Engine Backfire at High RPM

I am working on some tuning on my 302 Maverick, hoping I can get some other ideas here. Here is what's happening: the engine runs and idles great but has a tendency to backfire through the carb at high RPM (~6k). I don't think timing is the issue, I have it set at 16 BTDC @ idle and ~35 BTDC @ WOT. Before I changed springs in my mechanical advance, it was 16 @ idle and ~45 @ WOT but seemed to behave the same in terms of the backfiring issue. As far as I can tell, it seems to be running out of fuel at the top end. I can see fuel in the bowls so I don't think it is a fuel delivery issue either.

While I was messing with it I noticed that my vacuum secondaries only open if I hit the throttle hard; if I gradually bring engine speed up, the secondaries do not open. I changed to a lighter spring on the secondary diaphragm and it didn't seem to make any difference.

Motor details: 302, headers, aluminum dual-plane intake, Holley 570 CFM street avenger, Comp 31-242-3 cam (Duration 268/280, Lift .509/.512).

 

Valve float? Do you have the correct springs for the cam?

 

I had the same carb on my 302, similar cam and crappy aluminum heads. I had to install the white spring (lightest) and increase the main jets from 54 to 60 or 62s (bad memory) to get my spark plugs a nice carmel color. Now I was not having a back fire issue but a top end power issue. I would read your plugs to see if you have a lean or rich issue. Take it out some were secluded, Open her up and kill the motor, Coast to a stop and pull a couple plugs. White its too lean Black its to rich. If you have the glass float windows the fuel level should be about half way in the sight glass, If you have the plugs, adjust the float till the fuel starts to flow out the hole. What ignition do you have? this could also be the issue.

 

My first thought was valve float too although my only experience with that was at 17 and ended with a bunch of bent valve instead of backfiring! Regarding the secondaries opening, you really won't get much action free reving the engine as they open based on load. Only way to watch what they are actually doing would be a camera while driving it or run it on a dyno.

 

Also check valvetrain geometry. If it had bolt down rockers the pushrod could be a little long causing the lifters to pump up and hold the valve open at rpm.

 

Valve springs.

 

Usually weak/mismatched valve springs will act like much like a rev limiter before backfiring occurs.

 

Something else to look at is the fuel filter. If it is at the end of it's lifespan, the car will idle and mid-range OK, but the motor will lie down when wide open. It may make it through first gear OK and then lay down the longer and harder you run it.

 

Also, when considering fuel delivery, make sure your fuel lines are not partially blocked.

 

Intake valve or valve spring but, if a plug wire were to "arc" to another plug wire it could fire it at the wrong time giving the same result. Worth checking...