i think the issue is not the weber regulator but the flow check valve in the tanks themselves.
http://www.propane101.com/valveopennopropane.htm
Hi George,
http://www.weberindia.com/support/faq/faq.aspx?id=1
I should have put this in but I couldn’t find it again until now. The bold section explains why they work fine with the small bottles.
This is from the Weber web site it explains what’s happening and how to fix it but not why it’s such a problem with the Qs. It does appear that it is the regulator shutting down
Frequently Asked Questions My LPG gas grill won't get hotter than 120 to 150 ℃, even with all burners on high. Why?
Since 1995 all regulators (the part that attaches to the gas cylinder to regulate the flow of gas) have included a safety device that restricts the flow of gas in the event of a gas leak. This safety device can be inadvertently activated in two ways, putting the grill into what is commonly called “bypass”. The first way for the device to be activated is to leave one or more burner control knobs in the “ON” position when the LP cylinder valve is opened. The second is not to wait long enough to “start” the grill after opening the LP cylinder valve. The safety device in the regulator is activated each time that the LP cylinder valve is opened. The device resets itself when the gas pressure equalizes between the closed burner control valve and the regulator, through the hose. If a burner control knob is turned on before the gas pressure can equalize, the device will remain in “bypass”. The length of time necessary to wait to “start” the grill after turning on the LP cylinder valve is dependent on the length of the hose and outside air temperature. It is always good practice to wait a few seconds after opening the LP cylinder valve before turning on the burner control knob to start the grill.
Keep in mind that the safety device reacts to a gas leak. If a grill is in bypass, the gas connections and hose should be tested for leaks with a soap and water solution.
If the grill is in bypass, after checking for gas leaks, do the following to get the grill out of bypass:
•The Weber Q100, Weber Baby Q, Weber Q200, and Weber Q cannot go into bypass when used as sold.
•When using an adapter hose with the Weber Q/Baby Q grills, the grill CAN go into bypass just like any other LP grill. Follow these steps to correct it:
•Close the LPG cylinder valve
•Turn all burner control knobs to the OFF position
•Now, start the grill by doing the following:
•Open the grill lid
•Turn the LPG cylinder valve until it is completely open
•Wait at least 1 minute. Due to the length of the hose (6'), it takes longer for the gas pressure to equalize.
•Turn the burner control knob to HI/Start
•Press the igniter to light the burner
•Turn any remaining burner control knobs to High
•The grill should preheat to 260-290 degrees ℃ in 10-15 minutes.
•NOTE: Cold weather can delay the time it takes to equalize the pressure.