Idles OK, won't run well at higher RPM. I have been there and done that. All of the above posts give good things to look for. A couple of things learned the hard way follow.
1, Use a vacuum gauge to set timing on a running engine. The BB is good to start. With modern fuel and possible flywheel changes, the best way to set timing is with a vacuum gauge. Adjust for maximum vacuum. Idle only. Advance line can stay on. It is ported with the stock carburetor. After you get the best vacuum, dial it back a bit.
2, Recheck the firing order. I know, sounds like dumb stuff, however, I have crossed plug wires and others have done as well. I have also had good starting engines run with the order all messed up. so, check. turn to the BB. with the valve cover off, wiggle the valve on #1. Loose? Good. Tight? Try #6. Loose? 180 degrees out. No problem, just turn engine until the BB shows up again(one complete turn). Where the rotor points is now #1. You can reset the Dizzy if you don't like where the rotor is pointing, but you don't have to. Just start with #1 there and go clockwise 1,5,3,6,2,4. Reference:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1230.htm. Start with these and see what happens. All inline 6 engines, 216,235,261 have the same firing order.
