Are you sure its a '66? I know some Canadian Pontiacs used a 261 into the 60's but otherwise that motor went out of production at the end of 1962.

Otherwise first thing I would do is pull the plugs and 'read' them. Are they oily or sooty?


1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...