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Carburetor Swaps

I’ve been told that I need a new carburetor on my truck which is a 1984 Chevrolet C20 3/4 ton. It has the original carburetor and that’s been rebuilt twice 9 years apart. It’s an '84 Rochester quadrajet on a 350 engine. I have a usable spare quadrajet that came off an '84 305. My question is if I swapped them would I experience any issues in my truck’s driveability? Meanwhile I’ll look for a professional to rebuild the original if it’s still possible.

If your spare carburetor is good, you should experience no issues whatsoever. They’re the same carburetor.

If there is an electrical connection to the carb other than the choke heater, then it’s a feedback smog carburetor with a computer controlling or trying to control the fuel mixture…These can be a nightmare to successfully repair…A Quadrajet made before 1980 will not have these “features”…

If the spare carburetor has been sitting around for awhile, its seals could be shot by now so it might not work.

If your spare carb has been sitting around for a while, especially if it’s been used before, chances are it’s not going to work very well. Seals may be dried out or gummed up. I’d rebuild it and then put it on. Time was any corner garage could rebuild that carb for you, but it seems that it’s a lost art these days.

To mark9207, thanks for the info, I didn’t know they’d be the same, I thought they’d have different metering rods. Caddyman, yeah, they’re both electrical, and that may be one of the issues on the one on the engine now since I can smell a slight gasoline smell coming out of the exhaust at times.

Asemaster, agreed, some of the mechanics that have done work on the truck have tried to sell me on Holly rather than rebuild my Rochester.

Rebuilding the electronic Q-jet requires some special tools and installing it requires some specific expertise and working knowledge of a digital multimeter with a duty cycle function. And, there will be no certainty that the carburetor will operate correctly unless live data can be monitored.

And also, if the Q-jet is replaced with a Holley or other non OE carburetor the distributor must be replaced with an earlier model that is not ECM controlled. The most successful way to deal with engines with Electronic carburetors is to retrofit them with both the carburetor and distributor from a 1975 to 1980 model.

Just make sure it’s electronic rather than just an electric choke heater before you swap distributors. Pickups had much easier regs to meet.

That era of carbs GM was making modifications almost daily.

I owned a 84 GMC S-15. There were two different carb models (2SE and the e2SE). Within each model there were MANY different variations. Most were compatible with each other…but some weren’t. Not sure if GM was having the same issues with the Rochester Quadrojet. Hopefully not.

Odds are the 305 QJ is a bit different from the 350 QJ with minor metering rod or jet changes. However, it should still be be a bolt on and work fine.
There were more variants of the QJ than I can even remember; as in hundreds or even thousands.

If you choose to go with another carburetor, you might consider the Edelbrock 1406 which can be found used on the cheap and new at about 240 dollars. These carbs are reliable, simple, and very easy to tweak or rebuild if it ever came down to that.

When I said they were the same thing, I pretty much meant they were both 750 Quadrajets with electric chokes and, if they came off mid '80s Chevy C/K trucks, they should not be electronic feedback. I have never seen a Chevy truck of this era with an electronic feedback carb, but I suppose some could have come that way. Sorry if my info proved to be faulty, but generally, if you replace a 750 Quadrajet with another 750 Quadrajet that looks the same, it will work. I also agree with ok4450’s recommendation of the Edelbrock aluminum four barrel. These are simple, rugged, reliable, easy to service carbs that can be had with the same electric choke your truck came with or a manual choke if you prefer.

My search Re the carburetor indicates that 1985 and 1986 U.S. emissions light trucks used the Electronic(feedback) Q-Jet. The California models may have required that carburetor in prior years, though. There is some contradictory information and my books, for the most part, were for 49 state spec vehicles.

The Quadrajet was the best 4-barrel carb ever made…Once you understand their design features and weaknesses, they almost equal fuel injection in their performance…The only real weakness is the plastic float used in later versions and the welch plugs in the float bowl leaking and draining the float bowl overnight, making the first start of the day difficult…Both of these problems are easy to overcome…But once you get into electronic mixture control solenoids, all bets are off…

Once you understand their design features and weaknesses, they almost equal fuel injection in their performance..

Performance…yes…Efficiency and and emissions…NO.

My 67 Malibu SS with a 327 had the Quadrajet with the 2-speed power-glide transmission. When I stomped on the gas…and opened up all 4 jets…you could actually watch the gas gauge drop. Luckily gas was under 50 cents a gallon back then.

I think Caddyman has a point though; especially about open road driving. A properly setup QJ will provide as good or even better fuel economy than a TBI and some MPI setups.
A 91 Caprice with a TBI 350 is rated at 19 MPG and I’ve owned a number of older Chevys with 350s and QJs that all got 20-21 MPG on the highway. A few people I know have even managed to eke out near 25 depending on transmission and rear axle ratio.
Generally speaking on performance, it’s been dyno proven many times that a carbureted engine will outperform a fuel injected one at the very upper RPM range; not that many people drive around anywhere near that range though.

Heck, the 59 Corvette I had with a 283 and 4-barrel (granted it was a Carter) would tick off 21 MPG steady as a clock at a 70 MPH cruise and that was with a set of 4:11 gears in the back coupled with a 4-speed manual.
With a 5-speed and some highway friendly gears there’s no telling what it would have gotten.

The non electronic Q-jets were great carburetors that were easily repaired and rebuilt and offered exceptional fuel mileage and performance. I rarely removed them from the intake to make repairs. The vacuum secondary enabled the Q-jet to perform well on a wide range of engine displacements and performance options. For most applications replacing a Q-jet with any other carburetor results in a disappointed driver.

The Q-jet’s tiny float chamber delivered precise fuel metering even under extreme G-loading and tilt angles…The small, high velocity primaries deliver excellent fuel distribution and mixture control…The huge vacuum operated secondaries could be set up to deliver a seamless transition from high-vacuum cruise to zero vacuum WOT operation. The term QuadraBog was coined by those who adjusted the secondaries to open too fast, too soon, or worse, modified the linkage so they opened manually as the gas pedal was mashed down at low RPM’s…