Author Topic: Rochester Quadrajet Carburator (Read 7588 times)

SFRT - Frenchy · « **Reply #15 on:** February 28, 2006, 04:50:45 PM »

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.

Skuzzy · « **Reply #16 on:** February 28, 2006, 04:54:57 PM »

Bet that plate is either gone, or burned through.

nirvana · « **Reply #17 on:** February 28, 2006, 05:00:25 PM »

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.

GtoRA2 · « **Reply #18 on:** February 28, 2006, 05:03:08 PM »

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)

GtoRA2 · « **Reply #19 on:** February 28, 2006, 05:05:11 PM »

Quote

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.

GtoRA2 · « **Reply #20 on:** February 28, 2006, 05:06:43 PM »

Quote

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!

Captain Virgil Hilts · « **Reply #21 on:** February 28, 2006, 06:22:12 PM »

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.

weaselsan · « **Reply #22 on:** February 28, 2006, 07:08:26 PM »

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

culero · « **Reply #23 on:** February 28, 2006, 08:09:33 PM »

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)

lazs2 · « **Reply #24 on:** March 01, 2006, 08:48:51 AM »

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

culero · « **Reply #25 on:** March 01, 2006, 07:54:19 PM »

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

paulieb · « **Reply #26 on:** March 02, 2006, 01:30:52 AM »

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.

Captain Virgil Hilts · « **Reply #27 on:** March 02, 2006, 01:44:12 AM »

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.

lazs2 · « **Reply #28 on:** March 02, 2006, 09:04:45 AM »

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

Roscoroo · « **Reply #29 on:** March 02, 2006, 11:00:40 AM »

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)