Quadrajet (4-barrel) Carburetor

Introduced in 1965, the Quadrajet is the only four-barrel carburetor that performed well during the 1960s, through tighter emissions in the 1970s, and lent itself to electronic feedback in the 1980s. The Quadrajet takes advantage of its “spreadbore” design to offer very good fuel economy and outstanding performance. Spreadbore means that it has small primary bores and much larger secondary bores. Most versions have 1-3/8” diameter primaries (very few had 1-7/16” primaries) and all have giant 2-¼” secondaries. During normal driving, the engine operates off the small primaries. Following the laws of physics, the higher the velocity of air through the venturis, the more efficiently you can meter fuel. With the small primaries in this carb you can take a 5,000 pound Cadillac or Olds 98 and achieve 16-17 mpg, which is not bad at all considering the heft of the vehicle. When the driver gets in a hurry however, the huge secondaries open and you can pretty much watch the gas gauge move. The “air valve” type secondary had two controlling features.

• The secondary throttle plates are directly controlled by the gas pedal. You can bang them open on demand

• The air valve plate at the top would open in reaction to the open secondary throttle plates at the bottom. The opening rate is crucial

Very few people understand the secondary operation and a lot of nicknames developed, such as “Quadrajunk”. This could not be further from the truth. The “Q-jet” is an excellent overall performer, and is very tunable to do whatever you would like to do with it. It has been said that Rochester Products manufactured about 120,000,000 Quadrajets in about 30 plus years. All 120 million had some issues:

• Are hard starting when hot

• Are prone to hesitations and flat spots on acceleration

• Rough idle, due to gasoline differences from 40 years ago

With modern tuning techniques however, just about every one of these issues can be rectified. Your mighty Q-jet can still thrive in today’s jungle!

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