I HAVE ONE OF THE FIRST MAC 21'S WITH A SPP PIPE ON A LAWLESS LEG RUNNING A 437 PROP.IT IS MOUNTED ON A HTB 290. I CAN NOT GET IT TO GO OVER THE LOWER 40'S. I HAVE TRIED DIFFERENT PIPES AND LENGHTS AND STILL NO SPEED. THE MOD ROSSI I HAVE HAS NO PROBLEM RUNNING HIGH 40'S TO 50 MPH.DOES ANYONE...