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Quadrajet carb


Guest exbcmc

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My '73 GMC 350 cu has a quadrajet. I had a carb guy tell me he has a carb he can sell me that will out due this carb by 4 mpg. It's some carb from Japan that he swears by. (I'm sorry I don't know the brand) He showed me the bottom of the thing and the "holes" are much smaller than the larger two quadrajet secondaries. He said that for just driving around, it is great, just don't pull a trailer or boat etc, which I will not.

Any thoughts? Not only on the Japanese one, but on any such as the Edlebrock. Another mechanic said "Keep that quadrajet! It's worth a fortune!"

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The quadrajet s a fine carb with very small primaries and very large secondaries. Driven moderately it returns fine mileage but hard driving opens those secondaries resulting in great power with the resulting poor mileage. GM used the quadrajet on millions of vehicles throughout the years with very few problems. One area they did experience was weeking core plugs in the bottom of the carb. Check with your local shop to see if this is occurring.

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The quadrajet s a fine carb with very small primaries and very large secondaries. Driven moderately it returns fine mileage but hard driving opens those secondaries resulting in great power with the resulting poor mileage. GM used the quadrajet on millions of vehicles throughout the years with very few problems. One area they did experience was weeking core plugs in the bottom of the carb. Check with your local shop to see if this is occurring.

The leaky plug problem really only applies to the 1966-67 Qjets with the sheet metal plugs. 1968-later carbs have spun-in solid aluminum plugs that have eliminated this problem. On the list of likely problems with a 1973 Qjet, the plugs are WAAAAAAY down on the list.

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The large secondaries are often misunderstood.

The Q-jet is truly a variable CFM carburetor. Basically built in 3 sizes (although there were some smaller smog sizes in the 80's). The three common sizes:

(A) 750 CFM (most pre-1975)

(B) 800 CFM (most 1976 and newer, plus some 1971 and newer Buicks, and SD Pontiacs)

© 850 CFM (the single booster performance 1971 Pontiacs)

But these figures have to be qualified.

(A) 750 CFM is actually 150 fixed, with an additional 600 variable.

(B) 800 CFM is actually 200 fixed, with an additional 600 variable.

© 850 CFM is actually 250 fixed, with an additional 600 variable.

The amount of CFM on the primary side is dependent on the opening of the throttle plates.

The amount of CFM on the secondary side is dependent on the opening of the airvalve EVEN IF THE SECONDARY PLATES ARE FULLY OPEN.

As the secondary metering rods are connected to the airvalve, the amount of fuel passing through the secondary side is dependent on the airvalve rather than the throttle plates. Long story condensed is that the Q-Jet is only going to open the airvalve for the amount necessary determined by the needs of the engine (assuming the airvalve spring is not fatigued and correctly adjusted), even though the operator has his/her foot on the floor and the throttle plates are wide-open. VERY efficient design.

Of course, if the airvalve spring is fatigued, or badly out of adjustment, or the choke pull off is defective; the carburetor is not going to operate at its potential, but that is the operator's fault, not the carburetor's fault. I have yet to see a self-diagnosing and correcting carburetor of any brand.

And Joe is correct on the well-plugs (although a very few of the early production 1968's still had the pressed-in design). The later spun plugs are much more reliable.

Interesting (at least to me) that Carter and Rochester shared design information on the Q-Jet and TQ rather than the typical "lawyering". The Q-Jet came first, and Carter built lots of them for GM under GM license. Carter found that the 150 CFM primary side was too small for the larger V-8's and marginal for the smaller V-8's, and made the TQ's 200 and 250 CFM on the primary side. Rochester incorportated the 200 CFM primary about 1976. Carter also tried the single booster for higher output (Carter made a 1000 CFM) in 1969; and Rochester used it on the performance 1971 Pontiacs.

Like I posted earlier, unless your Q-Jet is completely worn-out, I would suggest rebuilding it yourself. If it is totally worn-out, then I would suggest having a professional rebuild it.

