The problem has always remained the same. It starts after the 3rd pull and runs fine at all throttle position for 4-5 minutes, then starts to run erratically sounding exactly as when the fuel has run out of the carburetor and then quits. However, there is plenty of fuel in the carburetor and even if you try to flood the carb it still quits. If you try to restart it, it will start after 2-3 pulls but will die within 15-20 seconds. If you wait a few hours, it will start up again but only run for 4-5 minutes. The time for it to quit is very predictable. I am not sure where to go from here. At this point, I fear that as the engine warms up something expands and starts leaking but it is not visible from the outside.
Any help is appreciated.
Marcelo

I had exactly the same problem with an EFNR. I replaced the barrel and never had a problem since. I suspect a hairline crack in the transfer port which opened up as it heated which is not visible to the naked eye.

Thanks for your reply. I am wondering now where it would leak to. The compression has stayed at 125psi even when the engine is warm and after it has failed. However, this intake port is on the back side behind the piston so if there is a leak there it would not be reflected in the compression. I may remove the barrel, heat it up with a torch and see if I can tell anything, mostly for my own interest.

Could you post the serial number for us? And confirm carburettor type please?

Can you confirm the bit about plenty of fuel in the carburettor after it dies - have you (immediately after dying) cut off the fuel and had a look by undoing the float chamber)

Apart from the filter on the tap and the one at the carburettor banjo are there any other filters?

The serial number is F 639F7. I have a Villiers carburetor. I have checked the fuel level after the engine quits by shutting off the fuel tap and removing the nut on the bottom of the float chamber. Fuel will pour out around the bottom nut.
The fuel supply has 2 filters which are clean (fuel tap and banjo).

Immediately after it dies does it still have a good spark?

Yes it does. I have checked this twice.