Replace your air and both fuel filters. Clean the banjo bolt that has the line coming out of the intake. Check all your linkages. Make sure you are getting full movement when you press the pedal. When you press the pedal to the floor you should max out the linkages. There is a throttle stop on the rear of the injection pump. Make sure that is hitting when you press the pedal to the floor. Make sure the all ball sockets on the linkages are clean and lubed, as well as all the pivot points. Check the throttle bushing on the fire wall. It's what translates the pedal movement to the linkages. Valve clearences aren't going to hinder you that much. While is should still be done, especially if it hasn't been done in a long time, I wouldn't expect it to limit you to 40mph. Also make sure you shift points aren't too low. If the car is shifting through the gears way too fast it will make it extremely hard to excelerate. You can adjust this with the bowden cable which is attached to the linkages on the valve cover.

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