How can you find out what rod/jet/hanger combo you have in your quadrajet carb??Will the carb stamped numbers help if the rods/jets are still factory??Thanks

 

Assuming you're talking about secondaries, forget the jets - they're fixed.

Secondary rods & hanger are easy to remove. One small screw on the top of the hanger. Get your magnifying glass out and read the markings on the hanger and rods.

What kind of q-jet do you have?

 

Its a noncomputer 800cfm quad off a 80 301 turbo trans-am.If you need numbers from the carb let me know and i'll pop the hood off tomorrow and get them.The A/F meter i have is a summit 10 bar digital(1 bar being leanest--10 being richest).At idle it reads no bars at all.Cruising it shows 8 bars.At WOT under boost it shows no bars at all.

I understand a bit now why im leaning out at WOT is because the vacuum of the turbo(draws thru the carb) is sucking my front metering rods closed so all im running on is the rear barrels.So i need to lean up the front set and pig rich the back set.Or is there a way to keep the front rods from closing??If so what do i need and where can i get them??Thanks

 

800CFM on a 301? hmm, figured that carb was only found on large displacement engines... well turbo I guess...

Well the primary metering rods are pulled down due to vacuum right? and your turbo acts like it's at high vacuum, ie low rpm, and closes them? what about turning your bottom stop for the primary power piston higher up? That might wreck your idle though...
I guess putting in thin (rich) secondary rods would work... but that'll only help after 1/2 throttle or so right? Could you bend the secondary tang to engage the secondaries much earlier? Bring the primary power piston up, then close off your idle mixture screws??? just throwing out ideas...

hmm, that's a real head scratcher. Let us know how it turns out, i'm personally very interested in your setup, and would love to see some #'s when complete.

 

I thought that the stock carbs for the 301 turbo had a boost referenced power piston.

 

I figured that would be the right way to do it, but I didn't have any clue whether that's what the factory did.

 

Thats what i thought too but it dont seem to be working.The carb was rebuilt at the same time as the engine and turbo.But the engine didn't last but around 100 or so miles before detonating and bending pushrods.All was rebuilt by/through the same company.The engine was tore down and diagnosed as the wrong pistons causeing too much compression.But maybe that company screwed the carb up also which ran it lean as **** and cause the detonation.

I've run the carb casting numbers and they come back as a 800 cfm quad specificly for the 301 turbo but that don't tell me what the rebuilders changed.Idle sucks anyway atm and adjusting the metering screws have no affect on the idle or throttle response so im stumped there also.I read something about attaching a checkvalve between the intake and carb to keep the primaries open underboost but cant seem to find details on it.

 

IROC- you're in deep weeds. I'm going to shoot you an email. I do QJets like they're my own children and this is a DAMNED cool project you've got going. I'd love to help you out with the carb. I'm sorry to hear you've got problems, but you're going "where no man has gone before" on this project. I can't help you with the turbo or the basic engine rebuild (although I've got a little experience with Poncho motors) but I CAN make that carb perform right.

Please feel free to ask about about me on this board regarding my QJet experience. Also over on Nastyz28.com, Camaroz28.com, Montecarloss.com or anywhere you want to check up on me. I've done QJet carbs for many board members and I think my reputation so far is without a single complaint.

I just think this is a really cool and different kind of project and I'd love to be a (small) part of it. No charge but for any necessary parts and shipping. I want to be a hero on this one.

 

Just noticed you don't list an email address on this board. Please feel free to contact me at mine.

 

Damon i appreciate the offer.I have considered that offer for the last few days.I cant help to think id still basicly be in the same boat when the carb got back.I think it would be real hard for you to diagnose and tune it without the engine under it.The other thing that bothers me is ive built this whole car from ground up by myself(except the uppipe,downpipe,crossover,and haveing the the exhaust manifold welded up) and i was still there and helped fit it and stuff and i wouldv'e done that myself if a had the tools.I feel if i send the carb to you and you modify/fix it that i'll lose that "I Built That With My Own 2 Hands" feeling and thats real important to me.That was the main reason the car was built--too many bet me i couldn't do it.

But i do know a way you can still be the hero here!!The info i need would be----How can you determin what hanger you have(all i see was part numbers and dont know how to cross reference);What is the richest rear rods;What is the richest hanger;and if that dont work i need a picture and instructions of how to install a check valve setup.If i can find that info i should be able to tune this bastage and maybe get some numbers.

 

Fortunately, carb calibration is a lot easier than tuning FI. Carbs are, byt their nature, abut 80% "self-tuning" to whatever engine you put them on. You may need to tweak the jets. idle screws, etc., a little depending on what engine is underneath it, but it's not like you're going to be off by a mile like you can easily be with FI on a modified motor.

Secondary hangers are simply listed by letter (stamped into the hanger on top), B through V. B is richest, V is leanest, but the hanger is really a "fine tuning" aid, the secondary rods make the biggest difference. Start with a hanger around "G" and you'll be in the realm of sanity. There is a listing of ALL secondary rods ever made for the QJet on this website's tech articles section that I compiled. At the end of the day, for performance use, there are 2 sets of rods that are "go to" choices of mine- DRs for a "middle of the road" kinda secondary calibration and DAs for a richer mix across the board. 99% of all QJets will work fantastic with one of these or the other. That's not to say that whatever you have in there already is junk, but these 2 sets of rods cover VAST variety of performance applications very well.

