Agree with pump or tank.
Drive with cap off to test. Sometimes you think the cap is vented but it's malfunctioning. Don't two carbs take more pump volume than stock pump?
Agree it's a fuel delivery problem so...lots of other stuff besides a tank vacuum lock or pump.
4. Dirty filters
5. Pick up sock clogged
6. Out of gas
7. (no.3) Carb adjustment of all kinds, float level, etc
8. Bad rubber hose somewhere
9. Manifold Vac leak
* Timing and possibly electrical issues.
The key is the slow progression which points to Fuel Delivery. Lots of things between tank and venturi.
Test and troubleshoot and call me in the morning
