Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Flat spot/bog in 1st gear, after shifting gears

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2Low8s:
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)

2Low8s:
Forgot to add my carb is #17080512.  Too late to edit/add to my post.

Cliff Ruggles:
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.......

2Low8s:
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. 

Cliff Ruggles:
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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