backfire under wide open throttle

[Broton2]

I’m having some interesting issues with my 76 K20, it’s got a new distributor, plugs, wires, and fuel pump. It starts to backfire out the top of the carb as soon as it goes to engage the secondaries, a very rapid popping. It idles and runs great but as soon as the secondaries go to kick in it pops like you wouldn’t believe. It’s got the OG quadrajet that I rebuilt after I had this issue with no change. I just timed it and have been stumped on it for a couple days now.

 

[fast 99]

Did you adjust or check the secondary air valve spring? Depending on application about 3/4 to 7/8 spring wind up. Setting should be in the carb kit instructions.

Fuel pressure and volume? Condition of fuel filter?

This is just one area to start. Ignition or mechanical issues could create the condition as well.

 

[Bextreme04]

Backfiring is going to be timing related or a mechanical issue 99% of the time. How did you set your timing? Have you run a compression test on your engine? Intake valves not sealing all the way could cause a backfire into the intake under heavy loads.

 

[77Dmax]

Has the motor ever been apart?

 

[Bextreme04]

You should also look at the condition of your damper. If the damper slipped and you just set timing before this issue started happening, you might have set your timing to a completely nonsense value. You can verify the mechanical TDC of the engine with a piston stop tool pretty easily and they are only a couple bucks.

 

[77Dmax]

76-77 was the prime era of flat cams chevy engines. Pull the valve covers and see how far they are moving. It'll be obvious.

 

[fast 99]

If valve train issue is suspected [broken valve spring / flat cam] check vacuum. Very likely reading will be irregular.

Weak coil is another possibility.

 

[1lejohn]

It could be running lean. The added air just makes the problem worse. Does it clear up once more fuel flows into the carb?

 

[Broton2]

Motor has 3k miles on a rebuild and good compression on all cylinders. I ordered a OEM style distributor as I think the “performance” one on it is causing timing issues. Timed with a timing gun set at 8 degrees. The fuel is fresh and filter is not clean but I suspect no issues. Thanks all as I dive deeper into this.

 

[Rebuildman]

New motor with flat tappet cam? Suspect cam lobes/lifters wiped out on break in.

 

[Oldsnut]

I have seen a weak or dying coil do something similar. You might try replacing the coil before tearing it apart.

 

[peats]

I would confirm correct plug wire orientation.

 

[Roostius_Maximus]

Check the oil filter for your cam lobe.

 

[Edelbrock]

One thing you could try, would be to close the choke part way and see if the problem goes away. If that solves the issue, then your running lean without the choke. If it were me, I would try replacing the carb with a cheap one from Marketplace or Craigs.