Today, I began rebuild/restoration of Quadrajets for a guy that has two '70 Grand Sports--one a GSX, being restored. He's having the engines done by a local speed shop--to stock specs. He also wants these carbs done to stock specs, recolored, linkage plated, etc. The carbs are #7040246 LK, which is an 800 cfm Quadrajet for a '70 Grand Sport only. Upon disassembly of the first carb, I noticed it has #68 main jets and AU secondary metering rods. That surprised me a little, as all the Pontiac Quadrajets I've done had .070" or larger jets in them. So, my first question is what is the stock main jet size for a manual trans '70 455 Stage I Grand Sport? Is the AU metering rod OK? Also, how many turns do you preload the secondary air flapper spring on this engine? Also, do any of you have a good source for parts for Quadrajets--such as choke pulloffs, throttle plate screws/shafts/bushings, main jets, metering rods, etc.? Thanks for your help.
I would try Greg Gessler for parts. http://www.Gesslerheadporting.com/ http://www.QuadraJetParts.com/
A stock '70 carb is 750 cfm; not 800. Stock '70 metering for Stage 1 Manual and Auto tranny is 68 jets; 42b rods; AU secondary rods. Spring windup is 1/2 turn.
The quickest way I know of to get yourself into BIG trouble rebuilding/restoring a Q-jet is to install the wrong jet-rod combination into the carburetor. Those early Buick units use very small main airbleeds, and installing the wrong (larger) jets in them will wash down the new engines in short order. They should be completely/correctly rebuilt, with ethanol compatable parts, and ran on a test engine to verify fuel delivery BEFORE being placed in service on "fresh" engines...IMHO. It is NOT possible to effectively replace those jets and metering rods without freeing up the APT system in the baseplate. The tolerances on replacement parts are simply too "loose" to exactly position the metering rods in the jets like the factory did for precise control of the part throttle A/F ratio. Those units also used different primary metering rods, which are tapered slightly on the upper section, with a gentle taper between steps. You would not believe the number of early Buick carburetors I get in here that have had HUGE jets installed into them to help correct a lean part throttle situation, simply because the builder/tuner did not know about the factory APT system, and how to free it up to provide fine control of the A/F ratio with the stock parts. So they start installing aftermarket jets and metering rods, which are NOT exactly the same as the stock parts, and end up going 3-4 numbers larger on the jetting simply because the metering rods are not the correct style, nor are they correctly positioned in the jets. I would also add, that over 30 percent of our business is getting in some of the prettiest carburetors I've ever seen, plated and often painted, that don't work worth a ship. In every single case it's the same thing, lots of time/detail went into making them pretty. Then they are filled with sub-standard parts, incorrect parts, and not correctly calibrated for the application. I actually LOVE this sort of work, as the hard part is already done for us, we simply get to go in and set it up exactly for what it's being used on. Something else one must consider when doing this sort of work, is that most, if not all of these engines were subject to emission standards. It is rare to have all of the emission control devices in place and operational. Currently available fuel lacks BTU in comparison to what they were originally tuned for. The ethanol added to it also makes it harder to burn. It is not uncommon to have to add some fuel across part of, or all of the load/speed range. It's exactly how much fuel to add, and where, that makes for a good end result........Cliff
Thanks for the quick responses! I believe the carbs I'm working on were not modified, especially now that I know .068" jets were stock and the AU secondary metering rods are still in place. The secondary metering rod hanger has a "D" stamped on it. The APT lever in the base moves freely. I've not found anything about adjustment or calibration of the APT system, so I intend to leave it alone. I know the '69/'70 Pontiac RAIV Quadrajets were rated at 800 cfm. Aren't the Buick Stage I Quadrajets also 800 cfm?
After 70 yes they were 800 cfm. Before 71 they were 750 cfm. The major Buick racing team from back then tested these head to head. Doug says there was no measurable difference in resulting performance.
