Don Terrill’s Speed-Talk

engine is a fresh 350 chevy with 9.3-1 compression, small camshaft 206/212, .440's lift, on a 112. dual plane performer intake, vortec heads (unmodified), headers. maybe 280-300 hp. timing is 11-12 initial, 32 total. Vacuum advance is hooked up. HEI ignition. Carb is a new quadrajet from national carburetors....a direct replacement for the truck it is in, 1985 chevy 4x4, full size. When i was in the shop the other night the air temp was about 44 degrees. The truck seemed to idle fine (although too low....) at about 650rpm out of gear, with a very mild surge every 8-10 seconds. it revved up fairly snappily, and seemed to be in the ballpark. Now today on the other hand, the outside temp where Im working on it is 13 degrees, and it definitely has a lean spot right off idle, and seems to recover from the gear change HORRIBLY, until it reaches say 4000+rpm, when it seems alright again........Im assuming lean, once again. The idle mixture screws seem to have little to no affect. Ive had them from 2 1/2 turns out to 4 1/2 turns out and notice very little if any change.......Overall runs horribly. Now it was suggested to me to look for a vacuum leak, but I seriously doubt that there is one, as i don't believe it would have idled happily last night at 650 rpm for so long, but i will look and spray around for one.......any ideas on what to do to make this thing wake up? know the heads flow much better than the stock 1985 castings, and I know the cam is a "little" bigger, but is it big enough to require completely different fuel metering?

try putting the shit factory emission <stovepipe> from exhaust to air cleaner with correct 120* sensor in air cleaner

having headers, and none of the original equipment here, I might be s.o.l. on that.......ugh.

rewguy wrote:having headers, and none of the original equipment here, I might be s.o.l. on that.......ugh.

Call Dean Oliver at Deano's Carbs. (870) 830-5470

http://www.deanoscarbs.com

Where's the vac advance hooked up to? Full manifold or timed port?

And are you tuning with a vac guage?

Quick and dirty test would be to disconnect the vac advance and jack the initial timing up to around 18-20 degrees or so. Just don't rev the snot out of it and keep it under 3,500 or so to see if the flat spot goes away.

The vacuum is ported vacuum. I have done exactly that....Ive had the initial everywhere from 8 degrees to 24 degrees........dead spot regardless.......just feels flat/lean/dead at the bottom.........constantly in and out of the accelerator pump at low rpms to keep momentum.

there use to be a part throttle adjustment on the originals I think it is right on top of the carb and under a plug, I kind a forget, but you can play with that a bit... also adjust hanger and or go to a thinner needle. Look up Lars Grimsud, cliffs hi performance and Danny Ashley they all specialize in Q-Jets.

aluminum intakes don,t seem to work very well in cold weather , try putting the cast iron intake back on , they hold more heat for better cold weather driveability , make sure cross over passages are open

no crossover on this one, lol.....vortec heads/intake.......

rewguy wrote:The vacuum is ported vacuum. I have done exactly that....Ive had the initial everywhere from 8 degrees to 24 degrees........dead spot regardless.......just feels flat/lean/dead at the bottom.........constantly in and out of the accelerator pump at low rpms to keep momentum.

So, you've tried heavy initial settings with the vac advance disconnected?

Having a commercial reman'd carb might be a big part of the problem. Most places seem to really screw up the calibrations to make them "universal" (as in "universally run like crap on every application, but work everywhere").

When you say "lean spot right off idle", I assume you are talking light part throttle city driving? Are your throttle blades exposing the entire transition slot when the idle screw is set for desired idle speed? I assume you are using a choke?

Sounds to me like you've got idle system problems. Time to look at the IFR, idle air bleeds, down channel restrictions, etc. Maybe some of the guys smarter than me can chime in, but at very light throttle openings, the idle system can be a huge factor in drivability. All the jet changes, power piston spring, metering rod, and APT changes in the world aren't going to help a dead spot right off idle.

groberts101 wrote:

rewguy wrote:The vacuum is ported vacuum. I have done exactly that....Ive had the initial everywhere from 8 degrees to 24 degrees........dead spot regardless.......just feels flat/lean/dead at the bottom.........constantly in and out of the accelerator pump at low rpms to keep momentum.

So, you've tried heavy initial settings with the vac advance disconnected?

yep....Im gonna try it once more, right now, with 20 degrees initial, and the vac. adv. plugged.

novadude wrote:Having a commercial reman'd carb might be a big part of the problem. Most places seem to really screw up the calibrations to make them "universal" (as in "universally run like crap on every application, but work everywhere").

When you say "lean spot right off idle", I assume you are talking light part throttle city driving? Are your throttle blades exposing the entire transition slot when the idle screw is set for desired idle speed? I assume you are using a choke?

Sounds to me like you've got idle system problems. Time to look at the IFR, idle air bleeds, down channel restrictions, etc. Maybe some of the guys smarter than me can chime in, but at very light throttle openings, the idle system can be a huge factor in drivability. All the jet changes, power piston spring, metering rod, and APT changes in the world aren't going to help a dead spot right off idle.

Yes, I have a choke, I have checked the transition slot exposure a few times, at least three times, I have backed the idle speed adjustment completely off and started from scratch, to INSURE that the transition slot wasn't over exposed. As far as "lean spot off idle", I basically mean, it seems crazy lean at low rpm, period. If I were to be rolling 7-8 mph, and grab even 2/3 throttle without gently feathering the throttle to coax out accelerator pump shot, it would die immediately. If I'm accelerating at 2/3 throttle and shift second gear, the truck will almost die when it hits second gear, and third.......unless i get back in the accelator pump.....

As mentioned there is a plug on top in the front of the carb,remove it to. Access the metering rod hanger adjusting screw I am willing to bet if you raise them your problem will go away.These rods control part throttle mixture,just keep raising them a little at a time till 6our problem is gone.I learned this the hard way when as a young buck I rebuilt a quad and had them adjusted way to low the thing idled OK but was lean as heck and wouldn't hardly pull itself. These are the same rods gm added a solonoid to and cycled up and down from rich to lean 60 times a second to get the average a/f mixture they wanted at cruise with the computer controlled qjets.

Of course the above is assuming the reman company didn't butcher it and the rods and jets are reasonablyclose.