Quadrajet about to be tossed.

[TCONNELLY]

I've tried about everything I can think of to get this carb to work right. I had the carb professionally rebuilt by a respected local specialist that has been rebuilding quadrajets for the last 50 years. He replaced the throttle shaft bushings and did a full rebuild. I've also changed to an HEI distributor and new plugs and wires, new ground straps. I've tried every combination of timing and idle mixture adjustment I can think of. The problem is with the idle. It's fine when cold, but as it warms up it starts to stumble and miss. It's very annoying sitting at a light with the engine shuttering erratically. Not so bad that it will stall. It usually runs quite well just off idle and while driving. If I don't give it too rapid accelerator it's quite crisp while driving. The last thing I can think of trying is a fuel pressure regulator. After that I'm out of idea's. If any one can offer any other things to try, it would be greatly appreciated.

Secondarily, are there any Holly's that would be a direct bolt on replacement? Or what would be closest to working. I don't want to get into another tuning nightmare, with changing jets etc.

The engine is a stock 402 BBC, in a 1972 C20.

[geezer#99]

90% of carb problems are ignition problems!!
What's your timing curve like?

[1970 charlie]

Are you running ported vacuum or manifold vacuum and as for the carb Holley did make a spreadbore carb I think it was a 650 cfm

[snj8198]

Are you by chance using any soft line anywhere in your fuel line? If this carb is a recent reinstall it's possible that junk got into the carb from the fuel line. Try blowing compressed air through the idle circuits and check your filters and that little inlet screen.

And as geezer said, be sure your idle timing is set right i.e. have enough initial timing, and are using full manifold vacuum for your vac advance.

Good luck.

[kenn]

Hello
I'm no expert and have had my share of Qjet probs and I'll bet you have tried all the usual fixes. It looks like your very close to getting it. I had to get the book on these and am still having my probs too. I think when your choke starts to open you are leaning out your idle circuit. If your mixture adjustment screws are not solving the problem it might be that the holes in the tubes that feed the mixture screws need to be opened up a tad.
To be honest thow I cant seem to get my own carb correct, I just thought that I would let you know that your not alone trying to get these things right.
K.E.N.

[Dean'smeanmachine]

I know it sounds crazy, but it may not even be your carb. i had a similar problem and like you , i thought it was the carb. turns out it was actually a bad points unit. just something to think about. check your timing too if you got a light

[54blackhornet]

My advise is don't give up on the Qjet. I had mine off my 71 GMC rebuilt by the Carb Shop in CA. Throttle shaft bushings replaced and well plugs resealed and all new ethanol approved seals and other wear items replaced. Ran like crap when I reinstalled it ? Had two other people who are familiar with them adjust it, still ran sh&$ty ? Installed HEI next, now it runs great when warm outside but still have choke issues. I must say it get tariffic gas milage compared to an Edelbrock

[davepl]

Either the guy who "rebuilt" it didn't do a great job or the problem lies elsewhere. Never hurts to have a known good spare carb on hand for some lazy troubleshooting, but I wouldn't condemn the QJet unless you know its bad.

I'm guessing vacuum advance is disconnected, or connected to manifold vacuum, such that you're not getting tip-in advance. Total random guess though.

[Dirt's72]

Is this the original carb to this truck? Not certain but idle circuits may be different from sb numbered to bb correct carbs. Did you go back and ask the guy whom rebuilt it and what did they say? I had my 72 c30 402 quadrant rebuilt by a local rebuilder and it ran good with points and I changed to hei and still good. I did have them install the carb after they rebuilt it just to be sure.

[68Gold/white]

Originally Posted by 54blackhornet

My advise is don't give up on the Qjet. I had mine off my 71 GMC rebuilt by the Carb Shop in CA. Throttle shaft bushings replaced and well plugs resealed and all new ethanol approved seals and other wear items replaced. Ran like crap when I reinstalled it ? Had two other people who are familiar with them adjust it, still ran sh&$ty ? Installed HEI next, now it runs great when warm outside but still have choke issues. I must say it get tariffic gas milage compared to an Edelbrock

X2
Rebuilt can mean many things...
Many don't care for Quadrajets, but they can be about the best possible carb for daily driving and performance...BUT they have to be tailored to your specific engine/vehicle, EVEN if it's original to the vehicle!!!

