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Old 10-09-2007, 10:48 AM
BogiesAnnex1 BogiesAnnex1 is offline
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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.

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