Nicholson I tried making a gravity fed cooker on the back side of a pic cooker but I used a 3/4 valve instead cause the 2 1/2 was so pricey but I had trouble with my fire so you think that was the problem also it was about a foot under the actual fire
Sometimes even with a bigger valve establishing initial airflow can be an issue. I found that big valve of mine for $5 on ebay. You may need to buy, or make, a little blower system to attach to your valve. Usually once the coals are pretty well established it will burn just fine on its own, even with a smaller valve.
Getting it burning isn't so much the problem with the top open it's when I close the top when it's stops burning like it suppose to, guess installing a 2 1/2 inch nipple couldn't hurt anything to start with, thanks for the help