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Dave, Backfiring at high revs is very likely to be an ignition problem. I would also check my valve clearance, that can be a problem too. You should be able to reset the distributor by rotating it, and resetting the wires, if you have enough clearance to turn the distributor 1/8 of a turn. What you do is pick a wire for reference, rotate the distributor 1/8 of a turn, to where the next wire is exactly where the original wire was. Then you reset the wires where they will be in the original place they were before you moved the distributor. Otherwise if there isn’t enough room to rotate the distributor 1/8 of a turn, you will have to remove the distributor, and reset it. I would also check the distributor cap and rotor for defects, this can cause a backfire too, if they have problems, like cracks, or carbon trails. If you have any more questions, as always, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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Thank you Dr. ...I will rotate the distributor to where I can operate my vacuum advance so that it clears the firewall and reset the firing order wires accordingly (common sense thing) but how can i check my unit (undamaged new delco distributor with built in coil) that the spark feed is good throughout the revs? It did not back fire on this setting previously. Could this unit be faulty and how do i verify without taking to supplier ect??
I have stripped and checked no blockages on my brand new Holey carb, it starts no problems but when I rev .. the backfiring is still a problem. deffinately spark headache?
Dave. I took most of the day off, sorry it took me a while to get back with you. Normally spark problem will be caused by old, or worn ignition system parts. With a new HEI distributor the chances of spark being the problem are significantly less. I would get a spark tester and rev it up then look at the spark see if it drops off, or gets weak. I would also wonder if you have a full 12 volts at the power in terminal of the distributor. You need to check that, because if any resistance wire, or ballast resistors are in line, you could be starving the ignition system for power. Basically you need the same voltage at the ignition that you have at the battery. New doesn’t mean good, so the distributor could be bad. If the voltage to the distributor is good, and you are 100% sure the firing order is correct, my bet is that the ignition is the problem. If you have any more questions, as always, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
If I win with your advise I will add a bonus. Thank you for your service.