Forgot to mention that a worn throttle body can cause a vaccume leak and a lean condition , also (Im no impco guru) but model J vaporizers are mostly mounted in 4 cylinder applications ,you are trying to feed 6 (1970s ) cyls.A vaccume leak ,if bad enough could introduce enough unwanted air into the manifold to lean out a regulator at the edge of its output limits. Ensure your vaporizer is equipped with a blue secondary spring (highest output pressure). If there is no leak at the throttle shaft ,the proper spring is installed,and the previously mentioned fuel adjustment does not work enough, you may need to go to a higher volume reg.