Thread: Popping back through carb

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  1. #1 Popping back through carb 
    Little Brit
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    As the title states, My Holly equipped 3.5 has started back-firing through the carb quite dependably starting at a little over 3,000RPM. Most noticeably at full throttle. Though a surge-y misfire can be felt at smaller throttle openings at that same 3,000RPM. Engine is mechanically quiet and has a smooth 850-900RPM idle, also runs well otherwise. Distributor is a magnetic breakerless SS42 series Mallory firing a Mallory 29219 coil wired straight 12V, not through a ballast (per Mallory's instructions for this distributor/coil combination).

    Several months ago before this problem showed up, noticed all lights self dimming, brightening, dimming, brightening, repeat. I thought I had a grounding issue and ran a separate ground cable from the battery in the boot to the ground strap in the engine compartment as well as smaller gauge ground wiring for ignition and alt. Seemed OK for a bit but since the lighting deal was intermittent, I'm thinking it was just coincidental. Do not really drive the car much in the dark but did finally notice that as the lights were going nuts the volt meter was pegged, then coming back to normal, then pegging again. A lights off daytime ride showed the same high voltage readings. Also I should mention the car just did not run as crisply when these voltage spikes were happening.

    A new voltage regulator wired into the Lucas alternator got the charging voltage consistently back to where it should and but for the mysterious back-firing through the carb, all is good. Should also mention that with the secondaries wired shut, the issue was the same and at the same RPM.

    So,,, Here's my $10,000 question. Could the overcharge conditions caused by the bad voltage regulator cause the distributor control module to go bad in such a way as to work at lower RPM but break down at higher RPM? Anyone have some insight on this? Woody? The distributor's from you.
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  2. #2  
    Little Brit
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    Nothing from The Wedge Shop eh? Oh well.
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  3. #3  
    Do you still have the air pump hooked up?
    The Wedge Shop
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  4. #4  
    Little Brit
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    Quote Originally Posted by The Wedge Shop View Post
    Do you still have the air pump hooked up?
    Thanks for the come-back but no this is an originally FI car that never had an air pump.


    What do you think about damaged module? Can one fail like this or do they sign off like light bulbs?
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  5. #5  
    Is it still an FI car? We've seen a lot of strange stuff happen to the cars from ageing ECU's, so yes certainly could be from that, especially if it was sudden.
    The Wedge Shop
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    www.thewedgeshop.com
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  6. #6  
    Little Brit
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    No, not still FI. As the first thread states, it is Holley equipped. In fact it is a 4160 390 CFM atop an Edlebrock Performer Manifold. This combo was well sorted and tuned in the past. It has recently started on it's own with no other changes in specs. Just the overcharge issue as stated in my original post. I know it's a lengthy post, but it pretty much describes the conditions.
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