I agree with Turbojack, Rum Runner and Bulldog. When the timing marks on the harmonic balancer of a Chevy V8 are at TDC, your distributor rotor will be pointed at #1 or #6.You're at #1 if you pass the "finger test" or both rocker arms for #1 are not under tension (both valves closed). I'm very suspicious of how the cam was installed. Some timing chain setups allow you to advance/retard the cam by moving the gear on the crankshaft to a different position. The cam sprocket also has a mark that has to be at 6 o'clock when #1 is at TDC. Popping thru the carb tells me your timing is retarded or your timing isn't advancing. I've lost exhaust lobes on the cam that also causes backfiring thru the carb. Good luck!

Since you said MSD back there, it sounds to me like an ignition problem. Did you use solid wire plug wires? Can you disconnect ALL of your MSD equipment and run on the dist & coil? A failing coil will do EXACTLY what you are describing, have you made sure the polarity is correct or substituted the coil? Hell, if all else fails, drop a GM HEI large cap distributor in it for test purposes. 1 wire to 12 volts and wala... test unit. Below is how to trouble shoot MSD ignition, but not specific to your problem


The following test will determine if your MSD is producing a spark.

White Wire Trigger:
If you are using the White wire (points or electronic amplifier) of the MSD to trigger the ignition, follow these steps.

1. Make sure the ignition switch is in the Off position.
2. Remove the coil wire from the distributor cap and position the terminal so it is approximately 1/2" from a good ground.
3. Disconnect the MSD White wire from the distributor points or the ignition amplifier.
4. Turn the ignition to the On position. DO NOT CRANK THE ENGINE.
5. Tap the White wire to ground several times. Each time the wire is pulled away from ground a spark should jump from the coil wire to ground. If spark is present, the ignition is working properly.

If there is no spark:

A. Inspect all of the wiring.
B. Substitute another coil and test again. If there is now spark, the coil is at fault.
C. If there is still no spark, check to make sure there is 12 volts on the small Red wire from the MSD when the key is in the On position. If 12 volts are not present, find another 12 volt source and repeat the test.
D. After inspecting the test procedures and inspecting all of the wiring, there is still no spark, the Ignition is at fault.

Magnetic Pickup Trigger:
If you are using the 2-Pin Magnetic Pickup of the MSD to trigger the ignition, follow these steps.

1. Make sure the ignition switch is in the Off position.
2. Remove the coil wire from the distributor cap and position the terminal so it is approximately 1/2" from a good ground.
3. Disconnect the MSD Magnetic Pickup connector from the distributor.
4. Turn the ignition to the On position. DO NOT CRANK THE ENGINE.
5. With a small jumper wire, short the Green and Violet magnetic pickup wires together then pull the jumper off. Each time the short is removed a spark should jump If spark is present, the ignition is working properly.


good luck my friend...