One important thing...Has or was it rebuilt through some company that sells their carbs through a parts store??? If so, you may need to find another carb to start with. THe rebuilders "Johnny Cash" carbs with all kinds of mis matched parts, no tell what you've got, if so...

First thing you need to do is find the carb number. Email number to Cliffshighperformance.com He stays very busy and cannot take many phone calls. He can send you all the parts you need to corectly tune/rebuild your carb. Buy his book and you will be able to do the work yourself!!!

[slikside]

What type of choke, if any, are you running? Is it adjusted properly to be fully open when warmed up?

[Redline novdog]

May not help ,but I'll ask anyway. Are you using a full 12volts at the HEI distributor ? and not the original resistor wire? If so I wondering if the distributor has a funky module in it! My $.02

[dirtyjim]

hopefully your 50 year carb rebuilding expert check the q-jet to make sure it wasn't warped before rebuilding it. if it runs bad when hot it could also be your coil going bad.

around 20 years ago I was working for a small auto repair shop and I needed to rebuild the junkyard qjet on my 84c10. I rebuilt it then drove next door to the gas station a filled the truck up. the truck immediately starts pouring black smoke out the exhaust, but it still ran great. I pulled that carb down and double checked everything at least 3 more times and several other guys at the shop looked it over too and couldn't find anything wrong. boss just shakes his head and says you ain't touching a customers carb ever again. it ended up being a tank of bad gas and all the smoke went away on the next fill up. drove me nuts for a week though

[jlsanborn]

Don't abandon the Quadrajet! See if this link takes you to a PDF file by Lars Grimsrud. If not, a quick Google search will get you there. First and foremost is to find the model number and make sure it's calibrated and adjusted to it's original specs. Most of the time, your done after that.

http://www.google.com/url?sa=t&rct=j...bzMPzm-iHyQqTw

[D.PASSMORE]

Issues that I've worked thru with QJETS: You need to establish that your ignition system is good (all parts) and you have it properly timed (the correct way). As mentioned, you need to know that you carb is rebuilt correctly. Your fuel supply is not contaminated and you are running correct fuel pressure. Manifold is sealed to motor, good carb to manifold gasket. Unhook all vacum lines and cap and ensure all are capped/plugged on the carb. Fire it up, Ensure divorce choke is not closed, I usually start 2 1/2 turns on mixture screws, get the ideal screw turned in a little more than usual to try and get a smooth run at about 1200 rpms. Let it warm up and start trying to back everything down to a good ideal. This includes playing with timing a little. What you looking for is different sympotoms to lead you to the problem, process of elimination. One thing that I've had to do is change jet/needle combination when increaseing proformance with a head, cam or manifold upgrades. Lars G. write ups and information is spot on when doing these kinda changes.

[TCONNELLY]

You guys have me motivated to persevere with the Quadrajet. I appreciate all the suggestions. I like the spread-bore concept in carburetors, so, want it to work. The guy who rebuilt the carburetor is local and rebuilds your actual carb himself. It's not sent out anywhere. He's a local guy on Long Island who was recommenced by multiple people.

The numbers on the carb body are as follows:
7042206 QA
0402
From what I have been able to find, the numbers match up to a 1972, 49 state, chevy with auto.

When I bought this truck last year, everything seemed to be very original, including the points distributor. I swapped out the points for HEI, and saved the original distributor to keep with the truck. I wired the HEI to direct 12 volt source off the fuse box, which I verified with a voltage meter.

I also tried different timing setups, ported vacuum, manifold vacuum, more advance and less advance. I also adjusted the idle mixture needles for richer and leaner settings and each different timing setup.

