what the hell have I done wrong

well I've tested the bulb with petrol and tubes and a jam jar and it certainly pumps in the right direction but also spits fuel out of the inlet - which causes the flooding carb
 
Yes it sucks the air / fuel thrugh the carb then pumps it into the tank.
The tank is and should be pressureised so the engine can work when the tank is lower than the carb.
Pinch off the return line & press the bulb .
It should not move much if the non return valves / valves are working.
If you can depress the bulb fully with th e outlet to the tank blocked off then the primer / purger is defective
If the primer can back flow into the carb it will depress the metering needle so pressurised fuel can squirt into the venturi depending upon the actual model carb and where the internal check valves are.
 
Sorry but even if the primer does back flow into the metering chamber it will not open the metering needle as the diaphragm must be pulled down and not pushed up. It will however will send fuel through the main nozzle and the mixture ports.

The metering diaphargm also only opens the metering needle to control fuel flow. For fuel to flow either the tank must pressurize or the fuel pump diaphragm be pumping fuel due to the impluses from the engine and the metering diaphragm pulls in due normal vacuum created in the carburetor ventura so the amount it pulls depends on how strong the vacuum is.
 
Just trying to have correct info posted. The better you understand how these carburetors works the better you can troubleshoot their operation.

Let say you have one you just rebuilt (installed a complete carburetor kit in) that still doesn't work. What would be the cause of no fuel in metering section even through you primed it? Now the engine would start and die.

Matter of the fact I just repaired a Stihl 028 chainsaw with similar problem. The only difference is no primer. The beast was very hard to start and was later dying after 15 minutes use. Now what was the two causes of this problem?
 
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OK, you have me on that one.
I can not see how the carb would prime / purge and then not have fuel in the metering chamber as the only way in is through the pump & metering needle.
Cubes that start OK then peter out have usually been either an impulse loss or a tank pressure failure.
Had them fail both over pressure cause the vent was blocked or under pressure cause the lines or cap were leaking.
The fist chainsaw that had me beat for months was an over pressureization which caused the carb to flood out.
In desperation I popped the colour tune in so knew it was flooding but over pressurization was not a situation I was familiar with back then.
 
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