72 LtOne,
As far as the carburation, a sticky choke plate or misaligned plates could cause the problem. To realign the plates requires filing-off the plate screws depressed or staked material in order to back the screws. This should be done with the carburator off the engine (in case the screws fall through into the manifold). Don't forget to re-stake the screws once they have been tightened or you risk injesting them and cause damage. The newer Model 4150's have a choke plate inside a slited shaft and do not use screws.
The ignition wire suggestions are valid -check them!
A failed or nearing failure acceleration pump can also cause the problem. What is happening is that a lean condition is being made worse by the part throttle acceleration. To check, simply remove the air cleaner and check down the air horn and look for the shooter on the primary side. Whenever you activate the throttle towards opening the plates, you should see a short stream of fuel coming out of the shooter's two nozzles. If this is not the case, then something is blocking the fuel from coming out. Also, slack between the acceleration pump lever and the throttle mechanism will cause a delay on the fuel and create the lean condition. The pump lever screw (the one with the spring) should be adjusted by holding the throttle wide open and fitting a 0.015 - 0.20" feeler gauge in betwen the lever and the screw head while the spring takes-up the slack (between the nut and spring).
Having had the backfires quite often, I would say that your power valve is also shot. This explains the "running like cr@p" comment on your part. It is located on the primary metering block and to remove it you need a 1" wrench (after removing the fuel bowl and metering block). A backfire protection for your power valve is available, and assuming that you have a relatively old Holley 4150, I would suggest that you obtain one and install it. Holley and Mr. Gasket make one that requires a bit of drilling, but the kit comes with the correct size drill. Or you can obtain the
Power Valve Shield and just change the carburator body bolt close to the middle of the primary plates (when one looks at the carb while removed from the car and up-side down). By the way, to drain the carb while off the vehicle simply turn it up-side down on an pan to collect the fuel.
A clogged or partially clogged main jet could cause the condition, but this situation is more rare. You would have to have alot of junk in the bowl for this to happen, but, you never know, if no filtering is between the pump and the carb, then its a more likely situation.
