Quadrajet questions - Chevelle Tech
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post #1 of 15 (permalink) Old Feb 17th, 15, 1:26 PM Thread Starter
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Quadrajet questions

I have a 454 in a '71 Chevelle with what looks to be an old Quadrajet on it but the choke looks to be removed [ see attached photo ]. The engine starts and runs OK with it set up this way. It does seem to run rich however....
Couple of questions since it's been awhile since I have had to mess around with a carb ....
1. Is this a common mod that is done to these carbs, removing the choke from the primaries ?
2. What causes the air doors on top of the secondaries to open? That vacuum actuator on the front side ? IT doesn't appear to have enough stroke to open the doors... is there something else in the linkage I can check that does that ? When I rev it the doors stay closed.
3. I did do an adjustment of the idle screws and it seems they were set to about where they should be, but the engine seems to run rich based on the smell of the exhaust [ thank goodness this car is smog exempt ]. What else can cause it to run rich? Worn out jets ?
4. If I want to replace this old Quadrajet, what would you recommend for a stock 454 LS5 with headers ?

Thanks !!!
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post #2 of 15 (permalink) Old Feb 17th, 15, 4:42 PM
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leo
 
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Re: Quadrajet questions

sunrise7(since you didn't put your name in there) Some folks may remove the choke air blades, but not a usual mod. On the secondaries, I don't think that just opening the throttle to WOT from idle sitting in your driveway or garage will make the secondaries work.

If you need your Qjet reconditioned, I use and recommend www.customrebuiltcarbs.com Phil is the owner and answers the phone himself. Just in case that isn't the correct carb for your car, he may have the correct one, you can do an inquiry on his site, good luck.

Leo Paugh
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post #3 of 15 (permalink) Old Feb 17th, 15, 5:14 PM
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Jim
 
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Re: Quadrajet questions

That carb looks to be in rough shape parts missing, etc. also it looks to be a later 75 up year carb, so it's not the correct one if that's what you're after. Find the number stamped on the drivers side should start 170.

Jim

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post #4 of 15 (permalink) Old Feb 17th, 15, 6:18 PM
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Re: Quadrajet questions

What is the number on the carb? Looks like a Pontiac? Also that is a electric choke. Are the wires missing?
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post #5 of 15 (permalink) Old Feb 17th, 15, 8:11 PM
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Re: Quadrajet questions

https://www.youtube.com/watch?v=JrQBCLdyDC8

These videos are a little "redneck" but they are informative and tells you parts people mess with that shouldn't be messed with.

The 72's did NOT have elec. choke so I don't think the 71's did either - it doesn't appear to be the original carb

Steve
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post #6 of 15 (permalink) Old Feb 17th, 15, 9:40 PM
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Bob
 
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Re: Quadrajet questions

I think q-jets are very good carbs for street use. They have always worked great for me when set up properly.

You may get some different ideas here, however I do not see any reason to remove choke blade for street use. If it were a race vehicle, that may be a different story, but I'm sure a race carb would be the way to go for that. As a technician, I see parts removed like that from those not performing the correct repair to the vehicle, instead someone might remove parts thinking it may help something.

Choke should be put back to working order in my eyes.

Secondary air valve or flap at top opens by the engine drawing enough air through it when the secondary throttle blade is opened. Normally if there was no way to delay the opening of it your vehicle would lose power from opening too fast. So, the linkage you see that has the diaphragm and hose on it... That keeps the secondary flap from opening too fast. At wide open throttle you start losing vacuum to that valve and it will allow the flap to open slowly.

You need full manifold vacuum going to it. Normally that hose gets plugged in ti the port right next to the diaphragm that you have plugged off. If it is getting full vacuum then its ok. With the engine running at idle the linkage should keep that flap closed.

It may be hard to get the flap to open just by reving the engine, the vehicle need to have a load. That flap also is what allows the control of the secondary metering rods to allow the fuel to the engine. (opens the jets in a sense).

It may run rich, due to the carb not being original to the vehicle, smaller jets in the primaries may help. Idle system has air passages and metered orifices that all work together.

