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post #1 of (permalink) Old 01-18-2003, 07:38 PM Thread Starter
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Holley Carb - fine tuning performance question

I have put a Holley 80457S Carb on a AMC 360. This model has the power valve blowout protection. I have been adjusting the carb using tips from TR. I have the carb running well up to a point. In the span of tuning the carb the floats have been set using TR's instructions. I have a smooth even idle. I moved up three sizes on the accelerator pump nozzle, adjusted the pump cam and added a quick change vacuumn secondary spring kit. I am getting what amounts to a single backfire through the carb during heavy acceleration.
As I accelerate extremely hard I get one backfire through the carb. This usually occurs after the engine is up in RPM's. Another way to repeat this is to cruise at @50 t0 60 mph and then attempt to accelerate - as if passing a another car.
I have gone through my Holley book. What I am thinking is that the power valve isn't providing enrichment at the right time and I should change it out to another size. I run the engine with a vacumn guage attached. The bacvkfire appears as the vacumn readings drop. I want to say it's as the reading goes below 6 hg. The idle reading is a smooth 18 hg. I forgot the cruise reading on the gauge.
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post #2 of (permalink) Old 01-19-2003, 02:05 PM Thread Starter
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Re: Holley Carb - fine tuning performance question

I just changede the power valve from a 6.5 to an 8.5. This should have provided fuel sooner. The end result was that the backfire through the carb got worse. I ended up with a series of rapid backfires in the carb. I did notice that the vacumn pressure did not drop as low as the previous runs. It's drivable so long as I monitor the vacumn guage to keep the pressure @ 8hg.
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post #3 of (permalink) Old 01-19-2003, 04:23 PM
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Re: Holley Carb - fine tuning performance question

Sounds to me like more of an ignition problem than carb.

Defective or loose plug wire - but not completely open -
causing higher voltage that under acceleration crossfires
to another plug, tracking in the cap, bad or dirty wire
insulation, or possibly even the plug wires are laying too
close for too long a distance causing crossfire.

Could even be wrong distributor vacuum.
Remote posibility too - wrong or mickey mouse plugs, or
gap too wide.
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