The choke arm to push rod alignment is somewhat sensitive. Is the push rod properly aligned? Ideally, in an inverted engine set-up, you want to push rod to hang straight down.

If it's off a 3/8" fore or aft, towards the firewall or away from the firewall in other words, it may cause enough bind to cause incomplete closing of the slide baffle. A misaligned pushrod can cause lifting of the baffle off the carb body. Or if misaligned sideways, pulling on the pushrod may not allow full mechanical movement of the arm to the stop.

Are you opening the throttle to full when choking?

Two or three of us have had some issues with original carbs not drawing enough gas to even pop. On my original carb I had to resort to taking the plug out and squirting gas into the cylinder. A quick call to Todd fixed that problem; he replaced the unit.

Ed also let me use his hi tork starter in the very early runs before I knew how to run the engine. That worked great too. But once I figured out what to do and what to expect, reliability has been unsurpassed

Some of the carbs may not be as clean as needed from the Walbro factory. Some guys have been successful by taking the carb apart, carfeully cleaning everything and reassembling the unit. That does the trick. I suggest you contact Todd Syssa and explain the problem. I'm sure it's a simple fix