Originally Posted by T-bucket23
What is the timing set at. Usually a backfire through the carb is either timing, lean mixture or valves that are to tight. You may also want to be sure the HEI is being fed at least 12 volts. Shoud be the same as battery voltage. Point system often has a resistor to cut the voltage down.
Also check the firing order, pay close attention to 5 and 7 as they are next to each other on both ends and are easy to mix up
This reply, posted above, has the best questions that need to be answered before any helpful info can be posted.
Originally Posted by Blazin72
A 1981 Chevy pickup had points? I thought they were all electronic by then? Have you checked your timing with a timing light? Have you checked to see if the outer ring on the balancer spun on the hub? If that happens you will never get your timing right. What condition are the timing chain and gears in?
This post from Blazin72, has another excellent question that needs to be answered before helpful info can be posted.
To the best of my knowledge an '81 Chevy was equipped with a computer controlled HEI. Setting the timing with a computer controlled HEI is not done the same way as a non-computer controlled HEI.
To the OP. Look at the HEI you have. Does it have a vacuum advance cannister attached? If it does, then the HEI is for a non-computer controlled system. If it does not, then the HEI must be run with the computer to perform correctly.
More info is needed.