Bike won't run

I've owned my 72 Combat for over 25 years now and it's never missed a beat, until recently.

I switched to a single 32 MM Amal a few years ago for ease of tuning. A month ago it began to get hard to start and then would only start and run if the choke was fully applied. As soon as the choke was released the engine stopped.

Considering the carby was 40 years old , I decided to buy a new "premier" amal carby which I fitted this afternoon. Guess what, I still have the same problem, the bike will not start or run unless the choke is fully applied.

All manifold bolts are tight, carby is bolted up tight and there do not appear to be any air leaks.
The throttle slide and choke slide are operating correctly
Compression is good
New spark plugs
Boyer ignition
Fuel taps are not blocked

????

Condensation water somewhere getting into bowl?
Pilot jet crud?
Fiberglass tank dissolving resin deposits?

Hi Hobot, carby is brand new today, no condensation I can see, have a steel tank

regards, Neil

know you checked it but a bike only running on choke is classic sign of a manifold leak, especially if 2 different carbs yield the same results- have you tried starting the bike and spraying either carb cleaner (carefully) on the manifold to see if it speeds up or a bit or WD40 and see if it decreases-will help tell if you have a leak?

Good point for me to keep in mind Mike, air leaks, thanx.

Check fuel flow to carb.

I agree with a vacuum leak. I assume you've got a 2-into-1 manifold. Is there a spigot on the manifold? If so, is the boot airtight? O-ring on the carby flange is good? No cracks in the manifold or manifold-to-head gaskets? carb-to-manifold flanges are not warped? Manifold-to-head flanges are not warped?

If you have a spigot on the manifold, did you hook up a vacuum gauge? If so, what vacuum reading did you get? Should be in the 24cm range, minimum.