I'm new here and built my bike which is a Kent beach cruiser with a HT motor. I'm having a problem with what I think is the carb. At high speeds the engine will cut out for a few seconds. If I let off the throttle a little it is usually fine. I thought it may be spark related so I replaced the plug and wire. Also at times when I let off the throttle the engine revs and other times it just sputters and dies. What could it be?