Single cylinder intake backfire (popping) - Team Camaro Tech
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post #1 of 10 (permalink) Old Nov 14th, 05, 11:19 AM Thread Starter
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Single cylinder intake backfire (popping)

Swapped the stock 327 cast iron intake and 2bbl rochester for a Performer and Quadraject setup on an original '67 327 block that I recently rebuilt.

After driving it a bit (maybe 50 miles), I developed a severe intake backfire popping noise, complete with shutter in the gas pedal from the butterflies. Things I have checked and the results:

-Verified timing to 8-10 BDTC with vac advance off.
-Verified vac line connected to ported vac, not straight vac.
-Engine runs, but makes an individual pop backfire to intake, my guess is just one cylinder is popping.
-Intake backfire = too lean, right?
-Used carb cleaner to wet around gaskets without much luck in sealing a gap.
-Went ahead and changed the intake gaskets to Mr. Gasket ultimate sealing type with raised port edges and all.
-Sealed all bolts, intake valley with RTV
-Carb idles at ~800rpm in drive
-Compression checked all cylinders - all within 5-10 psi of each other
-Checked distributor again to verify not 180 off, but engine runs fine with all other cylinders
-Verified correct plug wires to corresponding cylinders.

etc, etc.

In reading through previous threads, maybe I have the valve rockers on too tight . . .

What else might I check?


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post #2 of 10 (permalink) Old Nov 14th, 05, 11:43 AM
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Re: Single cylinder intake backfire (popping)

Cracked distributor cap?

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post #3 of 10 (permalink) Old Nov 14th, 05, 12:09 PM
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Smile Re: Single cylinder intake backfire (popping)

I have a 327 with 10:1 compression, it pops like that if I use low octane fuel. Maybe the gas stations in your area have switched to their winter fuel and it is causing your problem. Try a tank of Premium fuel and see what happens. I've had good luck with Amoco "Gold" fuel.

69 Camaro -originally a LM1 car. 327, Edelbrock E-streets, hooker, DUI performance distributors, ultradyne cam, stewart, TRW etc. Vintage Air, AGR steering, Corbeau GTSII seats, 700R4 transmission, 12-bolt w/Eaton 4.11:1
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post #4 of 10 (permalink) Old Nov 14th, 05, 12:51 PM
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Re: Single cylinder intake backfire (popping)

Pull the rocker covers off and start the engine. Watch to see if some of the rockers arent moving as much as the others. Flat camshafts cause that kind of popping back through the intake manifold and was quite common in older Chevrolet engines.

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post #5 of 10 (permalink) Old Nov 15th, 05, 07:18 PM
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Re: Single cylinder intake backfire (popping)

Yeah, cam lobes can wear down and cause strange things. This kinda sounds like a broken valve spring to me.

Make sure and triple-check the basics like firing order 1-8-4-3-6-5-7-2 and valve adjustment too. Cracked cap, as mentioned, could do it. So could plug wires shorting out, move the wires around so that none are touching.

Does it make the noise at a fast idle? If so, disconnect one plug wire at a time and you should be able to locate the bad cylinder, then you can focus in on that one...........

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
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post #6 of 10 (permalink) Old Nov 15th, 05, 09:43 PM
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Re: Single cylinder intake backfire (popping)

You could have a carbon track in your distributor cap. You might not even see it and the cap does not need to be cracked for this to happen. The track will conduct spark from one post to another nearby. The symptoms will be worse as you open the throttle and load the engine. Swap in a new cap and see if that solves the problem. Inspection may not show anything without a magnifier. It sounds like you're getting spark where it doesn't belong.

The other suggestions here make perfect sense also.
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post #7 of 10 (permalink) Old Dec 15th, 05, 08:52 PM
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Post Re: Single cylinder intake backfire (popping)

I've seen this type of thing before. It is probably in the valvetrain. A lobe "wiped out", or a bent push rod. Get those valve covers off, and have a look. Check for excessive valve lash, and idle the car while looking at the rockers, one may not be moving as much as the others. Also inspect all of the valve springs very carefully, one may be broken. I believe the latter was mentioned earlier. Good Luck!
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post #8 of 10 (permalink) Old Dec 16th, 05, 12:01 AM
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Re: Single cylinder intake backfire (popping)

you mention a tight rocker, did you adjust them before this occurred? if so, thats the first thing i'd look at. it definately sounds like valvetrain, i.e. a missadjusted valve, or a broken spring, or a wiped cam lobe.
good luck with it.


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post #9 of 10 (permalink) Old Dec 16th, 05, 07:01 AM
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Re: Single cylinder intake backfire (popping)

By some chance,, could one of the intake bolts being too long, tighten down on push rod and kept excessive pressure on lobe? It has happened. Popping thru intake indicates and intake valve not sealing for what ever mentioned reason.. Was the lobe on intake side?

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post #10 of 10 (permalink) Old Dec 18th, 05, 12:00 AM
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Re: Single cylinder intake backfire (popping)

seen the same thing from the idle air bleeds in a holley having a bit of dirt in them,a few hard jabs of the throttle at 3500 rpm while driving cured that one with a loud pop!lol!don't remember where the air bleeds are on a q-jet?
Eric B

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