If you decide to dig into the carb, feel free to shoot me an email about the primary side calibration and whatever you find in there and I'll see if I can shed some light on things for you.

It's very difficult to diagnose these things over the internet since there are many things that can be "wrong" on a QJet but not be visually obvious to someone who is not familiar with them. Dresite this being a turbo application (which I have never done before with a QJet) the basic principles of operation are the same. What I'm saying is that if the carb will idle your buddy's stock 350 jsut fine, it'll idle your boosted 301 just fine, too. The differences only happen when you're into positive boost- at or near WOT, and even then, the basic operating principles are the same.

 

I have a new never installed quad so i might just configure it for use since it appears the turbo quads power piston isnt performing as it should.Not sure if the chevy style fuel inlet will clear my carb hat yet though.I pulled it apart to see the rod sizes in it.Primaries are 53p and secondaries are DA with a N hanger.I looked on that list on the tech part of this site like you said but it dont have DA listed--just AD(typo?).The turbo quad should be rich from the start so what ive decided is to lean up the front for cruising and go full rich on the rear then work my way down.As soon as weather and health permits i'll go pull the rods and hanger outa the turbo quad and see whats in there.So outa that list on rear rods--translate and tell me which is the fastest and richest.Im gonna be on the hunt for a B and a G hanger.

 

You can get a F-hanger off most any olds 307 engine. That would be in alot of GM full size cars and cutlass.

 

The reason i want to go Full Rich from the start??If i put in the richest rod/hanger combo and my guage still doesnt show rich enough--then i have to do the check valve to the power piston thing which i still dont understand how you get it to the power piston!!If it shows rich enough then i work my way down through the rods fine tuning.

 

You can bend your hanger to be whatever you want, make it come in really early if you want...
DA rods (as Damon mentioned in his post), are about as rich as you can find easily. Go junk yard hunting, they're VERY easy to remove and check the stamp on...
You could also bend the tang to open your secondary blades earlier too....
but yea, grab as many rods as you can, and bend a custom hanger, then go testing. Good luck, i'm dying to see how this runs...

 

Pulled the rear rods from the turbo quad and they are DX rods on a L hanger.So i really need to find some rods and a hanger.From what i can understand from that chart(i finally seen the DA on the chart btw)that AD rods are the richest??And the DX rods were real close and they were lean as ****t.There is a scap yard locally that has about 5-55 gallon drums full of Quads so im gonna see what i can get there.I really need the pecv setup it looks like but i cant find details on it.Anyone know where to get this info??

 

DX are definitely about as rich as you can go. I've never actuall run across a set of rods that rich in a stock carb, so I'd say that there is something wrong inside the carb if it's truly still running lean. Power piston not functioning, wrong primary jets, rods, other weirdness.

I have no idea on the PCV setup. I'm sure the factory must have put a check valve or something in the system somewhere or you would be pressurizing the crankcase through the PCV valve every time you went into boost, which is definitely a no-no. I am not familiar with the turbo system in your T/A so I can't offer any advice from personal experience on that one.

 

well yea, when he goes into boost the vacuum jumps up and the power piston seats and goes lean. Correct? The turbo intake is below the carb, so the carb just see's heavy vacuum... This is supposed to be fixed with a different power piston setup meant for this... but you don't have that right?

So it's like having no primaries at all, and having to make up for it in boost by having huge secondaries....

Would it be possible to lean up your primaries even more, then possibly drill out your secondary jets a bit more?
That way the extra flow when not on the secondaries will be offset by the leaned up primaries, and into boost you'd have even more secondary flow.

 

No, not supposed to work like that. When you go wide open the pressure above the carb and below it will be almost identical, whether that's in a N/A rengine or on one where the carb sees boost. The carb should NOT be a large restriction to airflow once it's wide open. Maybe an inch or two of vacuum differential, but certainly nothing like the 5-6" of vacuum that would be necessary to suck the power piston down again (unless, for instance, the secondaries aren't actually opening.....??)

I'm talking about the power piston simply stuck in the down postion, the wrong primary rods being used (B vs. K style), the rods dropping off the hanger and wedging in the jets, the power piston itself being the wrong kind for the carb, etc. Basically, the system simply not operating as Rochester intended it to.

Point of reference......

I have a Qjet on top of my roots-blown 383. It's making power in the mid-high 400s right now. It doesn't suck the power piston down due to too much vacuum under the carb, either. My motor is similar to this turbo setup in one important respect: air is drawn THROUGH the carb just like it would be in a N/A setup and then compressed before being fed to the engine. You should be able to draw engough air through a QJet to make WELL over 500HP before the carb itself becomes any kind of major restriction to airflow.

