Engine \"stumbling\" The motor in my 73 K20 has had a "stumbling" problem for a while and I just cant figure it out. The motor only has 50,000 miles on it. It does it mostly in 3rd and 4th gear when the motor is under load. It does it the worst in 4th gear on acceleration and while pulling a hill. It just seems like its missing and it jerks the whole truck. Ive rebuilt the carburetor twice and everything with it is good for sure, and it has new plugs and wires, distributor cap and rotor button, fuel filter, and air filter. There is a clicking noise coming from the engine somewhere, my dad thinks its a rocker arm or something, we are going to adjust them all tomorrow to see if thats it. I checked the vacuum advance and its working fine. A new coil and adjusting the valves are the only ideas I have left. Does anyone have anything else to add that could be causing the problem?