Mailbag: Troubleshooting Detonation Problems on a Chevy 427 Big Block

big block chevy valve springs

(Image/Hot Rod)

You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re troubleshooting a backfiring problem on a 427 big block Chevy.

Q: I have a 1974 Olds Omega with a 427 cubic-inch big block Chevy. The engine has an Erson cam (292-degree duration, .514-inch lift), an Edelbrock Performer intake, a Carter 750 cfm carburetor, MSD 6A ignition and Blaster 2 coil, Holley electric fuel pump set at 7 psi, headers, and a Flowmaster exhaust.

At about 4,000 rpm, the engine backfires through the carburetor. I have been told the compression (11:1) is too high. I tried some VP “red” race fuel, and that seemed to help. I tried adjusting the timing and the carburetor, but that made things worse. I even pulled the timing chain to make sure it was okay. I have tried everything I know to try, and I still have the backfire. Any suggestions?

A: Are you running the valve springs recommended for that Erson camshaft? If not, we think the valve springs you have aren’t suited for the cam, and should be replaced with the manufacturer-recommended springs. Also, we suggest you continue to run the VP race gas. With 11:1 compression, you’re going to need it all of the time to prevent detonation.

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  1. Roger Hensley says:

    You most likely have bad lobes on a couple of exhaust lobes on cam. Did you say (sig erosion). Maybe head gasket between two companion cylinders.Double check firing order.wires.what is your actual fuel ptessure and volume (running).look at all plugs and check if fouled.never ask questions till atleast you do a compression test starting with all plugs out first. So when you have questions also make sure that you have answers to at least these questions.
    R oger Dennis Hensley master tech ASE L1. X1. C1

  2. “With over 55 yrs. of experience”—-unfortunate that your help ended with your last sentence.

  3. First, check to see if 11:1 compression is the real problem by doing a compression check with all plugs out and carb butterflies held open. Under 185 to190 PSI should not be a major issue even with iron heads and pump gas. Assuming that not the problem, check distributor advance curve, vacuum advance, and spark plugs wires not running parallel. Backfires can also result from lean carburetor conditions, so check carb float levels, etc.

  4. Everyone is saying the timing and fuel how about the engine temperature if you have anything over a 160 thermostat take in out then work on your total total timing on pump gas a total of 28 degrees should work ok if you’re still have pinging problem you may want to pull your heads check for sharp edges and polish them off

  5. With that kind of compression, i’m betting a head gasket blew between two cylinders.

  6. David DeThample says:

    If the problem happens at higher RPM, could be weak valve springs

  7. The only way a backfire up through the carb can happen is when two events occur. First the plug has fired, either at the right time or too soon (another issue). And second, an intake valve had to be open. Thats the only way a fire can spread out of the cylinder. I suspect an intake valve is not seating properly. Maybe a bad valve, maybe a bad seat, maybe a weak spring.

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