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Old 01-26-2008, 03:19 PM
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Backfire, what the hay?

Finally got my 350 chevy running, pertronix ignition, taylor plug wires, double hump heads, comp 280 magnum cam, balanced, etc. I have an rpm air gap with a 750 cfm holly vacuum secondary carb. I got it timed, double checked that
all the valves were adjusted correctly and it's starts and runs pretty well. 14 degrees initial 32 total, to stay out of detonation, (11:1 compression) Anyway, I'm getting some backfire action, I don't have a clue what might be causing it, could it be carburation problem?? Any hints would be mucho appreciated.

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The Tim Guy

I just read a post by old skool that said what I have is an "after fire" not out the carb but through the exhaust. I've been goin crazy adjusting float levels but they seem to be right. It ran perfect (no after fire) before I replaced the ignition (one pertroix ignitor, with another). Anyway would float level give me too much fuel. Its happening under 3 grand so I don't think the secondaries are opening yet.

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Old 01-26-2008, 03:45 PM
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I went through this on a buddy's car. There are a couple of things that cause the backfire through the carb. 1. Advanced timing, especially when the engine cranks real hard at startup when the motor is hot. 2. Stuck or sticking valve, or bent pushrod, and 3. Too lean of mixture at the carb. I'm leaning towards the lean carb. Pull the plugs and see what color they are. You may have to go to slightly bigger jets on that Holley. Doesn't sound like a plug wire because that's all new, but check it anyways.
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Old 01-26-2008, 05:38 PM
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backfire

Sounds like you have a nice combination, however you failed to mention your vehicle specs. Your vehicle, if automatic, should have a 2800 rpm stall speed convertor,
headers, and a higher numerical rear gear ratio simply because this camshaft does not start working until 2000 rpm. You can tune this carb until you wear the threads out, but the motor wants run above 2000 before wide open throttle. Also the optimal timing should be 36-38 degrees total with this set-up. I've set up 292's (060 over 283) up to 496's and I've only found 421 & 434 SBC and 470 & bigger BBC run "happy" at 32 degrees. If you build a performance motor, you need performance gas.

Hope this helps
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Old 01-26-2008, 06:01 PM
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what the hay??

The 350 is in a 52 pick em up (international) with a muncie 4 speed. Like I said,
when I broke it in I didn't have this problem. I had a bad leak so I took the carb apart and check the metering block etc., all was ok. When I put it back together I had the float level waayyyy off, fuel everywhere. I finally got them set and now I have this "after fire" (not through the carb) problem.

The Tim Guy
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Old 01-26-2008, 06:17 PM
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afterfire

Do you have any exhaust leaks at header and collector flanges? If you're not having "backfire" under acceleration, but you have "after fire" when you leave off the throttle, you could be running rich. If everything is OK but you have exhaust leaks, you will have "backfire" in the pipes when you let off the throttle.
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Old 01-26-2008, 10:35 PM
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Quote:
It ran perfect (no after fire) before I replaced the ignition (one pertroix ignitor, with another). Anyway would float level give me too much fuel. Its happening under 3 grand so I don't think the secondaries are opening yet.
theres your problem, i had pertronix in my old buick before the hei dist. it would diesel on me and make it hard to start when warm, i have had no problems with the crane piece like the pertronix. change it to something else and give it back.
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