ok....had this happen to me and the simplest solution usually works great.....had issues with mine the exact same way and it was a frustrating to say the least. called one major engine guy...he asked me where my fuel filter was located. I had placed it up on the top near the pump. I wanted to be able to see the fuel flow. he told me the carb was starving itself because the fuel filter was up too high and it was taking all the pressure it could to pull the fuel up the line to the pump and carb. thought "yeah right" shouldn't make a difference where the filter is located, but I went with his recommendation to move it down next to the frame rail...that way it wasn't hard for the pulse to keep it full and the fuel could be pulled up easier to the pump. it was the difference between night and day!! fuel has no problems being pulled through the filter way down by the frame and at the same time, it flows better up to the carb. and I went through all of it! replaced the carb, the lines, the filter and even the pump....and it was all because I wanted the filter up where I could see the flow......