Mailbag: Curing Misfire in a 396-Powered ’68 Camaro

Q: I have a 1968 Camaro with a 396 engine that’s .030″ over and has a port-matched Edelbrock Air-Gap intake manifold, 750 cfm carburetor, and an MSD 6AL ignition control, Pro-Billet distributor, and Blaster coil.

I recently installed new lifters, rocker arms, and a COMP Cams 268H cam. The engine runs strong right up to 5,500 rpm and then begins to misfire. It would wind up to 7,000 rpm with the old flat tappet cam without any problem.

I’ve installed new ACCEL 9000 ignition wires, changed the distributor weights and springs, and checked the timing, and it still misfires at 5,500.

Is it possible the ignition control, coil, or distributor when bad during the cam change? Or even worse, could the new cam be the cause of the problem? Please help!

A: While your COMP Cams 268H cam stops making power at 5,500 rpm, it shouldn’t cause a misfire at higher rpm. It should just stop pulling. We like to check easy things first, so make sure your MSD 6AL ignition control is getting a full 12 volts. Your battery could be weak and that could be the cause of your problem.

Next, we’d suggest checking your fuel line for kink or flat spots. We’ve seen fuel lines become damaged when cars are lifted for service.

Finally, check your valvetrain. Make sure your valve springs aren’t binding, that your pushrods are the correct length, and that your rocker arms are the correct ratio (they should be 1.7).

Hopefully, one of these suggestions will correct your problem and have your ’68 spinning happily over 5,500 rpm once again.

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  1. Bob Berndsen says:

    While on the subject of misfires, I have a 72 Mustang, rebuilt 351C 2V. I don’t know the extent of the rebuild other than ir has an Edelbrock Performer intake for 2V heads and a Holley 600 cfm 4 barrel carb.I bought the car 2-1/2 years ago with the engine already rebuilt. The issue is I get a misfire only when the engine is cold. If I let it warm up for about 10-15 minutes before I drive it, no misfire and it runs great. It has a Summit Racing HEI distributor with a Pertronix Flame Thrower ignition control module. I have replaced the HEI coil, rotor, distributor cap, plugs and wires. What could be causing this?

    • Daniel Wilson says:

      Bob Berndsen…a healthy Cleveland engine can be a strong performer. They respond well to the typical induction and exhaust upgrades.

      You didn’t mention if the misfire while cold is from idle on up or at a specific rpm but since the problem goes away at normal operating temperature, it’s most likely something wrong of a mechanical nature due to metal expansion properties.

      Since you’ve already replaced most of your ignition components, use a timing light to verify proper ignition events. Most Clevelands like advanced ignition timing so as a starting point, try 6 degrees before top dead center at idle with any vacuum advance disabled and the vacuum source plugged. Use as much advanced timing as your engine can tolerate without pinging or pre-ignition symptoms.

      Use a compression tester to check each cylinder when cold and again when fully warmed up. If any cylinders test substantially lower than the others during the cold test then you might have a sealing issue with the valves or rings.

      Assuming that your Holley carb is equipped with a functioning choke, make sure it operates as intended during the cold to hot transition. It would also be a good time to verify normal fuel pressure at the carb. For the recommended fuel pressure you can contact the Summit technical staff for expert advice or check the Holley website for information on your specific carburetor. I wish you well.

  2. Sounds like maybe a lifter is not letting the valve open when cold because its not pumped up. Assuming its hydraulic. You can shoot the exhaust manifold with a infrared temp gun to see if all cylinders are firing or an old school trick is pour a little water on them right at the port to see if they sizzle.

    • Nick Gavriles says:

      I have found many problems using the methods you suggest. An old fantastic or household cleaner spray bottle with water adjusted to stream or ir temperature gun will tell you a lot of useful information.

  3. I agree with one point; the first thing to double check is the valve spring department. I smell valve float.

  4. patrick mockus says:

    choke setting / timming off /

  5. Did anybody say to check the rev limiter in the 6al? I believe one of the chips is a 5500 chip.

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