With a hat, if you hold it at partial throttle for a second or two (like you are to hit your launch RPM), it will go lean. The butterflies let in way more air than the spool lets in fuel. So, when it's launch time, you end up too lean and get a stumble.
To help with it, you can change the angle of your arms to make the barrel valve arm quicker in relation to the butterfly arm. Stand the barrel valve arm up more vertical, the butterfly arm more horizontal (or both) and experiment with that. I bet you'll see improvement.
You could also use an enrichment valve or something to make things richer until you launch. Not sure how much that would help however at such a low RPM.
Spud