Correct, even Pontiac "Ram Air" units were simply recalibrated standard "750" cfm castings. The first large cfm castings showed up in the 71 Buick 455 carburetors. Pontiac answered with their own "800"cfm castings, used on the 455 "HO" engines in 1971, and the 400 4 speed application as well. They were the standard 750 cfm castings with the outer booster rings removed. Flow testing shows them closer to 828cfm. Pontiac didn't follow Buicks lead with larger castings that used both boosters until the 1973-74 455 Super Duty carburetors. The 71-74 Buick 455 Q-jets are the only large cfm divorced choke style carburetors other than the 1971 Pontiac "HO" carburetors. The 73-74 Super Duty castings were hot air choke models. Even better news for the smaller 750cfm castings. I have customers running mid 9's with them, at over 150mph, so they have great potential. I've also back to back dyno and dragstrip tested a 750 cfm carb against later 800 cfm carbs. On my own 455 making around 500hp (at that time), the dyno numbers were dead even to apprx 4500rpm's. From 4500rpms to 5600rpms, the larger cfm carburetor made a few more HP at every rpm, increasing to about 10 more hp at peak power. I backed up that testing at the track, where the larger cfm carburetor was worth .02 seconds in ET and .30 MPH faster than the smaller one. Not really that much difference in actual use. That testing was done back when my daily driven car ran mid-11's around 116-117mph.....Cliff
Just put a rebuilt 74 Ponitiac Qjet on my mild 71 Buick 350 and it seems to run fine. Electric choke is a plus!
Thanks to all for the info. I'm glad I recorded the APT dimension on each carb before disassembling anything. I noticed they vary significantly between each carb. From the info I've gathered in the past weeks, maintaining the factory setting on the APT is the best you can do for good driveability. I remember years ago fiddling with the APT on my '77 Pontiac 400 Quadrajet. It ran the best with the factory APT setting. Drilling the access hole for measuring and adjusting on that carb was not difficult, but when the APT is in the baseplate and the guy wants the carbs to look absolutely stock, I'd rather not modify the freeze plug access point.
"Thanks to all for the info. I'm glad I recorded the APT dimension on each carb before disassembling anything. I noticed they vary significantly between each carb." This is EXACTLY why changing jets and rod in those units in completely hit and miss, as the APT in the baseplate precisely controls which portion of the metering rod in in the jets at cruise (high vacuum). Now the "trump card". At best 1 power piston in 20 will not be bent from previous "rebuilds". We end up adjusting just about every single one we get in here to get the metering rods exactly even, then installing an external APT screw to fine tune them on our test engine before shipping them out.......Cliff
So, is there a fool proof method to free up the APT on these carbs? I ground a "special" screw driver (width & thickness) to fit directly into the slot on the adjusting screw, and the screw snapped apart inside the base plate. It ain't coming out - no way no how!
ou: ive been there. maddening. I find a mix of PB Blaster and LOTS of heat works (MAP + O2)--->sometimes. they can be difficult.
My success rate is close to 100 percent. I use a product called Kroil, and NEVER try to turn the screw until it's been soaked for quite a while and some heat is applied to it. If you try to turn them without heat, 99 percent of the time they snap right off.ou: .....Cliff
Hi Cliff, what is the diff or advantage between the secondary metering rod's that i have? My carb is a 72 & is original to my 350 on my skylark. One set is stamped cz & the other is cu. I think I got them mixed up from a second carb that i have from a diff 72. One set has a fat taper & the other set has a skinny taper. Thank's--Lee
You have to consider the application, and how the carb is set-up? A big percentage of the total fuel comes from the primary side of the carb, and the secondary airbleed tubes have an effect on A/F at full throttle as well. The more fuel (larger jets/smaller MAB's) that you bring in from the primaries, the less you will need from the secondaries (basic rule of thumb). Without a dyno at your disposal, the tuner must either change rods and evaluate "by the seat of their pants", or make some dragstrip runs and look for best ET and MPH. I much prefer the dragstrip, and have swapped out secondary metering rods on back to back rods across the entire range (rich to lean) countless times. Tuners should read this closely. Going from "pig" rich to so lean the engine started to surge slightly the entire range of ET is seldom much more than a tenth, and LESS than 2mph. These days I run my own carb a tad "fat", and just race it. I don't swap metering rods between runs unless it's a "test and tune" night and I want to play around with the A/F some. I sit at my pit site and pet the dogs and talk to folks instead. I watch folks all around me dumping the fuel bowls on their Holley carbs after every run, searching for the second and a half or so their combination isn't giving them! Kind of neat that if/when I decide to make a metering change, it takes seconds, I don't have to take the carb apart, and no fuel spilled anyplace. I can also speed up or slow down the secondary airflaps for good or "soft" tracks, in seconds.:TU: ......Cliff
Thank's Cliff, so i'm going to assume that my stock carb came with the fat rod's & i'm just wanting it to remain stock :TU:
So Cliff, How much richer than Stage 1 stock in a Q-jet would be good with ported standard heads and port matched stock intake, a 118 type cam, a half inch open plenum wood spacer, just under 10:1 compression and 1 7/8 full length headers? I'm running 73 jets and 44 rods and a BH secondary rod. I've always thought this was a bit fat (my throttle response could be crisper) and that there is more power in leaner metering. For quick reference Stage 1 is 68 and 42 with AU secondaries. I have a bunch of jets and rods for playing around with this but have left the combo alone for 16 years since installing headers.
