Aces High Bulletin Board
General Forums => The O' Club => Topic started by: SFRT - Frenchy on February 28, 2006, 08:06:21 AM
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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?
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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
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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
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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
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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.
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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
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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
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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 :D 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 :aok
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.
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What Start said :aok
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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
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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.
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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 :D 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 :aok
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.
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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.
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Good tip, thx.
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Does it have the original intake?
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Yes, that's why I'm not sticking a newer Edel/Holly. The whole car/engine is still stock complete with the time correct cali emission crap.:D
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Bet that plate is either gone, or burned through.
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Yes RTR, flyweights are in the distributor.
Would myou guys reccomend an amateur mechanic rebuilding a carb? I've thought about doing it (step dad usually has a few laying around in his shop). That Rochester looks complicated, but seems like it might be fun, if not just an interesting experience.
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Unless things have changed there are only a few good aftermarket intakes.
Warrior racing has one.
The stock steel and Ram Air Aluminum Intakes are not bad, and are better then the edelbrock aluminum ones, (unless edelbrock changed them in the last few years).
I am pretty sure Pontiac did something to the post 70 intakes though, the runners may be more restriced, as a cheap mod that no one would notice, swap in a 68 to 70 4 barrel intake. (67 isnt as good, and pre 67 wont bolt to a 67 and newer heads).
Just keep all the original stuff in a box.
Do you have a shop manual, 10 psi is not that low, the show manual for 68 said as low as 6 psi at idle was fine. (for a 400)
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Originally posted by nirvana
Yes RTR, flyweights are in the distributor.
Would myou guys reccomend an amateur mechanic rebuilding a carb? I've thought about doing it (step dad usually has a few laying around in his shop). That Rochester looks complicated, but seems like it might be fun, if not just an interesting experience.
]
An amature can rebuild a rochester 2 barrel, holley or Carter AFB, but I would not run a qaudrajet that had not been build by someone who knew what they were doing.
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Originally posted by Skuzzy
Bet that plate is either gone, or burned through.
If he is feeling brave and only has to drive in warm areas, he can just fill the area of the intake that is used to heat the carb with epoxy, then the plate doesnt need to block that section off. But He will still need a good carb gasket! ;)
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The screws you speak of are the idle mixture screws. Screwing them IN leans the idle mixture, screwing them OUT richens the mixture. ALL Q-Jets work that way. There are no "reverse" Q-Jets like there are "reverse" Holleys.
There IS an adjustable part throttle (APT) screw. It controls the off idle fuel mixture, in light throttle conditions. On the old style Q-Jet, this screw is behind a plug in the baseplate between the two idle screws. In a late style Q-Jet, it is under a plug in the airhorn, in front of the choke horn and the bowl vent. Either way, you'll need a special tool to turn the APT screw. On the OLD style, turning the screw IN richens the part throttle mixture, screwing it UT leans it. On the NEW style, turning the screw IN leans it, turning it OUT richens it.
If the APT screw has not been previously adjusted, you have another problem.
You MAY have a stuck power piston, or too stiff a power piston spring.
If you just had it rebuilt, take it back.
If you do not have experience with the Q-Jet, and you do not have someone right there who does, leave it alone. You can easily wreck an expensive Q-Jet with inexperience, and do permanent damage.
Oh, and rebuilt Q-Jets bought at the store ain't worth a damn. Holley, Tomco, and the rest of those morons throw away all those precision Rochester parts, and put theirs in, they drill holes they shouldn't, and plug holes they shouldn't. About 90% of those store bought rebuilts will not control fuel.
The Q-Jet is second only to the real Weber for precision quality fuel metering. The only thing better is fuel injection.
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As has already been pointed out the screws are only for setting the idle. Your problem is probably the jets. They are located in the float bowls and come in different sizes. They meter the total amount of fuel introduced into the intake. In time they will wear and give the same effect as moving up to larger (richer) jets. If you have a local speed shop check with them they should be able to help. If the car is stock and in decent condition, I would try and keep it as stock as possible. These Qjets are getting hard to come by. A new set of jets runs about 7 bucks, You'll have to use trial and error, they start at .0065 and move up from there. Here's a good site.
http://www.carburetion.com/quadrajet.asp (http://www.carburetion.com/quadrajet.asp)
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Frenchy to answer your original question, the screws control the amount of airflow through a siphon circuit that sucks gas from the bowl into the intake manifold. Turning them counter-clockwise backs them out allowing more airflow through the siphon therefore more fuel into the engine, clockwise shuts this down. As someone else pointed out, using a vacuum guage is the most accurate way to set them - back each out in small increments until you reach the maximum level of manifold vacuum at idle.
Savage is right, too bad your original carb isn't available to overhaul. There are many ways to screw up a Q-Jet rebuild. Leaky core plugs have already been mentioned, in addition there's hooking the float valve lifter clip on the float improperly causing a slight flooding condition, misaligned metering rod piston or dirty bore causing sticking etc etc.
