Ok, so here's an update.

I threw a new battery in it, that solved half my problems. Here's a more specific issue now that I can observe it while it's running:

It starts like a champ, idles like a champ. As soon as I rev the throttle and it hits 3-4000 RPMs, the engine dies. It starts to die, and it's almost like the more throttle I give it, the faster it dies.

I'm at a loss...

 

that sounds like the fuel flow is being restricted

 

check your fuel filter (i always forget about those htings)

 

when you checked the carbs did you take out the pilot and the mains to see if they were clogged. The mains are kicking in at the higher rpms, so if they are clooged there would be a problem.

Also what about the spark plugs? Pull them and see how they look.

 

try reving the motor with the cap open if it gose to red line then you know that the vent is cloged if not than it is in the shutoff

 

yup, basics works wonders at times.

 

make sure that you didn't pinch the vent line closed when you installed the tank. routing of hoses is very improtant. blow air through the vent line with the tank cap open !!!