Jon.

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The large secondaries are often misunderstood.

The Q-jet is truly a variable CFM carburetor. Basically built in 3 sizes (although there were some smaller smog sizes in the 80's). The three common sizes:

(A) 750 CFM (most pre-1975)

(B) 800 CFM (most 1976 and newer, plus some 1971 and newer Buicks, and SD Pontiacs)

© 850 CFM (the single booster performance 1971 Pontiacs)

But these figures have to be qualified.

(A) 750 CFM is actually 150 fixed, with an additional 600 variable.

(B) 800 CFM is actually 200 fixed, with an additional 600 variable.

© 850 CFM is actually 250 fixed, with an additional 600 variable.

The amount of CFM on the primary side is dependent on the opening of the throttle plates.

The amount of CFM on the secondary side is dependent on the opening of the airvalve EVEN IF THE SECONDARY PLATES ARE FULLY OPEN.

As the secondary metering rods are connected to the airvalve, the amount of fuel passing through the secondary side is dependent on the airvalve rather than the throttle plates. Long story condensed is that the Q-Jet is only going to open the airvalve for the amount necessary determined by the needs of the engine (assuming the airvalve spring is not fatigued and correctly adjusted), even though the operator has his/her foot on the floor and the throttle plates are wide-open. VERY efficient design.

Of course, if the airvalve spring is fatigued, or badly out of adjustment, or the choke pull off is defective; the carburetor is not going to operate at its potential, but that is the operator's fault, not the carburetor's fault. I have yet to see a self-diagnosing and correcting carburetor of any brand.

And Joe is correct on the well-plugs (although a very few of the early production 1968's still had the pressed-in design). The later spun plugs are much more reliable.

Interesting (at least to me) that Carter and Rochester shared design information on the Q-Jet and TQ rather than the typical "lawyering". The Q-Jet came first, and Carter built lots of them for GM under GM license. Carter found that the 150 CFM primary side was too small for the larger V-8's and marginal for the smaller V-8's, and made the TQ's 200 and 250 CFM on the primary side. Rochester incorportated the 200 CFM primary about 1976. Carter also tried the single booster for higher output (Carter made a 1000 CFM) in 1969; and Rochester used it on the performance 1971 Pontiacs.

Like I posted earlier, unless your Q-Jet is completely worn-out, I would suggest rebuilding it yourself. If it is totally worn-out, then I would suggest having a professional rebuild it.

Jon.

thank you!
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Carbking would be interested in your thoughts on the Thermoquad. When it was introduced in 1968 it was supposed to be the greatest performance carb ever, with advanced features like insulated phenolic float bowl, large CFM etc.

It never caught on and I always wondered why. Was it no good or what? I know Chrysler used them for years so they couldn't have been all bad. What do you think happened?

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Rusty - it was 1969 instead of 1968, but what is one year?

Bottom line: We have a few more than 150,000 carburetors in inventory, and there is a Thermoquad on my personal GTO!

The story:

The Carter thermoquad or TQ was introduced as a racing carburetor in 1969. The first production had press-in jets to make changing of jets easy; and it turned out to be much more difficult that normal screw-in jets. Approximately 10 weeks into the production, the press-in jets were changed to screw-in.

For 1969 and 1970, the TQ was sold only as over-the-counter for high performance/racing applications. But in 1971 Chrysler decided to use it on the high performance 340's.

In 1972, the mixture control of the TQ was redesigned to give more control over smog emissions at lower RPMs.

The 1972's were designed with an O-ring seal in the bottom of the bowl. Carter found that the O-ring, which worked well in the laboratory, failed miserably on the street; so the O-rings were replaced in mid-1972 by X-ring seals.

The O-rings are one of two items which have caused the TQ to have a bad reputation, although in my opinion, TOTALLY UNDESERVED!

