Todd, make sure you got the float adjusted correctly sounds like it's not shutting the needle valve. It's also possible that there is something in the seat that is not allowing it to fully seal. I'm not sure what the lift rod length is for a QA36A on a 1200 but Steve Bluniers method of measuring to make a lift rod for a blade should work for a snowthrower too. Drive the Cub up on some 2x lumber with the QA36A mounted measure the length between the lift point on the QA36A and the lift point on the 1200 with the lift in the lowest position. Here's some photos of Steve's 100 and blade for reference.
Here's a couple of photos of the "ramming speed" ™ lift rod that Steve made.
And here's an adjustable lift rod that I designed that is loosely based on Steve's fixed length lift rod.
Here's a couple of photos of the "ramming speed" ™ lift rod that Steve made.
And here's an adjustable lift rod that I designed that is loosely based on Steve's fixed length lift rod.