Carburetors #2: Rochester

well, like everyone else when i was in high school auto shop, some of the first carbs i was into were not by choice they were by necessity…..and they were the o so infamous quadrajet…I.E. quadrajunk carburetors. q-jet’s were not a bad carburetor for the most part. when they were right- they were damn near indestructable but when they fucked up….oh man what a piece a shit! and of course being that i went to high school in the early 90’s you can bet your ass i got to play with some of the E4M  electronic q-jet carbs when we had to make em work to get to school….ugh….and the dualjets….anyways i digress. this isn’t a tutorial on the history of the rochester carb division, just a bit on my knowledge of them.

lets start with the Q-jet. although they look the same for the most part over the years, they are all far from it. produced from the mid 60’s to the mid 80’s….and put on damn near every kind of gm car there was, when you run across one now you pretty much need to go by what the code is on the side of the main body to see what you exactly have. chevy and cadillac carbs had a side fuel  inlet to the right side of the engine/ BOP had a larger inlet the pointed straight out the front. as far as bolting up to the intake the carbs interchange- and the baseplate will also interchange with spreadbore carbs from holley. as far as exterior choke set ups they are all over the place. the cfm capacity was also all over the place from 500 to as much as 800cfm capacity from the factory and pretty much look identical on the outside.  most of the cfm capacity was dictated by the size of the rear barrels of the carb, the front barrels being the primary operating section of the carb and usually about 20-30% the size of the rear barrels. and most of the time you usually have to stand on the floorboard to open up the back barrels only to get a cloud of black smoke out the tailpipe. therefore when you miss on the cfm size like when you put a 750cfm off an olds on your SBC, it runs like a champ until you try to open up all 4 barrels.

the front barrels are  90% of a q-jets normal function. the float bowl sits at the front between the front barrels, directly behind the float,a vacuum operated metal power valve using metering rods  is located for main fuel metering.  the metering rods are forced into the center of the metering jets for the front barrels via vacuum. when engine load increases/vacuum decreases the rods pull out of the center of the jets allowing more fuel to enter the barrels. here now you can modify the q-jet for different applications with different size metering rods and jets….the jets are stamped with a number from the factory, as well as the metering rods. the air horn has to be removed to do this. it  is a real pain in the ass to remove the air horn compared to other carbs, it is not tuner friendly. ideally find the carb for your application to begin with and don’t fuck with it.

the rear barrels of the q-jet are very basic. simply 2 tubes hanging out  over two throttle blades and are fed by two jets at the bottom of the float bowl. the back barrels are openned through a progressive spring loaded linkage, usually start to open when the front barrels are damn near full open, and can be modified to open sooner in the rpm range with a bit of creativity. the primary metering device for the back barrels is the air valves located in the air horn directly behind the choke.  a vacuum canister usually hooked to the right front of the q-jet controls the rate at which the rear barrels take effect. this is resposible for the o-so -famous bogging sensation of the q-jet when you punch the throttle. a performance mod you can do to aid this is to drill the orifice in the vacuum can out a few drill bit sizes….or even let the rear air valves float freely without being hooked to the vacuum can at all.

the biggest problem with the q-jet in my book is the front throttle plate shafts. as they see almost all the work they wore out and got real sloppy in the base plate- which creates a vacuum leak that can come and go at any time and give you all kinds of shit like occassional detonation- backfire-hesitation-hanging throttle. back in the day there was no repair kit for this so we just cranked the idle mix screws out a bunch or partially wired the choke shut so we could get to school. also on the bottom of the float bowl supposedly the blocking plugs are prone to leak, but i never ran across one that did.

then of course there were the Q-jets which are referred to as the E4M carbs. basically they were an abortion. what it was is a failed attempt at making a carb work like fuel injection.  is in place of the vacuum controlled main metering valve it was an electric solenoid that pulsed the metering rods into the jets. the more often it pulsed the leaner it ran…the less frequent it pulsed the richer the mixture. this was, and is, know as pulse width. there are a throttle position sensor and an ignition control to the module in the distributor. modern fuel injection is based on the same principle of pulse width but it is much more efficient and solely relied on electronic injectors. these things were out of calibration and running like shit in lessthan 10,000 miles from the factory in some cases. they required a specialty tool for calibration called a sheppards staff if i remember right….and were damn near impossible to ever get right. your best bet was to usually hope the solenoid would short out and hope that the car didn’t smoke black.

another carb that i dealt with and still occasionally see is a dualjet rochester. it is basically a q-jet carb without the back barrels. variations of this were used from the late 70’s to the mid 80’s both with and without electronic bullshit. the non emmisions carbs were a pretty good carb, but the emissions carbs with the electronics were junk.

and then there is the very popular 2g and 2gv carbs. these are a very simple 2 barrel carb and also came in different calibrations and venturi sizes over the years for both big block and small block applications. there are no metering rods but a single metering valve in the bottom of the float bowl that was actuated by a separate vacuum actuated rod sticking into the float bowl from the air horn.  for the most part a very reliable carb and widely used in demo derby and street use. for all intensive purposes: not a good performance carb. to make serious power years ago they would use three of these things as a 6-pack to get enough cfm into the engine. now days you can get a large amount of cfm from a single 4 barrel so it becomes more of something for nostalgia than anything. if you are forced to use a stock carb for circle track or something like that, then there are places you can go to that will get the job done.  

these are the carbs that everyone claims to be a specialist so i will not elaborate but on a few things here:

– if you do not need a choke. make sure you plug the vacuum port of the base plate that draws hot air for the choke if you remove it.

-make sure the baseplate and throttle shaft are snug. also it seems that the throttle levers can losen.

-buy a rebuild kit from the referrence number on the side of the main body. this is what i do.  it will also show what it was originally for if you dig far enough.  you usually get the right gaskets as over the years the 2g did change slightly on some of the gaskets, but for the most part it didn’t change much, just enough to make it a pain.

–  only change jet sizes if you have a problem that nothing else can resolve. to make life easier you can drill and retap the jets to fit holley jets to make life easier.

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