So why are the O-rings bad, when they were replaced after only a few months? Easy. O-rings in manufacturer's quantities are about 90 cents per thousand. X-rings, in the same quantity are about $2.00 each!!! Guess what the enthusiast gets if the enthusiast goes to the auto parts house and buy a cheap kit, because "the dealer's prices are ridiculous!"? The cheap parts house kit O-rings will fail after a couple of months, and the enthusiast bad mouths the TQ!

The second issue which has plagued the TQ is "split or warped" plastic bowls.

The first time the enthusiast goes to rebuild his/her TQ, he/she removes the 8 screws around the perimeter of the carburetor and shakes the carburetor to get it apart (and it doesn't come apart). Now the enthusiast pulls out the plastic hammer and taps the throttle body, again trying to separate the body castings, but they are "stuck" together. Now the enthusiast pulls out that most destructive of all tools, the screwdriver, and starts prying the castings apart; AND THE PLASTIC BOWL SPLITS!

Now thoroughly disgusted, the enthusiast goes to the parts house to get a replacement bowl (they are not cheap!), and the dude at the parts house shows the enthusiast the two "hidden" screws that reside under the choke plate that the enthusiast didn't see and didn't read the instructions.

And later, a friend asks "How do you like your thermoquad". Answer - "It runs great, but the bowl warped on mine". Anybody that thinks this dude is going to own his/her mistake doesn't know human nature!

And so, the TQ, in the opinion of a few that understand them, is one of, if not the finest of all street carburetors; and to those who simply parrot what they have heard, it is a "piece of ....".

The biggest current problem with the TQ is many parts are available only mail-order from that grumpy old hill-billy in Missouri ;)

Like I stated earlier, I have one on my personal performance GTO.

Oh, and Ford used several on performance EXPORT applications.

And here is a reference link to a satisfied racer:

http://www.dapa.org/drag-racing-with-jim-hand-part-16-a-q-jet-alternative/

Jon.

Edited by carbking (see edit history)
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So you can confirm they are a good carburetor. I always wondered why they never caught on with racers or hot rodders.

1) Most people lack the patience or skill to work on them, thus the "quadrajunk" slur.

2) Car magazines write articles that feature the products of their advertisers. Aftermarket carb companies bought (and continue to buy) advertising space. Rochester did not. If a Holley was featured in Hot Rod, it must be better... :rolleyes:

I'll also point out that there are a LOT of racers who used Qjets. I suggest the Roe book on Qjet carbs.

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Rusty - lots of pixels have been unjustly disturbed about what is the "best" carburetor for racers. I tend to believe any of the major brands (Carter, Holley, or Rochester) can be made to function within a fraction of each other based on the skill and knowledge of the tuner. The TQ does have a very slight advantage with the thermoplastic bowl maintaining a slightly cooler fuel temperature (slightly more oxygen), but only maybe 2 1/2 to 3 percent. As mentioned above, major disadvantage in parts availability today.

But for street, the spread-bore just plain works better than a square-bore for virtually all applications (heavy duty trucks are an exception). As both the Q-Jet and the TQ are spread-bores (Holley and Autolite also made spread-bores) both the Q-Jet and TQ are excellent for the street. As mentioned above, with an inventory of more than 150,000 carbs, I personally chose a TQ for my personal car. (I also have a set of dual genuine Carter AFB's on my shop truck).

For those that like the Q-Jet, the book written by Cliff Ruggles is required reading. And there are really good books on the Holley. Unfortunately, I have yet to see a good aftermarket book on any of the Carters. The best information on Carters is found in: factory service manuals, the Carter Master Parts and Service book, and the Carter Competition catalog.

Jon.

Edited by carbking (see edit history)
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I've owned a number of GM products that had Q-Jets on them and they were among the most dependable carburetors I had. I rebuilt the one that was on my 67 Cadillac only to see if it would run a little better but it made no difference whatsoever. Aside from that, I never needed to mess with them at all.

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Joe you misunderstand me.

I was asking why the Thermoquad never caught on like the Quadrajet.

