Quadrajet primary metering rods - question
04-24-2016, 01:54 AM
#1
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Quadrajet primary metering rods - question
In my C1 ('62) a 305 cui engine from 1977 and a powerglide transmission are installed. I bought it as it is and the last work to do is rebuilding the Rochester Quadrajet carb ( MV-Type).
The consumption before the rebuild of the carb was very high ( about 9 miles per gallon) although I was just cruising and the engine is not the most powerful version of the GM engine line. The ignition system is a HEI-distributor and the timing is correct, the spark plugs and the coil are new, the compression is very good on all cylinders. I assume that the rods are wrong and the carb was probably a used one from a bigger engine.
I have a new set of primary metering rods in a size of 0.037; 0.041 and 0.045 ".
My question regarding reduction of the gas consomption:
Which primary rod should I use, the thinner one or the thicker one?
With my Corvette I am normally just crusing at a speed between 60 and 80 mph.
I am living in Germany and we have no speed limit on highways. But for going faster for safety reasons I prefer my Porsche.
Thanks
Alfred
The consumption before the rebuild of the carb was very high ( about 9 miles per gallon) although I was just cruising and the engine is not the most powerful version of the GM engine line. The ignition system is a HEI-distributor and the timing is correct, the spark plugs and the coil are new, the compression is very good on all cylinders. I assume that the rods are wrong and the carb was probably a used one from a bigger engine.
I have a new set of primary metering rods in a size of 0.037; 0.041 and 0.045 ".
My question regarding reduction of the gas consomption:
Which primary rod should I use, the thinner one or the thicker one?
With my Corvette I am normally just crusing at a speed between 60 and 80 mph.
I am living in Germany and we have no speed limit on highways. But for going faster for safety reasons I prefer my Porsche.
Thanks
Alfred
04-24-2016, 06:35 AM
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The reigning authority here is Lars Grimsrud and his quadrant papers (do search). He's done a lot of valuable research and is what I used in setting up my q-jet. Mine is an MV also on a stock 255hp 1969 350. It was a mess! I'm now using .069 jets and .044 rods. Runs great and getting 17 mpg. Also there is Cliff Ruggles (cliffshighperformance.com) who is also an authority with his own website and forum.
04-24-2016, 08:50 PM
#3
In my C1 ('62) a 305 cui engine from 1977 and a powerglide transmission are installed. I bought it as it is and the last work to do is rebuilding the Rochester Quadrajet carb ( MV-Type).
The consumption before the rebuild of the carb was very high ( about 9 miles per gallon) although I was just cruising and the engine is not the most powerful version of the GM engine line. The ignition system is a HEI-distributor and the timing is correct, the spark plugs and the coil are new, the compression is very good on all cylinders. I assume that the rods are wrong and the carb was probably a used one from a bigger engine.
I have a new set of primary metering rods in a size of 0.037; 0.041 and 0.045 ".
My question regarding reduction of the gas consomption:
Which primary rod should I use, the thinner one or the thicker one?
With my Corvette I am normally just crusing at a speed between 60 and 80 mph.
I am living in Germany and we have no speed limit on highways. But for going faster for safety reasons I prefer my Porsche.
Thanks
Alfred
The consumption before the rebuild of the carb was very high ( about 9 miles per gallon) although I was just cruising and the engine is not the most powerful version of the GM engine line. The ignition system is a HEI-distributor and the timing is correct, the spark plugs and the coil are new, the compression is very good on all cylinders. I assume that the rods are wrong and the carb was probably a used one from a bigger engine.
I have a new set of primary metering rods in a size of 0.037; 0.041 and 0.045 ".
My question regarding reduction of the gas consomption:
Which primary rod should I use, the thinner one or the thicker one?
With my Corvette I am normally just crusing at a speed between 60 and 80 mph.
I am living in Germany and we have no speed limit on highways. But for going faster for safety reasons I prefer my Porsche.
Thanks
Alfred
04-24-2016, 10:45 PM
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Lookup your carb # as pamotorman suggested and then find the number on Lars paper, http://members.shaw.ca/crussel/quadr...ing_Paper.html
That will tell you what jets & rods to start with and go from there. Btw, you want a bigger diameter rod to go leaner.
That will tell you what jets & rods to start with and go from there. Btw, you want a bigger diameter rod to go leaner.
04-25-2016, 12:06 AM
#5
First step
As one who has specialized in the Rochester Q-Jet since 1970 I can tell you many variables figure into this. But first if you can get the car put on an engine analyzer with live scope then you can be sure everything else is good and to spec. If so, next step is to take it out and get it to full normal operating temperature, cruise at the speed you'll most be at on the road, "cut it clean" off and pull a plug and read it. That will tell you which way to go.