We recently rebuilt the 389 in our 65 GTO and it appears we have a problem. The engine idles normally but raising the rpm to say 1500 to 2500 and holding it at a constant rpm the engine shakes. My thought after talking to the guy who did the machine work that because he resurfaced the heads (and we have no record and he can't remember how much was removed) perhaps the valves are not closing all the way. Does anyone have any idea as to what would cause the shake?
Engine rebalanced during the rebuild?
Rocker arm nut has backed off.
Dead cylinder - rocker arm fell off or pushrod bent not activating a rocker/valve.
Worn/rounded cam shaft lobe and valves not opening as they should.
Could be an ignition/distributor problem.
Could be a fuel problem - bad gas/water in fuel, carb problem.
If valve problem - ie not closing fully, you would most likely hear popping out the exhaust or back through the carb - unless a valve is simply closed due to rocker arm off.
Typically resurfacing the head only takes off a minimum of material and is not the same as milling the head to raise compression.
These are just a few thoughts as it could be something else completely different. You could also have a very major problem as well. So very hard to be specific on such a general question and unknown.