Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

looking for some better mpg/setup advice needed

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Cliff Ruggles:
Head castings play a HUGE role in efficiency.  There were quite a few offerings for SBC's over the years of production.  Many are "door stops" having crappy combustion chambers and very restrictive ports.

A good set of "modern" heads and higher compression will certainly help, as will tight quench. Most engine builds are WAY too low for compression with WAY too much quench in them, so they are turds for power production and suck tons of fuel for "normal" driving.

The RPM intake and RV cam bring nothing to the table for mild engine builds and stock compression.  Unless the block was decked/squared, tight quench, etc, it's probably less than 8 to 1 compression if 76 cc heads were used on it.

Anyhow, for the carb, the PP will be down for all "normal" driving with high vacuum, so use the APT system to fine tune part throttle A/F for best efficiency/fuel economy.  The amount of timing added by the vacuum unit is a big player here as well, and low compression engines require more timing or at least like a lot of timing to be most efficient.

For sure I would make the APT accessible without taking the top off the carb, and verify the hanger arms are level and exactly even.  Smaller main jets for the low compression 350, and put one of our kits in the carb to get it up to par for this new fuel......Cliff

mike mc:
I don't know specifically what head is on the motor but west coast engines stated the comp rate was 9 to 1. hp dynoed at 310 at the crank. 360 torque. I have a feeling the primary hanger arms may be bent. the reason I say that is I adjusted the apt all the way down which should have leaned the mix out . it did not. I would think with a .75 jet and 61s rod you should be able to lean the mix out so much that it shouldn't run. I am at a point where if the temp outside is cool "60 degrees" it will scream with no bogs. and get around 15 mpg. if it is hot "95 +" it usually will bog late in the acceleration, right before it goes into the next gear. I would think that would indicate running a little rich.

Cliff Ruggles:
I agree, something amiss if you can't lean it out with the APT system using those parts.

At least 95 percent of the carbs we get in here for rebuilding have bent hanger arms for the primary metering rods.  It's AMAZING that if anyone got to the carb in it's life for rebuilding, that they bent the arms taking it apart or putting it back together, but it happens almost every time......Cliff

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