One problem with a Howell TBI is constantly running rich. I never had that problem like many others the did the install. I wondered why? I've had my EFI system now for about 6 years and it's worked well with only one failure. The difference between my system and others that I found was the gas lines. My lines were rusted. The Autum before the Howell install I replaced all the gas lines. I went to a 3/8" return line. This seems to be the trick for lessening richness. Follow my logic.
1. If your using the stock OEM line as a fuel return, it likely 1/4", or 5/16" and this builds up too much resistance to the flow.
2. The resistance translates into a higher backpressure at the regulator.
3. The backpressure can't be overcome by the regulator and it passes that increased pressure to the injectors.
4. The on-time for the injector is set by the comptuer.
5. A set on-time and higher pressure at the injector means more gas past the injector pintle during the on-time.
6. This increased the richness above the ability of the computer to lower the on-time.
The result is the TBI runs rich.
Soulition: Measure the fuel pressure at the regulator, both from the pump, and in the return line. Increase the diameter of the return line as necessary to restore the regulator input pressure to normal.