One of the first things that should be done when starting a motor installation is to carefully inspect the insides of the kit supplied fuel tank, before it is installed on the bike. I've yet to see one that couldn't benefit from a good cleaning. They come from China with rusty insides that will eventually clog even the best filter. Methods for cleaning the tank have been posted here numerous times. Acid baths, rocks and or sand, specially formulated rust busters, etc.
Another common misconseption is that you need fuel flowing to the carburetor like a fire hose. If it flows a steady stream,even a small one, it is enough.( the only exception is a highly modified engine but even then I seriously doubt that it could consume fuel faster than a steady stream.) If it just drips, then you have a problem. There have been reports of petcocks that were not fully open when the lever is in line with the valve and require the builder to 'find' that spot where fuel flows the best. The same for a fully closed valve (petcock). Something else to keep in mind when replacing the kit valve with an aftermarket style is the threads in the tank. They are a metric machine thread, not pipe threads as are most commercially available petcocks. A 1/8" pipe tap can be used to alter the kit threads as long as you don't run it in too deep.
As for fuel filters, don't rely on the fuel you get from most gas stations to be pure or by that I mean, crud free. If you mix your fuel in a container, after several fills take a close look at the residual gas left in it and you'll see some trash. That stuff will eventually build up in your bike's fuel tank and then either the carb or the filter. I'd rather catch it in a filter than having my needle valve stuck open with crumbs. Bairdco's advice is sound... filter your fuel before it goes in your tank. That's good insurance.
Tom