Tuning with secondary metering rods can be a frustrating experience. We've observed engines go lean on the dyno when installed metering rods with much smaller tips than the ones used in the previous pull. This happens because the meterings were not using the same tip length, taper, included angle leading to the tip, etc. Tuners often find themselves swapping hangers in conjunction with metering rod changes to get the fuel curve to go in the right direction. Then you have uneducated tuners who remove the airhorn and put bigger jets in. Guys, it ain't a Holley or Demon, ALL track tuning can be done externally, in seconds, and you'll never tear a gasket or loose a drop of fuel! When tuning the Quadrajet, the primary side is tuned first, for best throttle response right off idle, fuel economy, and power at heavy part throttle. Then you LEAVE IT ALONE, and go after the HUGE secondaries to bring the fuel curve where you want it. The only exception to this would be a "race" only set-up, where you've set the carb up to run straight off the jets. This allows for primary side jet changes a few numbers in each direction to "clean-up" the fuel curve some. Even with that said, I NEVER remove the top and change jets once I find the size that works the best for the combo I'm tuning. Back to metering rods. All of our customers who are setting records and running at the top of the pile have us custom machine a complete set of metering rods for them using the same tip length, and included angle leading to the tip. This allows the tuner to go in the right direction, and in small amounts, when tuning on the dyno or at the dragstrip. Dragstrip tuning is ALWAYS better than dyno tuning, because it is a "dynamic" environment. Dyno tuning can yield different results, simply due to improved fuel delivery, and the fuel not moving around in the bowl nearly as much. It still amazes me to this day how well a Q-jet works at the dragstrip. The fuel bowl may be small, but it is in the center of the carb, with the jets on the bottom. Once the venting system is brought up to par, they work FLAWLESSLY at the track, even on really fast cars. The second best feature is the adjustable airflaps. Once you figure out what you are doing there, it's next to impossible to induce a stumble, hesitation, bog, etc into the engine on the launch, even with the front wheels high in the air! One last comment on tuning. On quite a few occassions I've swapped out metering rods from "pig" rich to so lean the engine started to "surge" slightly, in .003-.005" incriments (back to back runs). It's rare to see much over a tenth deviation and less than 2mph for ALL RUNS! You've really got to love a Q-jet for that type of testing, metering rod changes take less time than it took me to type this sentence! I used to run Holley's, lots of them, and still remember my early tuning adventures with them. I've worn out the screws in the main bodies, and dumped fuel all over my engines countless times. I see LOTS of other folks dropping the bowls on their Holley carbs between rounds at the track, searching for that half second or so missing from the combination. I chuckle these days when I see this, and remember my early years at the track, and all the testing I've done since then. These days I run the SAME tune year round, just a tad rich in hot/humid weather, and pretty much dead on the money in "mine-shaft" air. Gives me more time to pet my dogs, talk to folks, and sit back and relax. In the entire course of a days racing, I add fuel and double check tire pressure between rounds. This keeps the car weight the same, and 60' times as well. Not much else to do other than help others out on occassion, and enjoy my day at the track......Cliff PS: I always leave the hood up as well, to help cool things down, and catch the "priceless" looks from folks who wonder by and check out the stock intake, q-jet carb, and the ET on the windshield!
