Check distributor vacuum advance with a vacuum pump connected to the diaphragm , watch to see it move. Also check points with dwell meter. Installing new parts with factory settings is just a start, you must fine tune them for maximum performance or economy.

Like Pete (Pontiax) said, fine tuning. Is the point gap set right?, Is the timing set to factory spec?, Was the distributor ever taken out and put back in? If it is off by a tooth, it will run but not run right. As mentioned before the vacuum advance, the rubber could be cracked and causing a vacuum leak. Vacuum leaks in general, especially at the carb base. Check all the vacuum hoses going in to the carb and replace cracked hoses that might cause leaks. The push rods were changed and my question is. Were the lifters properly adjusted? They could be too tightly adjusted or too loose. Either way this would effect the running. Even thought you did replace all the tune up parts, how do you know they are all in good working shape. You could have some bad wires or a bad plug. The PCV valve hose could be causing a vacuum leak. Put a vacuum gauge on your motor and see what it tells you. You could have late valve timing or late ignition timing and the worst culprit of all is a worn out motor. Another thing to look at is leaks at the intake manifold. Check the intake by spraying WD40 around the intake runners, while the engine is running. If your motor suddenly starts running better when you spray, then you know you have to replace the intake manifold gaskets. These are some that i can think of off hand, someone else here knows more. Good luck with the fine tuning of that fine 250 engine, i have had those engines and i love them. Cheers. George.

Double check the valve settings   and  rev the engine a lot    my neighbour does that whens he tuning his car

Hello, you need to check your timing with a timing light and if it's off, set it to correct advance (plug the vacuum line hose to the distributor with a screw while checking, the distributor will be difficult to turn(twist) try tapping after you've loosened the tightening bolt......also your new carb. has an idling screw to adjust the lean/rich mixture at an idle...for a test, turn the screw, usually at the front lower part of the carb with a spring on it, inward (clockwise) until the motor almost stalls, then go reverse one full rotation and see if it improves....good luck...TDM (4 speed)

Good for you. Glad to see you got it worked out. And now you know how to set your rockers . I bet some day some one is going to have the same problem and you will be telling him how to do it.

Well done

Ian