Those year carbs are nice in my eyes, they are the ones I run, even though they are not correct for the year, or stock resto type.

If it were me, and having the parts to do the repairs, I would try and get that carb back to working order. A much easier solution would be to have it replaced with a similar carb. You can put the correct year carb on, or get a rebuilt, or new q-jet, or go after market.

For an easy job and fix, I would contact a shop that specializes in q-jets and buy one that is set up for your vehicle. Then the jets, metering rods, idle circuits - should be all set up nice.


There are many ways to go about getting a new carb, to me driveability is important and why I like good working Quadrajets. Some do not...

Bob
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post #7 of 15 (permalink) Old Feb 18th, 15, 1:01 AM
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Tom Terrific II
 
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Re: Quadrajet questions

too bad some dunce hacked the choke stuff out of it. HOwever, you might be able to get by with it like that since you live in Cal. Most of that state stays warm enough to get by with no choke on a Q as long as you don't mind baby-footing it a little.

The secondary throttle on a Q is in the baseplate like most other carbs, the butterfly on top is an air valve. it opens a more slowly than the sec throttle to keep from leaning out the engine with fresh air with the giant secs open. That link from the sec air valve to vacuum choke pull-off is needed, don't remove it.

The smell of the exhaust is likely raw fuel/air mix caused by mis-fire at idle. Do you know how to do a cylinder balance test to find if there's a problem cylinder? This is often caused by a bad plug wire or vacuum leak.

Best carb for that engine is a Q, that's what fits properly on the intake manifold. There's reasons why Chevy used the Q on that engine stock.

Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.
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post #8 of 15 (permalink) Old Feb 18th, 15, 6:34 AM
von
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Re: Quadrajet questions

I'd find a junk (cheap) Q jet with hot air choke, which yours likely was before the elect choke conversion (hot air choke ones with the round plastic dial are easily converted to elect), remove the necessary choke parts and install them in yours to get the choke working again.

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post #9 of 15 (permalink) Old Feb 18th, 15, 3:10 PM
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Re: Quadrajet questions

Be carefull when doing any tuning on that carb because it looks to be a 1975 or newer gen q-jet and if thats the case i think it uses different length prim metering rods then the prior 74 or older q's use.

Anyway,a q-jet is a great choice for stock to mildy modded bbc street perf chevelle so if you want to get a great running carb with proper choke operation etc contact Sean Murhpy (q-jet expert!) and he will custom build/properly calibrate a q-jet to run well right out of the box for your setup.

Heres the link to his website.

https://www.smicarburetor.com/products/sfID1/28/sfID2/9

I have had Sean do mult q-jets for me inc replating for show cars i maintain for other people and he even did a great job a few Therm Quads used on later 60's early 70's Mopars too that are a spread bore design like the q-jet having sml primary & lrg secondary but has a black plastic body vs the q's metal body.

I have the stock q-jet on my car/motor that Sean replated that i tweaked a bit more for higher perf/more rpm & pwr on my 69 chevelle ss 396 with mildly modded #'s matching 396/now 402 bbc with approx 375-380hp/430-440 lbs trq,M20 4spd & 12bolt with 3.31's which runs strong for what it is,gets 12.7mpg mixed/15.5-16mpg open road driving.

Scott

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Last edited by SWHEATON; Feb 18th, 15 at 4:01 PM.
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post #10 of 15 (permalink) Old Feb 18th, 15, 4:12 PM
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Re: Quadrajet questions

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post #11 of 15 (permalink) Old Feb 19th, 15, 2:27 PM Thread Starter
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Re: Quadrajet questions

Thanks guys for all the great replies !!!!!! I will get the numbers off the carb when I have some time to get out to the garage, probably this weekend.... Even without the choke, the big block starts cold with a couple of pumps of the pedal.... It just seems to run rich to me based on the smell of the exhaust... Guess I should pull the plugs and take a look. Otherwise car runs great ....... BTW,, I drove this car, as is, across the country in 2013. New York to Cali on it's own power...... It was a blast !!!
It's a '71 convertible and we took the Rt 66 corridor as much as possible ....
But the carb would be a good upgrade ... to a better Qjet....
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post #12 of 15 (permalink) Old Feb 20th, 15, 1:46 PM
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Corey
 
