Author Topic: Rochester Quadrajet Carburator (Read 7106 times)

SFRT - Frenchy · « **on:** February 28, 2006, 08:06:21 AM »

Hey I have a 750 R QJet on my ontiac 73 455. I am runing too rich. I have the carb set at the factory settings (2.5 turns).

To lean my mixture, I need to use the two screews on each side of the fuel intake. I always thought that those screews were comanding the amount of fuel I allow in. Yesterday, a mechanic told me those screews regulate the amount of air, so I need to turn them to the left to lean my mixture.

So, I'm lost. Which side do I have to turn those puppies, and what do they control, fuel or air?

lazs2 · « **Reply #1 on:** February 28, 2006, 08:22:40 AM »

running rich? what you are talking about is adjusting the idle circuit. The idle screws do indeed control how much air is let into the idle circuit.. but it is only good for about 1500 RPM .

Secondary circuits take over at that point. At what point is the motor running rich? just idling or driving down the road?

I have a quadrajet.. it is an edelbrock 850 but is pretty much the same. quadrajets tend to have leaks... the fuel leaks from the bowl into the intake... some rebuilders epoxy the plugs.

If this is the case.... let the car sit for a few days and then go out and pull the air cleaner... look down the primary and open the throttle assuming that the accelerator pump is in good condition... you should get a squirt of gas... if you don't... you may have to pull the carb top off and look... if the bowl is empty then the plug is leaking into the manifold...

mine leaks slightly.. it doesn't make the car run rich but if the car sits for a week I have to crank it over till the bowl fills up with fuel before the accelerator pump will work.

Is the car harder to start after sitting for a day or so?

lazs

RTR · « **Reply #2 on:** February 28, 2006, 09:52:56 AM »

hmmmm...way too rich? Making black ugly smoke, running really rough?

I had a 79 TA with a Rochester quadrajet do the same thing. (bear with me, it was 20+ years ago).

Seems to me IIRC there is a set of fly weights in there. The springs wear, and the flyweights swing out way to early. This causes the carb to go very very rich. It was a quick fix for mine. Just replaced the springs.

That Carb is a great carb though IMHO.

Hope this helps or at least makes sense.

cheers,
RTR

Ripsnort · « **Reply #3 on:** February 28, 2006, 10:13:03 AM »

I had a 455 olds and that Rochester carb used to be known in its day as "the quadro-bog", because of the initial noise that it made with those secondary butterflies opened up. I re-built a couple of them in my younger days, but at a certain point, replacement with a better carb is the only option.

Once you start messing with the factory settings of the needles, you're going screw things up unless you're really experienced with such settings.

http://www.jason.fletcher.net/tech/carbtuning/carbtuning.htm

GtoRA2 · « **Reply #4 on:** February 28, 2006, 10:55:23 AM »

If it needs a rebuild, check out chevie and Pontiac forums to find a guy who knows what he is doing, most shops dont and will just **** it up more then it is now.

Used to be a place in Ontario CA that specialised in the called the Carburator shop, but I hear the guy who owned it got killed in a motorcycle accident and the shop went down hill.

There was a guy on the classic pontiac forums who would rebuild them, but I dont remeber the name.

Edelbrock sells them, I think new, but they are pricey.

SIK1 · « **Reply #5 on:** February 28, 2006, 01:08:19 PM »

The Roachchester is what we use to call the old Quadrajets. I never had much luck with them but I do know guys that could make them run. Not sure about Edelbrock making them now I do know that Edelbrock is making a version of the Carter AFB's, a much better carb than the Quadrajet IMHO and both are better than a Holley for street use.

The best way to adjust the idle air mixture is using a vacume gauge, but if you can't use the vacume gauge method, with the car idling turn one of the screws in until the engine starts to stumble then turn it out until the engine picks up rpm about 100 rpm above idle then turn the screw in 1/4 turn, repeat on the other screw. You may have to do this more than once on both screws. If you turn the screw in and the engine never stumbles you have a vacume leak. A word of caution do not reef the idle air screw down against it's seat a light touch is prefered here.

Before you tear into the carb make sure that all of the other systems are working properly, check your timing, plugs, wires, cap and rotor, if it is a breaker point ignition check the gap and dwell of the points. After all that check both the manual, and vacume advance, (the mechanical advance is what I think RTR is talking about). If everything checks out good it may be time for a carb rebuild.

Sorry for the long post.