I will try some more of the suggestions made here. Attached, are some pics of the carb, in case anyone can spot something that looks wrong. I removed the air cleaner to take pictures, so the line from the air cleaner is disconnected in these pics. Which is why the vacuum port next to the choke does not have a line attached. The engine is cold in these pictures.

Thanks!

[firedemon]

don't know why its running poorly but please put a double spring on the throttle . if the single spring brakes it could go to the floor and will scare the hell out of you at the least hate seeing these trucks on the end of a hook

[D.PASSMORE]

Looks normal, stock BB motor, again troubleshooting can be long and time consuming. You have to start at the basic - fire and gas, determine which one it is. If it's running Ok under power, that is a positive sign, but I would verify good sealed cylinders and valves, then head to the ignition system and timing. Or, take it to a reputable shop that knows how to work on these older motors. Ever used a vacuum gauge? They are cheap and can tell you how your motors is acting by the vacuum that it is pulling.

[geezer#99]

Can you turn either mixture screw in all the way without the motor quitting?
And I'll ask again, what's your timing like.
What initial?

[TCONNELLY]

Originally Posted by geezer#99

Can you turn either mixture screw in all the way without the motor quitting?
And I'll ask again, what's your timing like.
What initial?

Turning in either mixture screw will cause the motor to die.
I previously was using manifold vacuum with 18 degrees initial and 24 degrees with the vacuum line attached. I found the timing would dip initially when depressing the throtle. I found this produced a slight hesitation. I changed it to ported and left the initial timing at 18 degres. Total is 36 degrees. This seems to be working better for off idle response. I think this means that it's running off mechanical advance until steady state cruise speeds. Please correct me if that is incorrect. I also tried advancing and retarding the timing under direct manifold and ported conditions. These are just the most recent settings that were working best.
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[geezer#99]

Are you plugging off the vac line to the distributor when you set the initial?
Your numbers are confusing.
18 initial and 24 with vac attached says you have only 6 degrees in the vacuum pot.
Plus 18 initial and 36 total says you have 18 mechanical.
Your motor runs off initial plus a progressive timing from the mechanical and an added vac advance at cruise when the throttle is barely open.

[67chevy_hotrod]

Second larger throttle spring x2 it's cheap insurance

[special-K]

I'm just going to hop in and ask about the condition of your torque convertor. I may be out in left field, but it sounds like hitting it at a different angle might help. It shutters coming off the line and doesn't idle well. Hmmm... makes me wonder what is known about the torque convertor. I also think toward vacuum issues which keeps one eye still looking at transmission area. No one has asked, so now I have. If it's all good we have crossed that off the list

[TCONNELLY]

Originally Posted by geezer#99

Are you plugging off the vac line to the distributor when you set the initial?
Your numbers are confusing.
18 initial and 24 with vac attached says you have only 6 degrees in the vacuum pot.
Plus 18 initial and 36 total says you have 18 mechanical.
Your motor runs off initial plus a progressive timing from the mechanical and an added vac advance at cruise when the throttle is barely open.

I am plugging off the vac line to the dist., when setting the initial. The vac advance is adjustable. I dialed it back so that I could have more mechanical advance at idle. Previously , I had 12 initial and 12 vac, at idle. I found that the dip in advance when stepping on the gas, from idle, was noticeable. I recently , changed to ported vac, to eliminate all vac advance at idle. I found this gives a smoother transition , as there is longer a dip in the timing, when giving it gas from idle. The vacuum drop causes the timing to drop back before the mechanical picks up.
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[TCONNELLY]

Originally Posted by 67chevy_hotrod

Second larger throttle spring x2 it's cheap insurance

I do actually have a second spring attached to throttle. It's just not showing up well in the pic. It's dirty and in a darker part of the photo. It runs parallel to the throtle cable. It's attached to the bottom of the throtle running toward the rear. You can just see it, it you expand the photo.
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