rochester quadrajet - Impala Tech
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post #1 of 4 (permalink) Old 08-17-2014, 02:17 PM Thread Starter
 
 
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Location: rhode island
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rochester quadrajet

68 R/Quad off 396 camaro, manual trans. (7028210) jet size 71, primary rods, A over 98 or 4 over 98, difficult to read, secondary rods look like A over X. Set on top a 66 283, 60 over with mild cam. I know this carb was used on a wide range of engines and vehicles. What i would like to know is jet and m/rods sizes used on small block application. No AC, with p/steering, p/brakes, and 2 speed power glide. Also any thoughts on detent rod modifications?
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post #2 of 4 (permalink) Old 08-17-2014, 07:51 PM
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A QuadraJet designed for a manual transmission is going to be missing the kick-down lever you need that is attached to the driver side throttle shaft.

The other issue is the mid year 1968 production run started the 1968-'74 dual tapered metering rod that is not interchangeable with 1974 and up (electric QuadraJets) or the single tapered metering rod used from it's introduction in 1965 through 1968. Examine the rods for a double step at the end of the rod.

Rochester QuadraJets are great carbs when properly tuned. Problem is they are not the easiest carb to tune because part of the transition circuit and the main metering circuit are machined (requires more machining to modify) into the body and base plate of the carb. You can change jet sizes or metering rods up to ten numbers to compensate for a mismatched transition slot or air bleed. This complexity was the motivating factor that has me using Holley 4150 or 4500 series carbs today.

A further factor is that the carb is vacuum controlled! Not unusual considering all carbs rely upon engine vacuum to work. However the Rochester design differs from many other cabs by using straight manifold vacuum for some functions, but relying upon springs to counteract vacuum for other functions. If you change a cam such that the manifold vacuum varies the carb has to be tuned all over again (usually by a professional). This is because springs are linear and do not change the pressure exerted; while the vacuum with a different cam does, so what was a good tune with one cam potentially requires a trip back to the shop with a cam change.

Finally I think that the very few "Rochester Gurus" do not freely share information as there is a lot of contradictory information on the web; and it is difficult to separate the kernels of truth from the chaff that the "expert" posters on various boards throw up in the air. Though there are charts listing metering rods, main jets and required air bleed bore to work I do not know how reliable that information is. Haynes Motor Manual cautions against any changes to the carb and states you shouldn't attempt to modify it (just rebuild it), if that doesn't fit your need try a different carb off a different car. (these are old books published in the mid seventies)

I freely admit that I am no Rochester QuadraJet Expert (nor do I have delusions of ever reaching Guru status), so don't ask me questions I can not answer. This is because if I where using a Rochester due to class rules it would be tuned by a professional. I might add that I would gladly pay not to have to work on a Rochester QuadraJet; though I have had to rebuild a bunch of them (I swear carb cleaner dissolves those tiny spring steel hair pin clips; even when reinserted in the rods to keep track of them).

Big Dave
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post #3 of 4 (permalink) Old 08-17-2014, 08:54 PM Thread Starter
 
 
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Thanks Dave..I did rebuild it and it runs fair, perhaps a little richer than leaner, but we will run it for now. What is a good bolt on replacement ?
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post #4 of 4 (permalink) Old 08-18-2014, 11:31 AM
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If it works now you don't need a replacement. All carbs use engine vacuum to meter out fuel to air in a close (often fatter than optimal) ratio of 14.5:1. You want to go leaner than that down to about 12.2:1 for more power, but get too lean and you get into detonation. It was to protect the engine so that it will run for 100,000 miles that the factory chose to go too rich than too lean.

Unless you buy a "race" carb you will find the same fuel ratio built into any other brand of carb you choose to replace it with. So you gain nothing by swapping. If you change your driving style or add a bigger motor, then you could consider going to a bigger carb (one with a higher cfm rating). The Rochester is kind of neat in that the cfm of air passing through the carb grows with demand. So it is very tolerant to over carbureting a motor (a common mistake made by newbies who assume if a 650 cfm carb works well then a 750 cfm carb will work better).

If you did buy a "race" carb then the initial fuel ratio will be lower than for a street carb, and a race carb will offer screw in everything to tune the carb to your car's optimal fuel to air ratio for that afternoons pass at the strip. Trouble is tomorrows weather (humidity and temperature) will differ from today's, and the carb could be too lean when you crank it up to off load it from the trailer. Optimal tuning is do transient that it changes with the weather (as demonstrated in yesterday's NHRA national meet at Brainard, MI race way). I hesitate to tell anyone to buy a race carb as it is a lot of work to keep the carb in tune and if you don't know what you are doing it can make the car impossible to drive very quickly. (this is the reason half the world swears by Holley carburetors and the other half of the world swears at them).

Big Dave
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