My thoughts are you are going way too slow! At 30 MPH I am in second gear. I have a tachometer so I know how slow my engine is going. It does depend on the sprockets you have though. I never go less than 40 MPH and I don't shift into fourth until I am going about 50 MPH. Sometimes I take it to 100 mph in third just to make sure it can. I have on friend who says he never uses fourth gear. One time I did a trip 100 miles on back roads of southern Vancouver Island and I never used fourth gear. But seriously the Vincent is an amazing motorcycle that is pretty happy in heavy slow traffic and also good on the open road. Usually when I finally settle on my carburetor settings I have tried up and down from every setting possible, from pilots to mains. It takes a long time to get it right.