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Re: Quadrajet questions

Quote:
Originally Posted by sunrise7 View Post
I have a 454 in a '71 Chevelle with what looks to be an old Quadrajet on it but the choke looks to be removed [ see attached photo ]. The engine starts and runs OK with it set up this way. It does seem to run rich however....
Couple of questions since it's been awhile since I have had to mess around with a carb ....
1. Is this a common mod that is done to these carbs, removing the choke from the primaries ?
2. What causes the air doors on top of the secondaries to open? That vacuum actuator on the front side ? IT doesn't appear to have enough stroke to open the doors... is there something else in the linkage I can check that does that ? When I rev it the doors stay closed.
3. I did do an adjustment of the idle screws and it seems they were set to about where they should be, but the engine seems to run rich based on the smell of the exhaust [ thank goodness this car is smog exempt ]. What else can cause it to run rich? Worn out jets ?
4. If I want to replace this old Quadrajet, what would you recommend for a stock 454 LS5 with headers ?

Thanks !!!
I can only comment on a replacement carburetor, and that is a Sean Murphy, Stage II, QJet carb. I have used one on a recent and similar LS5 build, but with a modern cam (XE256), and it ran very well for me. Currently, I am building a ~500HP 454 and will use the same carb, which is going on the dyno this week.

Best wishes to you!

Corey
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post #13 of 15 (permalink) Old Feb 20th, 15, 1:50 PM
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Corey
 
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Re: Quadrajet questions

Quote:
Originally Posted by SWHEATON View Post
Be carefull when doing any tuning on that carb because it looks to be a 1975 or newer gen q-jet and if thats the case i think it uses different length prim metering rods then the prior 74 or older q's use.

Anyway,a q-jet is a great choice for stock to mildy modded bbc street perf chevelle so if you want to get a great running carb with proper choke operation etc contact Sean Murhpy (q-jet expert!) and he will custom build/properly calibrate a q-jet to run well right out of the box for your setup.

Heres the link to his website.

https://www.smicarburetor.com/products/sfID1/28/sfID2/9

I have had Sean do mult q-jets for me inc replating for show cars i maintain for other people and he even did a great job a few Therm Quads used on later 60's early 70's Mopars too that are a spread bore design like the q-jet having sml primary & lrg secondary but has a black plastic body vs the q's metal body.

I have the stock q-jet on my car/motor that Sean replated that i tweaked a bit more for higher perf/more rpm & pwr on my 69 chevelle ss 396 with mildly modded #'s matching 396/now 402 bbc with approx 375-380hp/430-440 lbs trq,M20 4spd & 12bolt with 3.31's which runs strong for what it is,gets 12.7mpg mixed/15.5-16mpg open road driving.

Scott
Scott, I agree that Sean builds a very good QJet, and I used a Stage II on a recent and very mild 454 (460HP). I was able to get almost 20mpg in a Chevy Caprice, 2dr. Landau, 200R-4, 3.08 geared, setup.

Good information for the poster, too!

Corey
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post #14 of 15 (permalink) Old Feb 20th, 15, 6:21 PM
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Derrick
 
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Re: Quadrajet questions

that's a late 70's carb with an electric choke housing from a mid 80's carb. looks to be a Chevy carb just going by the way the fuel inlet comes in from the side.

someone probably took the choke butterfly out because they couldn't get the rod to hook up to the linkage down inside the carb body- that can be tricky if you haven't done it before..

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post #15 of 15 (permalink) Old Feb 20th, 15, 9:30 PM
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Re: Quadrajet questions

Call Phil at the link I posted, He'll be able to help you get the correct carb for your car.

Leo Paugh
Maryland Chevelle Club #017
A.C.E.S.#3731
progress has little to do with speed, but lots to do with direction

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