Around here, everyone used Quadrajets. Remembering the favorable publicity about the Thermoquad when it was introduced, and knowing how well it performed on Chrysler muscle cars, I always wondered why it didn't catch on. To this day the Quadrajet is the favorite carb of the GM camp, at least in these parts.

Edited by Rusty_OToole (see edit history)
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Perhaps the reason you are getting poor gas mileage is that you have a 350/400 transmission. Upgrade to a TH700-R4/4L60 or TH200-4R and you'll get reasonable gas mileage with an electronic overdrive. There are plenty of conversion kits and lots of you-tube videos to help. Did this with my 1980 Chevy pickup.

And you're really not hot-ridding your vehicle.

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Perhaps the reason you are getting poor gas mileage is that you have a 350/400 transmission. Upgrade to a TH700-R4/4L60 or TH200-4R and you'll get reasonable gas mileage with an electronic overdrive. There are plenty of conversion kits and lots of you-tube videos to help. Did this with my 1980 Chevy pickup.

And you're really not hot-ridding your vehicle.

FYI, neither of those GM transmissions mentioned are "electronic" overdrive. Both the 200-4R and the 700R4/4L60 use conventional hydraulics to actuate the OD clutch pack. The only "electronic" aspect of either trans is the torque converter lockup function, which can be activated by a number of schemes that apply +12V to the activating solenoid.

A bigger problem is that neither trans is particularly strong. You'd need to get one properly beefed up, and by the time you add in the other required mods, you're talking $2500. A 4L80E would be the preferred OD trans, as this is really a TH400 with an extra OD gear set (the 4L80E uses many of the same hard parts for gears 1-3 as the TH400). Unfortunately, this is an electronic trans and requires a stand-alone trans controller, so again you're looking at $2500 or more.

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Guest exbcmc
FYI, neither of those GM transmissions mentioned are "electronic" overdrive. Both the 200-4R and the 700R4/4L60 use conventional hydraulics to actuate the OD clutch pack. The only "electronic" aspect of either trans is the torque converter lockup function, which can be activated by a number of schemes that apply +12V to the activating solenoid.

A bigger problem is that neither trans is particularly strong. You'd need to get one properly beefed up, and by the time you add in the other required mods, you're talking $2500. A 4L80E would be the preferred OD trans, as this is really a TH400 with an extra OD gear set (the 4L80E uses many of the same hard parts for gears 1-3 as the TH400). Unfortunately, this is an electronic trans and requires a stand-alone trans controller, so again you're looking at $2500 or more.

Thanks again to all for your input....

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And, while many do not like them, a less expensive method of improving fuel economy is to swap in a 4-speed manual transmission. Of course, mileage will vary with your driving habits. Not a Chevrolet, but my 390 went from 17->22 when the C-6 was replaced with a 4-speed manual (highway mileage).

Does require a different set of pedals, and some additional linkage, but these can often be found in a salvage yard.

Jon.

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I completely understand wishing to keep it original.

I would not have suggested the manual conversion except your original post was about changing carbs for better fuel economy.

And the work scared me as well, which is why I didn't do it! Went to the local transmission shop, and asked what I needed. Got a laundry list, and went to a salvage yard. I bought all the parts at the salvage yard, bought new clutch, pressure plate, and throw-out bearing at the FLAPS, and took everything to the transmission shop, along with the truck. Went back two days later, and drove home the truck.

Shop accepted the used C-6 in full payment for the labor.

Jon.

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Guest exbcmc
I completely understand wishing to keep it original.

I would not have suggested the manual conversion except your original post was about changing carbs for better fuel economy.

And the work scared me as well, which is why I didn't do it! Went to the local transmission shop, and asked what I needed. Got a laundry list, and went to a salvage yard. I bought all the parts at the salvage yard, bought new clutch, pressure plate, and throw-out bearing at the FLAPS, and took everything to the transmission shop, along with the truck. Went back two days later, and drove home the truck.

Shop accepted the used C-6 in full payment for the labor.

Jon.

I love it!

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