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Old 05-17-2010, 02:45 PM
v8hed's Avatar
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Too rich?

Car is running a 383 sbc with a 750cfm Edelbrock carb... when I activate the secondaries (mechanical), the car hesitates/bogs for a second, then accelerates cleanly. When blipping the throttle at idle, I can see a small puff of black smoke from the exhaust tips (sidepipes). Does this sound like the carb is running too rich? Accelerator pump adjustment?

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Old 05-19-2010, 09:21 AM
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Yes and yes..............too rich and adjust the accelerator pump as well.
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Old 05-19-2010, 09:32 AM
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A bog with mechanical secondaries is normal unless there is a secondary pump shot. That is why vacuum secondaries are popular. Tuned properly vacuum secondaries are just as effective as mechanical are, and are a lot more pleasant to drive. Back in high school we used to put a screw in the secondary linkage of our Holley carbs . We thought the bog then the kick was cool, we know better now.

Vince
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Old 05-19-2010, 02:31 PM
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The momentary black smoke from the exhaust when you punch it is most likely carbon in the pipes from excessive idleing with a rich mixture and or rich fouled spark plugs.
Fouled spark plugs will give a momentary stumble/missfire reguardless of the carb tuning state.
The edelbrock carbs need a agressive spark advance curve. Both in terms of keeping the idle and plugs clean and crisp throttle response.
especially with a big cam.
Dial in the advance curves correctly.

Get a afr gauge and o2 sensor to help you dial in the carb calibration.
Inspect the secondary side of the ignition,,, plugs. cap rotor coil wires.
Make sure the distributor is getting full supply voltage.
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Old 05-19-2010, 03:10 PM
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Do what F-Bird has said with the timing first if you don't already have it that way, if you still have a stumble/bog after that then my next move would be larger pump shot as carbs rarely have an bog due to being too rich. The bog is usually a lean "hole" in fuel delivery not being "covered" with enough pump shot, so the engine falls on its face(stumble/bog) and then catches up with itself and powers up the rpm band.
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Old 05-19-2010, 03:54 PM
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Thanks for the replies. I seem to have eliminated the bog by switching the distributor vacuum advance hose to the full manifold vac port on the carb (right side port) and moving the pump shot linkage to the bottom hole (was on the top hole). Still getting a small puff of black smoke (only just visible when looking down at a sidepipe when blipping) but the bog/stumble when going to WOT has now disappeared.

Not sure if switching the vac advance hose fixed it, or altering the accelerator pump linkage did the trick. Will try reverting either the vac advance to the original port or changing the accelerator pump linkage back to the top hole and see if the bog returns.

Timing at idle with vac advance disconnected is 10-12* BTDC. Total timing is around 34-36* BTDC.
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Old 05-19-2010, 04:26 PM
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Quote:
Originally Posted by 302 Z28
A bog with mechanical secondaries is normal unless there is a secondary pump shot. That is why vacuum secondaries are popular. Tuned properly vacuum secondaries are just as effective as mechanical are, and are a lot more pleasant to drive. Back in high school we used to put a screw in the secondary linkage of our Holley carbs . We thought the bog then the kick was cool, we know better now.

Vince

I'm guilty of the same thing Vince


Cole
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