It is necessary to approach this in a logical manner to locate the "real" source of the problem. Otherwise, you can spend huge amounts of money and still not solve the popping. Or, at best masking the problem.

An example of this would be: re-jetting a carb to compensate of a vacuum leak. While this could possibly stop the popping. The leak still exists and will eventually get worse and start popping again. This is called "fixing the symptom, not the problem".You would be far better off fixing the vacuum leak!

You see, popping  IS A SYMPTOM of a problem (not the other way around)! There are many problems which have the symptom of popping!  For example; a weak intake valve spring causes popping. But somewhat different than the popping of a vacuum leak. Bad plug wires cause popping but somewhat different than the popping of a lean condition of a carb. Retarded timing causes popping that is somewhat different than the above samples. But they all have one thing in common: "POPPING". And it would be very easy to work on the wrong thing that you perceive to be the problem.

Careful diagnostics is your only chance of locating and correcting the real culprit. Any other method is purely trial & error and luck.

Before you do anything to your engine, think about how & when the popping started.  Did you make any changes to the engine IE: change carb(s), manifold, distributor, etc...? If the answer is yes, then this is the most likely place to start looking! 9.9  times out of 10 this is where the problem lies...

Look for vacuum leaks, bad gaskets, crossed lines & wires, dis-connected vacuum lines, and loose bolts. Re-check your timing and firing order! Carb(s) could be mal-adjusted for your application. Or, carb(s) could be leaking air, and even bad carb(s) in need of re-working. 

In my experience, 75% of popping is caused by mal-adjusted air/fuel delivery. Whether it be vacuum leaks or bad adjustments. 22% caused by electrical timing and bad plug wires, and 3% caused by mechanical failure.          

Popping that is consistent (or continues) under a load suggests plug wire crossed and/or cross-firing, and even a mechanical weakness in valve springs or worn cam lobe(s).

Intermittent popping suggests a vacuum leak in close proximity of the carb(s), a weak accelerator pump, and/or a worn out butterflies in the carb(s).

SUGGESTED TOOLS:

Vacuum Gauge

squirt bottle with water

compression gauge

Using a squirt bottle with water and a vacuum gauge connected to the engine you can quickly locate a vacuum leak. Squirt around all the gaskets and watch the vacuum gauge! If you squirt a leak, your vacuum will rise and fall off as the water blocks and passes through the opening. You will also notice a definite change in the engine RPM as well.

If you don't find any vacuum leaks, and your electrical is OK, then you need to start looking at your air/fuel supply I.E.: Carb(s)

This gets really complicated! Especially if you are using multiple carbs. I will tell you that the popping is caused during the transition from IDLE to BOOSTERS!

The RED area within the transition region is where popping occurs. It takes a balance of three functions to make a carb run smoothly. Your carb goes from IDLE, ACCELERATOR PUMP, to BOOSTERS. If any of these functions malfunction late then engine popping WILL occur! You will notice that the accelerator pump over laps both the idle circuit AND the BOOSTER bridging the gap between them. If this bridging isn't performed properly, you will get a lean air/fuel mixture to the engine causing a momentary pop or popping until the transition has been completed. Sometimes the initial pop will damage your power valve, and sometimes prevent the boosters from ever boosting, thus causing MORE POPPING! And then what a mess!

OK, here's how it all comes together; (follow the chart...)

While idling you're pulling fuel and air through the idle circuit. When you begin to throttle, the manifold vacuum drops and the accelerator pump delivers a shot of fuel to prevent the engine going lean until the vacuum signal starts fuel flow through the boosters. Meanwhile if the vacuum signal is low enough the Power valve(s) will open, delivering extra fuel to the boosters. If the accelerator pump starts too late, a lean air/fuel mixture happens. If it is a minor late instance then you may not even notice it. If is really late, you will get popping and/or a slight bog. If the fuel pump doesn't work at all then a sever popping and/or stall will occur. You may not even activate the boosters. However, your power valve(s) will likely be open. This really won't matter, because the lean condition already exists.    

Boosters are timed by the fuel level in your float bowls coupled with the vacuum signal present at the boosters. So if your fuel level is low, then the boosters will be late in sending fuel. Thus generally causing a momentary bog. This can also happen if your power (Holley) valve is incorrect for your engine. Or metering rods in other carbs.

Accelerator pump duration is very important too! It has to be able to squirt long enough to allow the boosters to be in full operation.

Power Valves  are specifically designed to deliver about 10 jet sizes of extra fuel when the vacuum signal drops below a pre-determined point. If the power valve(s) open too late you will run lean while accellerating. If they don't close, you will run rich. So it is important to have the proper power valve(s) for your specific application!   

This fine balance between idle, acccelerator pump, power valve(s) and boosters is the difference between a great running engine and a real aggravating piece of crap.

So BEFORE you start messing with all the things you THINK might be causing your popping problems, consider all the potential things that cause the POPPING SYMPTOM!

      National T Bucket Association