I suspect you have more problems than adjusting the idle mixture screws will cure. Its probably too far to drive the car here, but if you want the carb checked out yank it off and bring it. I can inspect it and correct any obvious faults with it on the bench, too easy. From the way you describe the symptom, I suspect a flooding condition, if correct its probably easily identified by inspection.
Back in the day I used to bet people $100 I could disassemble and reassemble a Q-Jet blindfolded, with correct operation once installed. I never got a taker, unfortunately :)
culero (has a lot of hours on that carb)
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The Q jet I am using is an edlebrock performer RPM # 1910. It is very tuneable. there is a part throttle adjustment that is under an allen plug near the airhornm on the primary side. all the metering rods are adjustable and the tension on the secondary throttle opening is adjustable.
I can get up to 14 mpg with my 468 big block in my el camino with this carb and it has been trouble free for 3 years now. I do think there is a minor leak in the fuel bowl tho that will drain the fuel bowl in a week or so.
Holden... is this one of the things you were talking about with "plugging things that shouldn't be" ?
I don't know if they are even making this carb anymore. It is the only 850 cfm Q jet ever made so far as I know.
lazs
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That was Savage talking about plugging things they shouldn't, and no I'm pretty sure he didn't mean the fuel bowl leak.
Its common to see Q-jets have bowl leakage in the well plugs. Easy to fix. Remove the throttle plate, clean the well plugs and slightly abrade them with a wire brush, apply JB Weld epoxy and let it cure for at least 24 hours before putting gasoline back in the carb.
culero
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For some reason, the 455 Pontiac intakes have a good portion of the secondary bores blocked off. I know for certain that this was a problem from 1975-on, not sure about 1973. It wouldn't be a bad idea to replace the intake with either an Edelbrock Performer or a mid 1970s 400 Pontiac intake. I would go with the Edelbrock for the weight savings and improved fuel distribution. The person above who posted about vacuum lines causing your stumbling condition probably isn't all that far off. Replacing ALL the lines is the only way to know for sure.
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If you look in the main metering body, close to the front bolt holes, you'll see two holes in two blanked off wells. About 90% of the time, the morons at those "carb rebuilders" will plug those two holes. It makes the thing idle about 4-6% on the CO. And you CANNOT lean it out with those holes plugged.
There were several versions of the Q-Jet with 1 9/32" primaries. Depending on who you talk to, those were either 800 or 850 cfm. The Q-Jets with 1 7/32" primaries are considered 750 cfm.
If you can find them, use genuine Rochester parts for your rebuild. Make sure no one has taken the Rochester jets, metering rods, and power piston spring out. If they have, you can find the genuine parts on the 'net, and it is well worth the time and effort.
DO NOT hook the needle in the hole on the float bracket, it goes over the bowl side. DO NOT leave out the filler block that goes in the float bowl. DO set the float level at 5/16". Set the secondary air door at 3/4" to 7/8" turns from zero tension at rest closed. Put the link back on the primary choke pulloff to the secondary air door. At WOT, the secondary blades in the throttle body should lean slightly FORWARD. Put the accelerator pump rod in the hole closest to the pivot. Richer is NOT always better. Neither is opening the secondaries faster. DO NOT run more than 7 psi fuel pressure.
Search the 'net for the book on the Q-Jet by Doug Roe. It is almost gospel. It also has part numbers for Rochester parts.
There are Q-Jet equipped cars in Super Stock covering the quarter mile in WELL under ten seconds, at well over 140 MPH. They WILL run.
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Thanks captn... I was not aware that the Qjet ever came in anthing bigger than a 750 cfm... what did the 800 and 850 come on?
Mine is made by some other company I belive, the unlikely name of "acme" or some such "magnetti marelli powertrain USA" all new components not a reman.
I like that it has an electric choke too.
lazs
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the 800 cfm Qjet if you look down the primary's has a very very light bump in the throat .. they also had the 3 step secondary meter rods
it came on 455 's and 400 RA 3 eng's along with some 454 chev 's and later as an altitude compensating version .
So Frenchy I take it that you may need one of these puppys ..
Ive got 2 of them right now .. ones for my 72 lemans and the other is well .. extra .. i'll have to check the build year on them but one maybe a 73 vintage ..
I'll build it for ya and give it lotsa luv and the correct tweeks .
(i'm one of those Old school Qjet guys )
and yes ive ran 11.20's with a q jet in a 3600 lb car.
if your gonna fix yours i highly suggest that you check it for over tightening and that the secondary fuel plates arnt hanging up .. when set correctly a nickle will open them if dropped in the right spot...
Email me if ya want ..I off to another 12 hr day (im Tubbing a AMC rogue and setting up ladder bars w/ coil overs ,,, tring to maintain a stock looking wheelwell and trunk )
PS check your timing chain ... pull cap put socket on crank and move it if it goes more then 8 degree's befor turning the dist rotor ... thats 90% of ur problem along w/ the LOW OIL pressure ... youve got huge main bearings and they need 40 psi min . i run mine at 50 min ... 90 norm ... you may have nylon timing gear in the oil pickup screen clogging it. (even from the past)