I had a similar issue with 'Tater when I 1st got her running. As it turned out the carb that was on it was for a MUCH smaller engine. The venturi & jetting was totaly wrong for the volume of air the engine needed so it simply wasn't pulling enough fuel & would only run choked. Beautiful idle, but once I got it out on the road it gasped, stumbled, & died in a loud grotesque military manner. It was sad, really. BUT! With some help from the wonderful folks here on the bolt I mananged to track down a factory correct carb and she's run like a champ ever since.
So yeah, my point, I have a point! I'd look up the specs for your engine, see what carb is supposed to be on there, and work from there.
If you have a solid idle now, to verify your manifold leakage you can use carb cleaner spray. Just spray short shots at all the suspect points & if there's a leak it will draw it in and the engine will stumble. Super simple way to find manifold leaks, MY preference anyway.
