Both levels are High: This has by far the longest list of possibilities and warrants a proper approach utilizing the process of elimination. There are well over a dozen variables that must be checked, in the proper order. Usually this does end up being a carburetor-specific issue, but not always. In either case, it's the symptom where most complications due to unnecessary adjustments come from.

Starting with the simplest, the float bowl vent solenoid (FBVS) must be hooked up for the carburetor to vent. If the carburetor cannot vent, fuel will often siphon at high demand, which continues even when demand falls lower. A disconnected FBVS will usually cause an very erratic fuel level, but usually both bowls will be effected the same way.

The FBVS is the electrical cylinder mounted to the front of the airhorn. On older models it is mounted by three screws and has a green ground wire attached to one of them. These can be easily removed along with a brass bung inside the air horn, and then a penny can be inserted into the vent hole. It will stop at a ledge inside, and then needs to be epoxied into place. However, on later models, the solenoid is screwed into a much different brass bung that's pressed into place. Unfortunately, the vent solenoid plunger is trapped inside the bung and it's extremely difficult to remove them. Trying and failing will result in a very nasty mess.

If the FBVS is retained, it must be hooked up. They all have a single black & white wire which needs to be hooked up to an ignition-switched 12V+ source. Fuel tank venting can also be attributed to erratic float bowl fuel levels, though not usually a high level.