 

huh, I was just going by:
The turbo increases the vacuum seen at the base of the carb and "tricks" it into thinking it's at low RPM high vacuum, and closes the primaries.... Is this wrong then? Is high RPM lower vacuum no matter what? just more FLOW, because of the turbo? I'm a bit confused.... -J

EDIT: Therefore IROC_turbo, maybe you are going in the wrong direction....

 

When you're wide open there is very little vacuum under the carb unless you're drawing a MASSIVE amount of air through it. An 800 CFM carb flows 800 CFM at only 1.5" of vacuum. You would have to draw jsut shy of DOUBLE that much air through the carb (around 1400 CFM) just to get 3.0" of vacuum under that same carb. To draw 5-6" of vacuum, flow would have to be astronomical. NO WAY is this little motor using that much air (unless it's making well over 1000 HP!).

There are other reasons you may wish to put a slightly stronger power valve spring in anyway, but sucking the power piston down at WOT (and ANY RPM) just ain't gonna happen. UNLESS your secondaries aren't opening. Then you can definitely draw enough vacuum at high RPMs to suck it down and lean out.

 

oh! ok, at low throttle there is a small venturi opening basically, this speeds up airflow, showing a pressure differential, ie. vacuum. That's what idle/off idle is and what seats your power piston. Then at higher rpm/WOT you have wide open bores and suck through all the flow value (for example) and have a low pressure differential (1.5").... But if you had a SUPER hungry motor, with a dinky small carb, you'd end up with a bigger pressure differential eh?

ok, so even with boost, as long as the "booster" (blower/turbo) are BELOW the carb, the carb just sees a bit more flow then usual, nothing to throw off the carb that much?
but the "booster" above the carb pressurizes it and is a bad idea generally? (at least for a q-jet?)

Damn! Now that's handy info! Maybe a twin turbo motor for me isn't that hard


ok, so, sorry to steal your thread IROC, but this is probably helpful info for you... Check the function of your secondaries eh?

 

Yeah, that's basically it in a draw-through application. You're have to be making a LOT of HP to draw any serious vacuum under a big-a$$ 800 CFM QJet with all 4 tin can lids wide open.

In a blow-through boosted application a carburetor (including a QJet) will STILL meter fuel correctly IF the fuel bowl (inside the carb) is pressurized at the same boost level as the outside of the carb. This is why in many blow-though carb setups you'll see they simply surround the entire carb with a big box. This assures that both inside and outside of the carb are always seeing the exact same boost level. It's still a lot trickier to tune a blow-through application than a draw-through one.

 

Ok update for you guys.After reading what you 2 have been posting i got curious and hooked a vacuum guage to the back of my carb under the butterflys.At idle they match at 19 or 20 vacuum.Cruising at 2000 they match at 1.At WOT under boost my boost guage shows 8(hooked to intake under turbo) and the vacuum guage is pegged past 30.So there is a difference of what the carb sees.

After some dissasembly of the carb AGAIN i noticed its not the main body that makes a turbo quad its the throttle plate and a one of a kind gasket.The throttle plate has a 2 extra vacuum ports on the front.One of those isolates the power piston to just work from that vacuum port and where the tiny hole on a normal throttle plate is for the piston to work from is solid(plugged)and the gasket makes sure it runs through the added port.The place that rebuilt all this stuff used a regular gasket so im gonna have to order that specific gasket to make it work through that port.I'll take some pics of all this stuff when i get a chance.

Btw the front rods are 49M.What rods can i get to go leaner on the front??

 

Almost forget--Factory used a PEVR(Power Enrichment Vacuum Regulator) to moniter the power piston and these parts are discontinued from what i hear.

 

Why do you want to go leaner on the primary???

I can explain the rods jets deal if wanted, like the power tip being .029 or whatever and how primary jets (only) effect WOT, and rods and jets effect part throttle etc...

huh, kinda an oddball carb for that application eh? Where are you getting that gasket? You're lucky it's still made...

oh! So you were measuring your vacuum/boost with a gauge on the dash, and now you're trying the vacuum gauge at the base of the carb? and comparing.
hmm... Maybe the dash unit isn't very accurate?...

oh and finally, 49 is a pretty fat (lean) rod already...
Are you sure it's M? Most of them are K, I know M exists, I just thought they were rare, or some sort older truck application?...

 

My 83 G20 van had 50Ms in it from GM.
M rods are not rare at all.

 

I got lucky on the gasket.Went to the scrap yard digging through quads for the rods and hangers.Came across a new looking quad that still had the rochester carb cap on it(a carb hat that plugs the carb).Decided to take it home with me.Got so many rods and hangers i cant count them all--2 throttle plates and that whole carb and hat for 5 bucks!!Took it apart to see what the insides looked like and it was shiny and not a spec of dust or corrosion.And got to looking at the gasket and it was the one i needed(What luck)!!Put it on and put carb back together and reinstalled it and used a 1 way check valve from the intake to the power piston port and she idles perfect FINALLY.Will have to pull it out on the road and take it for a spin soon.

Guages are accurate and new btw.That diff in the gauges pre-turbo and post-turbo shows that the turbo WAS sucking the primaries closed.So the carb was seeing 30+ vacuum under boost and closing the primaries.