Cam

StarOfAfrica2 · « **Reply #6 on:** February 28, 2006, 01:41:38 PM »

I remember when I was a kid everyone called them the "Quadrajunk" and advocated something along the lines of tossing them in the garbage. Holley was the big deal then, and quite frankly I've never seen a worse "out of the box" carb than a Holley. I came to prefer the Carter over anything. I remember in the late 80s or early 90s people began looking for ways to get decent (well, liveable anyway) fuel mileage out of their resto-mods and discovered a couple of guys rebuilding Rochesters. As complicated and confusing as a Quadrajet can be, they really are wonderful pieces of work when they operate as they should. Key is having someone rebuild it that understands what they are doing. Its not quite as simple as my old Holley 600 was. Properly tuned, they are great. Out of tune, or worn out, they are a headache.

This site has a pretty good walkthrough of the process.

http://quadrajetcarburetors.com/bushingkitinstructions.html

SFRT - Frenchy · « **Reply #7 on:** February 28, 2006, 01:48:22 PM »

Thank you, the carb is actually out of the rebuild place. The place deals only with carburetors, and have been recomanded to me by an ex-SCCA champion. It's called CFM Carburetors, owned by Steve Zapeta.

My engine has 140,000 original miles. You know is overdue for a rebuild

Anyway, the original 73 Pontiac Rochester was replaced by an Oldsmobile Rochester by some crook in LA. What steve did was to work the Olds carb to make it a "Pontiac Carb". According to him, Pontiac Rochesters had some slight differences that made them a little more "performance oriented: compared to the Olds.

Back to the problem. The old carb used to give me sputers when I was floring it from low speed. I also used to have the big black cloud of gas in the exaust.

Since I put the new carb, I gained a lot in throttle response. The secondaries finally open .... Brrrrrrrrrrrrrrrrrrrrr BROOOOooooooo!:p
But when I stop at a red light from 50MPH, the RPM are initially fine, after couple of seconds, the oil pressure fluctuates from 40 to 10, and the engine fights not to stop. I have to push the rpm a bit for the engine not to quit. I did change the iddle settings (far right screew) linked to the stopper of the accelerator cable, but to no change. The initial iddle at the stoplight is high, then the engine starts missing till "brrtt .. brrrt .....brrttt ........dead." I still have some black smoke when I floor it.

The engine having never been restored, it might be a vaccum leack of some sort messing up my iddle. This week end I'll stick a vaccum guage on my manifold and check if one my cylinder is deficient. I'm fairly convinced I need a valve job as when I'm getting high in the RPMs, the whole car starts vibrating. (Or maybe my cam lobes are worn ... or both). The guage will tell me all I need to know anyway.

To answer Larz, my Rochester is no exception with the fuel leack, since it's a real old one, not a Edelbk/Holley modern reporduction. I have to pump the gas 5 times, crank 2 secs, and on the thrid time the engine will come alive. This procedure is even written from the factory on the sunvisor

Anyway, I'll play with the screews. Maybe the "factory setting 2.5 turn" is not really suitable for this worn out 455. I'll do the old, turn to the left till the engine sound change, and enrich a bit.

How do I control my fuel input for when I'm above 1500RPM/black cloud syndrome? Maybe the carb is fine, but my intake valves don't open enought, so I don't get enought air in, and end up spitting fuel in the exaust.

As a side note, I have a lot of faith in those old QJets, as there were no better design to me:t They seem to do the job as good as those Edel/Holleys, when you cruise around the dragstrip.

SFRT - Frenchy · « **Reply #8 on:** February 28, 2006, 01:49:32 PM »

What Start said

RTR · « **Reply #9 on:** February 28, 2006, 02:22:31 PM »

DOH! Dopey me.

As I recall, the fly weights are the ones in the distributor? (Man I'm having severe brain farts over this!)

huh? hehe

RTR

GtoRA2 · « **Reply #10 on:** February 28, 2006, 03:40:03 PM »

Quote

Originally posted by StarOfAfrica2
I remember when I was a kid everyone called them the "Quadrajunk" and advocated something along the lines of tossing them in the garbage. Holley was the big deal then, and quite frankly I've never seen a worse "out of the box" carb than a Holley. I came to prefer the Carter over anything. I remember in the late 80s or early 90s people began looking for ways to get decent (well, liveable anyway) fuel mileage out of their resto-mods and discovered a couple of guys rebuilding Rochesters. As complicated and confusing as a Quadrajet can be, they really are wonderful pieces of work when they operate as they should. Key is having someone rebuild it that understands what they are doing. Its not quite as simple as my old Holley 600 was. Properly tuned, they are great. Out of tune, or worn out, they are a headache.

