You said you are running an automotive fuel filter. That may cause problems, but usually it is fuel starvation. The vacuum petcock is not a fuel pump. It is a simple valve that is either open or closed, depending on whether or not it is getting vacuum.
I appreciate the packaging difficulties you face, but a proper exhaust system is much more than some random tubes taped together. There is no constant pressure in the exhaust. When the exhaust valve opens, high pressure gas enters the pipe and travels down the pipe, through a collector, a mid-pipe, and a muffler , on its way to atmosphere. Behind that pressure wave, a low pressure area forms that helps scavenge the combustion chamber, allowing it to fill with a fresh charge. Complicating this elegant maneuver is a reversion wave. Once the high pressure wave exits the system, atmospheric pressure enters the system to equalize the pressure in the pipes. If this reversion gets to the exhaust valve before it closes, it can actually push some of the air-fuel mixture back into the carburetor, normally resulting in a lean mixture.
In general, shorter pipes will only work at higher rpm. The shorter, the higher, longer lower.
A collector allows for a broader rpm range of happy pressure pulses since the individual pipes can help scavange each other and reversion waves can be reduced.
As mentioned above, CV carbs are very sensitive to all this since the slides are wholly dependent on having proper pressure signals.
You have introduced a bunch of variables, all of which affect others. Mechanical slide carbs will eliminate some of the problems, but getting the engine to produce full power throughout the rev range, with crisp throttle response and smooth running, will likely take a great deal of experimentation.