Car Bucking

At this point, my initial thought is that's it's rpm related. 50mph - good, 70mph - not so good. It does sound heat related, and while i'll disregard the track use being how it's short lived, i have a hard time seeing how the extra rpm on the highway is generating enough heat to cause any difference? Unless of course something is right on the edge?

I'll step on the limb and ask, how old and what shape is the coil in? Are you using a paper carb gasket vs a good insulating type? With your new fuel system, what pressure are you running? And are the floats set to a low or high setting on the site holes, if a Holley? Are you running a vacuum advance distributor? How do the spark plugs look after a episode like this? I'm honestly not sure if you're going lean or rich at this point.

Just grabbing at straws for now, but I hope one of us can get you going smoothly for good. :thumbrig: