Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 2Low8s on July 14, 2022, 08:50:45 AM
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Hello,
Long time lurker, first time poster.
2 years ago, I purchased a Jet Performance 35002 qjet that I thought would be more tuneable and "fix" a lot of the issues I've experienced with the 2 prior Carter AFB and Edelbrock carbs. In stock "Jet" trim, it'd never ran better but was pig rich and fouling plugs within 15 minutes of idle time/tuning.
I've gotten it more dialed in after finding this community and (still) reading Cliffs book. Most of the raw fuel smell is gone, and it's "hitting" like fuel injection. The car has never ran so fast.
"Stock" 35002- 74 jet/50M Primary, DA secondary rods. Forgot to check which hanger. With this setup I had no control over the idle mixture screws. When I opened it up to check what jet, rods, power piston spring, etc, I found the air bleed holes were plugged with lead. I pulled those and gained SOME control of the idle mixture screws. I also jetted it down to a 70 jet. These 2 things helped a little bit with getting some control of the mixture screws, but I don't get any control until about 1/2 a turn from all the way in.
I am also experiencing a 'flat spot' part way through 1st gear, and right when I grab 2nd. The flat spot is there, thought not as bad, in 3rd, 4th, and 5th when I give it somewhere between enough gas to maintain speed at cruise and enough to increase speed going uphill. In first gear, I give it gas, let out on the clutch, and about 1-2 seconds after the clutch engages all of a sudden it bogs for a second, and picks back up. Does the same in 2nd but usually immediately after the clutch grabs fully in 2nd it flatlines and then is fine. I've got the APT turned out about 4 turns at this point, and the secondary air valve spring at about 7/8-1 turn from just contacting the arm on the shaft.
I did try moving from ported vacuum to manifold. It made cruising speeds require much less throttle, but made no difference in the flat spot and if anything made the mixture screws less responsive after turning the idle back down to about 700rpm. Initial timing is set at 14*, I have 34* by 3000 rpm, and another 10* vac advance (adjustable). I have an inline Holley AFPR set at 4.5psi as read between the post pump filter and the carb.
Car/Engine specs are:
86 Camaro Z28
T5 manual
40 over 305 with flat tops- roughly 9.5:1.
Weiand Stealth manifold w/ L69 Dual Snorkel air cleaner
416 heads- ported/polished, Manley race flo undercut/tulip valves
214/224 cam with .444/.466 112LSA
1.6 aluminum roller rockers
Long tube headers to true dual 2.5" exhaust (challenge on 3rd/4th gen F bodies)
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Forgot to add my carb is #17080512. Too late to edit/add to my post.
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The JET carb would be nothing more than a commercially "remanufactured" carb with better coloring and zinc work. The are "generic" units just like what you'd buy at Pep Boys. Lead plugs driven into the bypass air holes, cheap components, nothing "high performance" about them anyplace.
Your fuel pressure is really low for one of those carbs and jets are way too small for that carb number, or at least on original unit.
Difficult to help out with a "reman" because they open things up and drill out or smash down or put inserts in others, etc. Most have larger than stock idle tubes so not surprised it's "fat" at idle, they do that because you get less complaints and come-backs.......
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Good to know- appreciate the insight. I discovered/pulled the lead plugs and cheap secondary air valve spring that broke with less than 30 miles of driving- entirely neighborhood and stretch of vacant highway for secondary and APT tuning. At least the turd I'm polishing is prettier than most in that case :)
What do you recommend for fuel pressure, and what jet size for the 305?
My brother in law has a 'carb tackle box' for Qjet, edlebrock, and holley. I seem to recall him having a pen vise and drill bits in there. Sounds like I have some measuring and more tuning to do. Finished the book last night. Judging from my idle vacuum (14 average) I should work it towards the settings in recipe 2.
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I like to see 6-8 psi on the later carburetors with a .135" inlet seat and small float.
Can't recommend jet size as I don't know much about the carb or what was done to it. If all airbleeds and such are the stock sizes 73 jets would be a good starting point.......
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Appreciate it. I'll turn the pressure up to 7psi and do some measuring/checking in the next couple weeks and report back. Been studying (reading over and over) the parts of your book related to idle and modifying idle circuit. I really appreciate the book, and your guidance here!
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Per your advice and the book, I increased fuel pressure to 7psi, changed to the 73 jets, and set the carb up for 'recipe #2'. Car runs GREAT, but still fighting idle.
My idle tubes were .040, and I 'resized' them down to .037 for the time being with a drill bit and needlenose pliers, rolling the idle tube around the drill bit shank end and kneading the 'jet' side with the pliers. IDCR was just a tad larger than stock, and resized it to .052.
I measured the amount of exposed transfer slot which was under .040, maybe closer to .035. During a test drive we moved the vac advance back to ported, and now it will run ported for the first time without having a bunch of the transfer slots exposed/fully.
I still have no control over the mixture screws- once past 1/2 a turn out the exhaust smells rich. When I fully seat them, the rpm starts to increase and I pick up vacuum. Does that mean I need more bypass air, or should I resize/order smaller idle tubes (and IDCR?), and add bypass air if that doesn't yield mixture screw control?
1/2 turn out- vacuum fluctuates between 12-14"
fully seated- vacuum fairly solid 15".
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What sizes are your idle air bleeds? Solder the tubes shut & redrill.
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How big are the holes under the mixture screws? Remove the springs from the mixture screws and verify that you are able to shut off any fuel from the mixture screw holes.
Make sure you don't have any nozzle drip.
How much transfer slot is currently exposed at idle?
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77cruiser- Idle air bleeds are .070. Solder as in soldering for electric connections?
Cliff- I'll pull the screws and measure, as well as see if it shuts off fuel sans springs. I'll double check for nozzle drip, but did not see any prior to this point.
I have somewhere between .035 and .040 transfer slot exposed at idle.
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Pulled the mix screws and springs. Measured out to .089 with my #43, couldn't get a #42 in there.
Still ran fine screwed all the way in with no springs.
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77cruiser- Idle air bleeds are .070. Solder as in soldering for electric connections?
Yup try 33 - 35.
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Like 77 says, throttle down the idle tubes and try it.
I saw that you had removed the lead plugs in the main casting
that blocked the idle air bypass. Did you check that the rest of
the bypass passages were still functional? The right gasket holes?
on both sides? about maybe .050 inch holes???.
Might need more air...??
Pull off a small manifold vacuum hose or cap at hot normal idle and see what happens.
It might be a challenge getting a perfect idle with that much cam, rocker and
intake on a 305.
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You could make some of these if you want to make it easier to adjust.
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I got the Rona last week, so I'm taking advantage of the time to dig into the carb now that I'm feeling better and can function. I'll start out at .033 as well as resize my IDCR to .046. I'll walk it in to .035 if need be.
After looking more closely at my transfer slots I realized they aren't square and that's probably why I could turn the screws in all the way and not kill the motor.
I like your idea but those seem a bit overkill for me and above my fabrication skill set! I can glue metal together, but it's like me- Ugly but it works.
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Quick follow up.
Idle tubes resized to .033, and couldn't get the down channel restrictions out to resize. Rolled with the single change.
No changes.
Idle increases noticeably when I pull the vacuum cap that I connect my gauge.
Need more bypass air??
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Yes try more bypass air. Make sure all the original bypass passages
Are open. Both sides.
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Made sure both sides were open from baseplate to airhorn.
Opened up the bypass air (both sides) from .070 to .113, going .010 at a time in 4 steps and then testing idle functions at each step/increase. If I turn the screws in all the way vacuum drops and gets more erratic and it idles rougher but still can't kill it with the screws. At 1 turn out on both sides it idles smoother than before, and part throttle is much crisper, stronger and not 'jerky' when cruising. If I remove my vacuum gauge to create a leak the RPMs still increase, but not as much as previously.
New development though. On my test drive, WOT just isn't working, as if it's choking out all the way to redline. Maybe need to loosen my air door spring? I believe it's around 1 turn in. WOT was the only 'part' of the carb I had dialed in before!
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That,s PLENTY of idle bypass air. Now get the engine fully warmed up
and set the idle speed screw as slow as it will stay running and then try the idle mixture
adjustment screws again. If you still cant stall the engine with the screws in don't worry
about it. It should at least cause a stumble. Sometimes it just beeees like that.
If it idles OK run it.
About the new WOT issue, I don't know why that would just show up......
Do you have that Choke/air flap interlock still on the side of the carb....??
1 full turn is a lot. But if it was good before I don't know what would have changed.
3/4 turn would sound more normal if the rest of the secondary is OK.
If in doubt get a spring/cam kit from CHP.
Keep at it you'll get it whipped.
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Finally got more time tonight to mess with it. It kept getting better the more I increased the tension on the spring. Pulled the airhorn off and discovered a secondary rod had fallen out of the hanger. Looks like I'll be replacing those- the elbow that keeps it in the hanger is gone on the one that fell out.
After putting it back together, it ran like it should. I'll mess with the mixture screws again tomorrow to sanity check my calibration and then finish adjusting APT.
Just need it to stay below 90* outside so I can enjoy driving the car (1LE clone/AC delete car) topless to work. I sincerely appreciate all the help and insight!
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Last follow up. I swear!
Retuned with vacuum gauge after fixing the secondary rod, and got some mixture screw control. Got it 3 turns out both sides. Car runs better than ever.
Don't know much about that Justin feller or the Case family, but placed an order through Cliffs for parts I need and probably don't need, but Justin Case.
Wish I would have discovered Cliff, his book and forum prior to getting fleeced on Jet Performance. Cliff and everybody here's been right more than my wife.