<html><head></head><body><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:16px;">A little more information about the symptoms is needed.<div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:16px;"><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:16px;"></div></div><div>Where does it backfire? Intake or exhaust?. Do you have electronic ignition or the old points and Condenser? </div><div>A backfire from the carb tends to always be a ignition/timing type problem whereas a backfire from the tailpipe tends to be a carburetor problem. A MISfire at the tailpipe can be ignition as well. But you state the engine won't idle. So I am presuming it cranks, starts and runs good off idle. Dirty carbs, both Holley and Eddelbrock/CarterAFB will cause a poor or no idle concern. I'm making the assumption that the idle screw is not backed off and you have no vacuum leaks.. </div><div>Backfiring when a panel switch is used, and I'm taking a guess that you mean from the intake, and you have the original ignition system, could be caused by a defective condenser and or coil. The extra load pulls power from the weak coil and it collapses early. </div><div>This is all just a guess as we don't yet have all the facts.</div><div>Good luck</div><div>Trevor </div></div></body></html>