Check the intake / exhaust manifold seal at the head, 250's are a little hard to seal up and they can cause all kinds of running issues. While the engine is running ( idling ), use an open, non-burning, propane torch and wave it around all the ports at the head. Go around the top and up under the intake. If you have a leak, the engine will speed up due to extra fuel it pulled. For exhaust leaks, just use your hand and feel for hot air rushing out.
For testing, mark your distributor and head with tape and a marker, then turn it some and see how it acts, if it's worse, go the other way, if it's the same, put it back to the marks and move on.
At this point, you have checked air and spark and both working, so you are down to the carburetor. Be sure the vacuum advance is working, and be sure the mechanical advance is not stuck. Pull the distributor cap and give the rotor a twist, it should snap back into place each time. Be sure the PCV valve is new and work, and you have no other vacuum leaks.
99.9% of all 250's have hydraulic lifters, they are set like all hydraulic lifters, find zero lash and go 3/4 to 1 turn more of the lock nut. For us " racers " we go 1/4 to 1/2 turn to gain a few more rpm at high speeds { at least we think we do }. To find zero lash, rotate the push rod with one hand while tightening the nut with the other, when you have zero play between the rocker and push rod, you found zero lash. The lifter must be on the base circle of the camshaft, this is where the valve s are closed. There are many ways to set valves, but I like to set the first cylinder on the compression stroke, both valves are closed, and check lash. I then rotate the engine till the #2 cylinder is on compression and check it, then 3 and so on till all 6 are done.
To hear knocking at idle is not good!
