My 75 450SLC runs slow. Will not top speed of 25MPH for the first 20 minutes after starting. What could be wrong? Also lousy gas mileage 5 miles per gallon. Had tune up, didn't help.

Thank you

Cheryl

oldie
 

Haven't seen one of those for a while.
First, I assume the plugs and points and timing are ok.

1. there is a vacuum controlled mixture unit, or maybe a tear in the hose going to it. It usually mounts on the fender with about a 3/4 inch hose going to it.
2. another common problem is oil on the trigger points in the bottom of the distributer (not to be confused with the ignition points)