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Old 10-09-2007, 10:22 AM
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Exhaust Afterfire

I have a 468 BBC drag car with BG KD carb. I just replaced restrictive mufflers (very quite) with race mufflers. Immediately the car would not idle. Made slight increase in idle adj and car seemed to idle OK. Went to track and car backfired through the exhaust througout the run. Fuel pressure has always been high at 10 psi. Should changing just mufflers caused this. BG and Holley both say I'm running out of fuel (Holley Blue pump can't support 650 HP). Previous owner had pressure high to compensate fuel supply. Does all this make since to cause popping in the exhaust? Old school would say popping in carb is tuning, popping in exhaust is ignition. I'm confused.

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Old 10-09-2007, 10:48 AM
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Quote:
Originally Posted by weslyc
I have a 468 BBC drag car with BG KD carb. I just replaced restrictive mufflers (very quite) with race mufflers. Immediately the car would not idle. Made slight increase in idle adj and car seemed to idle OK. Went to track and car backfired through the exhaust througout the run. Fuel pressure has always been high at 10 psi. Should changing just mufflers caused this. BG and Holley both say I'm running out of fuel (Holley Blue pump can't support 650 HP). Previous owner had pressure high to compensate fuel supply. Does all this make since to cause popping in the exhaust? Old school would say popping in carb is tuning, popping in exhaust is ignition. I'm confused.
Old school probably covers 80% against 20% of the cases, which makes it not quite right some of the time.

You probably can't run more volume through the pump simply by raising pressure. It would be more appropriate to raise pressure from and excess of volume then drop it with a regulator before the carb, this would supply the carb with a consistent pressure volume all the time, where-as, what you have now is excessive volume and pressure at lower speeds that diminishes as speed increases. to accomplish a design with regulated supply will start with a pump that has excess capacity and a regulator. As it is the excessive pressure could cause the carb to run rich on the bottom end and steadily move toward lean at high speed. The lean out could be insufficient to overcome induction inertia so it's not backfiring out the carb. But a lean mixture also burns erratically and slowly which can result in after-firing at high RPM. Also carbs tend to lean out on the top end and if this is set up to deal with excessive line pressure, this issue could be additive.

Yes it could be ignition, but since you know you've got a fuel pump issue, I'd start there.

Bogie
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Old 10-09-2007, 12:12 PM
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Quote:
Originally Posted by weslyc
I have a 468 BBC drag car with BG KD carb. I just replaced restrictive mufflers (very quite) with race mufflers. Immediately the car would not idle. Made slight increase in idle adj and car seemed to idle OK. Went to track and car backfired through the exhaust througout the run. Fuel pressure has always been high at 10 psi. Should changing just mufflers caused this. BG and Holley both say I'm running out of fuel (Holley Blue pump can't support 650 HP). Previous owner had pressure high to compensate fuel supply. Does all this make since to cause popping in the exhaust? Old school would say popping in carb is tuning, popping in exhaust is ignition. I'm confused.
If all you changed was the mufflers and car was not popping before, it ain't the mufflers. Mufflers may slow it down a fuzz, but they won't cause backfire.

What else did you do? Any other adjustments,timing , valves,change plugs, etc.?
5&7 wires crossed?
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Old 10-09-2007, 02:28 PM
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Quote:
Originally Posted by dgcantrellsr
If all you changed was the mufflers and car was not popping before, it ain't the mufflers. Mufflers may slow it down a fuzz, but they won't cause backfire.

What else did you do? Any other adjustments,timing , valves,change plugs, etc.?
5&7 wires crossed?
Sounds like spark timing to me, too. What type of Ignition, MSD? How were the mufflers mounted, WELDED? I recomend pulling your engine and getting it DYNO tested with your complete ignition system.
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Old 10-09-2007, 02:39 PM
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It can't be the mufflers, the quieter ones would have causes more restrictions on your flow those would be the ones that caused the problems. When I took off my manifolds and put on headers the system would backfire also. Try tuning your carb and your fuel flow, just remember that every little thing you do to your engine will affect another part. I would go get it dyno tested, those guys over there know there stuff, often it just needs a little twist here or there good luck!
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Old 10-09-2007, 03:41 PM
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exhaust afterfire

I agree with Bogie's answer. Running 10 psi fuel pressure is just a tiny band-aid on your set-up, and will not make up for the lack of volume - volume being way more important. Now that the motor is ingesting more air (because you freed up the exhaust), the fuel demand is beyond the pump's capability to provide it.
Think of it like this - you will get more water out of a 2" pipe at 20 psi than you will a 1/2" pipe at 50 psi.
Bogie is smack on with his answer.
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Old 10-09-2007, 04:16 PM
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Back in the "old" days, I seen something enough times to know it can cause problems, but never heard of it in this case though, I have seen where the spark was dying which caused a backfire or a plugged fuel filter.
However I learned from what my older brother always preached, if you free up the exhaust you must richen the mixture to compensate. Backpressure which is just held back exhaust gasses in the cylinder after the exhaust stroke occurs, so less of the air fuel mixture can enter the cylinder due to the left over exhaust gasses. So in turn the carb is tuned for that exhaust system, then you have the exhaust system changed over to a low restriction dual. Now the system is free flowing, there`s little exhaust gasses left over in the cylinder, but the fuel mixture is still set by the old system, so no more fuel enters the cylinder but more air does, so the mixture is dead lean, and the carb cannot compensate for it, it has to be jetted up to be corrected. I seen in extreme cases the mixture is so lean it burns the exhaust valves.
Now I`m not saying this is the problem, but I am saying this does happen, when I changed the exhaust on my three wheelers to cobra systems from the dead stockers, I had to richen the mixture to get the lost power back. If the fuel pressure isn`t up to snuff then by all means get it back in order, then I would check the mixture, you can bet it`ll be on the lean side.
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Old 10-09-2007, 05:26 PM
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Bad plug wires can cause the same problem.
Look at them in the dark, a lite misting with water helps.
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Old 10-10-2007, 03:50 AM
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Exhaust Afterfire

Thanks for the tips and advice. To answer some of the questions... I have an all MSD ignition system with 6AL box. I didn't change anything on the car except the mufflers. Seems to make sense that I must have been on the borderline for the fuel system capabilities and when I changed the mufflers it pushed the fuel system over the edge. Your advice seems to agree with BG and Holley as well. I'm in the process of putting on a bigger fuel pump and regulator so that I can get enough fuel volume at recommended pressure. Will probably have to check carb jetting after that. Thanks.
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