This site has a pretty good walkthrough of the process.

http://quadrajetcarburetors.com/bushingkitinstructions.html

HAH
I agree, when a qaud works they are great! When they dont they suck. I have never liked Holley carbs either, everyone I have delt with has been a pain in the bellybutton I an swapped a Qaud back in.

I ran an buick qaud from a 68 skylark on the last two Pontiacs, I had it rebuilt by the carb shop and they did a few mods on it to keep it from leaking in the future, (bushing kit on the secondary shafts I think, been years) and some mods to how the secondaries worked. This was done before the guy at the Carburater shop died.

I liked the buick qaud cause it had a longer filter housing and it made it a bit easier to run the fuel lines.

GtoRA2 · « **Reply #11 on:** February 28, 2006, 03:41:05 PM »

Quote

Originally posted by SFRT - Frenchy
Thank you, the carb is actually out of the rebuild place. The place deals only with carburetors, and have been recomanded to me by an ex-SCCA champion. It's called CFM Carburetors, owned by Steve Zapeta.

My engine has 140,000 original miles. You know is overdue for a rebuild Anyway, the original 73 Pontiac Rochester was replaced by an Oldsmobile Rochester by some crook in LA. What steve did was to work the Olds carb to make it a "Pontiac Carb". According to him, Pontiac Rochesters had some slight differences that made them a little more "performance oriented: compared to the Olds.

Back to the problem. The old carb used to give me sputers when I was floring it from low speed. I also used to have the big black cloud of gas in the exaust.

Since I put the new carb, I gained a lot in throttle response. The secondaries finally open .... Brrrrrrrrrrrrrrrrrrrrr BROOOOooooooo!:p
But when I stop at a red light from 50MPH, the RPM are initially fine, after couple of seconds, the oil pressure fluctuates from 40 to 10, and the engine fights not to stop. I have to push the rpm a bit for the engine not to quit. I did change the iddle settings (far right screew) linked to the stopper of the accelerator cable, but to no change. The initial iddle at the stoplight is high, then the engine starts missing till "brrtt .. brrrt .....brrttt ........dead." I still have some black smoke when I floor it.

The engine having never been restored, it might be a vaccum leack of some sort messing up my iddle. This week end I'll stick a vaccum guage on my manifold and check if one my cylinder is deficient. I'm fairly convinced I need a valve job as when I'm getting high in the RPMs, the whole car starts vibrating. (Or maybe my cam lobes are worn ... or both). The guage will tell me all I need to know anyway.

To answer Larz, my Rochester is no exception with the fuel leack, since it's a real old one, not a Edelbk/Holley modern reporduction. I have to pump the gas 5 times, crank 2 secs, and on the thrid time the engine will come alive. This procedure is even written from the factory on the sunvisor

Anyway, I'll play with the screews. Maybe the "factory setting 2.5 turn" is not really suitable for this worn out 455. I'll do the old, turn to the left till the engine sound change, and enrich a bit.

How do I control my fuel input for when I'm above 1500RPM/black cloud syndrome? Maybe the carb is fine, but my intake valves don't open enought, so I don't get enought air in, and end up spitting fuel in the exaust.

As a side note, I have a lot of faith in those old QJets, as there were no better design to me:t They seem to do the job as good as those Edel/Holleys, when you cruise around the dragstrip.

Check the Power brake booster to see if it leaks when you use it, I had a fullsize blazer that would do the same thing, it went away after I replaced EVERy vacume line in the engine compartment including the big one to the booster.

Skuzzy · « **Reply #12 on:** February 28, 2006, 04:02:30 PM »

Check for the presence of a stainless steel gasket (sometimes they have the normal gasket material afixed to them in spots, with bare stainless steel showing) under the carbuerator. If this gasket is missing, you will get the symptoms you are mentioning. It is there to prevent exhaust gas from getting into the carbuerator.

SFRT - Frenchy · « **Reply #13 on:** February 28, 2006, 04:12:28 PM »

Good tip, thx.

GtoRA2 · « **Reply #14 on:** February 28, 2006, 04:48:00 PM »

Does it have the original intake?