I think one of the problems is that my grill is old and perhaps slightly warped, or I need to adjust the metal riser for a tighter fit because there were some small gaps that were potentially letting heat out. Additionally, I left the bottom vents open for oxygen, but perhaps that wasn't a good idea?
Let me start by prefacing that I don't have any experience with cooking pizza on a Webber, but I have used a BGE to cook pizza, and I have cooked a lot of pies on a gas grill. First and foremost, cooking pizza in a BBQ creates a ton of heat with little or no insulation to protect you or anything else near the grill. This is not what the grill was designed for. You need to thoroughly consider safety first before you do anything. If you are not absolutely sure it is safe. DON'T DO IT!
Is there a door for the opening so you can close it? It seems like the opening would let out more heat than a loose fit. As for the bottom vent
, yes, I think you need to leave it open, or you probably wonít have enough oxygen to get the heat you need. I think you want to keep the top vent closed
At a minimum, I think you need to 1) start with a lot more heat - wood fire on the coals maybe, and 2) figure out a way to hold the heat in. I could not get browning on the top until I figured how to manage these. Here is what I was doing to cook pizza in my BBQ: http://www.pizzamaking.com/forum/index.php/topic,9614.0.html
. I would also 3) get a thicker stone. Scott123 would probably be the best to ask for a recommendation here. I'd say a Fibrament BBQ stone that comes with the metal tray (I liked the metal tray because it reflected some of the heat - will explain below), but there may be a better solution.
You need high temperature (say 700 or so) on the stone, but you need it even hotter than that in the air above else youíll burn the bottom of the pie before you get browning on the top. This is why I liked the Fibrament on the metal pan. My grill put out a ton of heat from below which I needed to keep the air temp up, and if I didnít reflect some of it away from the stone, it would get too hot Ė 850F+ which is way too hot for the 2:15 or so bake time I needed to get the top right. You will likely have a similar problem. I also found that I lost a ton of heat through the metal top of my BBQ (and I didnít have to deal with a big opening in the front too). My solution was to insulate the top. Youíll see it in the link I gave you. It made a big difference. As a first attempt as this, if I was you, I would use aluminum foil to close off the air space in the lid. You could put it across the top of the riser or across the bottom of the lid. This will greatly reduce the air you need to keep hot, and create an air gap, albeit a big one, I think it will have some insulating qualities.
Can you feed the fire when needed to keep the temp up? It looks like there is a door on the side of the grill itself you could leave open if you slid the stone over to the side a little? This might work to your advantage in other ways as well. I would think another part of your solution might be to have the main heat slightly offset from the stone - i.e. the heat is not directly under the stone which might help to up the air temperature without overheating the stone. Youíll probably need to turn the pizza during the bake. Even if you donít offset the fire, the heat will probably be far from even especially with the big opening on the side.
The bottom line is you need to figure out how to make enough heat for a long enough time, hold enough of it in, and balance it so that the pie cooks evenly.
I hope this helps.