Discussion in 'The Venerable Q-Jet' started by HotRodRivi, Apr 13, 2020.
I see a whole lotta work - so how does it run?
It's a "Frankenstein" right to start with. Using the 1975 early APT center section is a mistake, they are HORRIBLE. Without going into great detail they only lasted one model year because of a fundamental issue with the castings that the engineers didn't realize until after they went into production.
I would get a later 1976-80 Buick, Olds or Pontiac main casting with the later APT design to replace it.
The plastic pull-off mounted to the later bracket doesn't work for chit and not as good as the original design as it doesn't have any material to unload the choke on a cold start and is NOT adjustable.
The clear epoxy is completely useless to seal up bottom plugs. 100 percent of the carbs that come in here using it fail a leak test, same with JB Weld. Get some Marine Tex or Devcon and if the plugs are actually leaking (most are not) remove them and tap for screw in plugs as shown in my book.
The metal screen in that airhorn "slot" isn't needed and should be removed so it doesn't go thru your engine. For really fast cars (hard launch) you need to use a factory baffle behind that slot or fill it anyhow.
Another complete waste of time/funds is to drill under the bottom plugs where the passages intersect for the fuel supply up to the nozzles. You might see a slight benefit shortening the tubes so they aren't that deep, but there isn't any restriction in that area that will effect fuel flow as it's at least 3 times the diameter of the small secondary disk inserts with a metering rod hanging in them.
I wouldn't use cross-drilled secondary nozzles either, they were only used on Q-jets with a long stop to limit cfm, like the Pontiac 301 units.
Nice selection of secondary metering rods. Once you replace the main casting with a better one I'd shorten the secondary air bleed tubes about 3/16", drill .036" holes in them and use AX, CE or DA metering rods for initial tuning. Make sure to modify the stop on the secondary airflap shaft so it opens to 1.300-1.330" as shown in my book or even better go to page 107 and grind the stop off an add an adjustment screw for fine tuning.
Also be aware that 1975 models are the bastard child for Q-jets. Some will have upper idle airbleeds in the main casting above the DCR's, others will have them in the airhorn. This complicates mixing/matching of the later center sections and airhorns. Make sure you don't end up with no upper idle airbleeds at all or two pairs, neither will work well and require further modifications.
The 1975-up front inlet non-CCC units, Buick, Olds and Pontiac will all be the larger castings. Often dubbed "800" cfm they actually flow same as the Edelbrock 1910 q-jets when the secondary throttle plates are set to open to a full 90 degrees and airhorn to 1.270" as shown in my book on page 118 . So they are "850" cfm units not "800" and capable of being used on some pretty high HP set-ups. Your title says "max effort" so the option is there to remove the outer booster rings to build an 897cfm unit. Probably not needed unless the power level is over about 750hp, but still an option. You still have to get all the fuel past one needle/seat assembly so fuel delivery needs to be up to par for the power level and a larger N/S assembly, preferable around .145-.149" if making over about 550hp.
If you have any specific questions or need any help just ask. Not trying to be critical here but a lot of flags go up before we've even got started with that particular combination of parts........FWIW......Cliff
I should have added text. I was planning on it. Have you seen double stamped sec rods. This main body had no passage drilled for port vac for dist or the secondary pulloff. So i drilled those in the main body. I eleminated alt compensator. It also did not have the needle height adjustment behind the power valve. So I installed the necessary threaded bush at proper height. So now it has that. Now it compares hole for hole to the pontiac main body it replaced. The sec pulloff worked great on that carb. Its still on the same airhorn. I like the plastic ones because you can drill the orifice as you know. Ive probably done everything you can do wrong to my carbs and still made them work. Ive been well past that for many years now. This one ive finally found your book. Now I understand all the air bleeds and hole sizes. Nice nail polish on page 72.
Yes Im a 4barrel rochester hoarder. For every set of needles I have every external and internal removable part from the carb they were on. To the bone.
I removed the rib accross the boosters. I slimed the bulky midsection. I will not remove the outer booster. Everything opens right. The drilled sec nozzles came from a turbo carb. Had ax needles and c hanger. Im using dx/dr with E hanger. I thought the nozzles were drilled either to get some fuel to go forward cylnders or for turbo lag. Just my guess. I have plenty of nozzles so I can go none drilled. I actually couldnt tell a differance. No dyno and Im a track virgen with a one legger. This is going on a 75 455 .10 over TA stg2 se heads, 290H cam, hydrol flat tap , spherical dish TA pist, B4B intake, 700r4, 3.08 one legger. Everything but zero deck.
Im runing mallory elec pump, return reg , 3/8 , 3/8 fuel lines , sending unit in and out 3/8. You can see the baffle first pic. I put the better Apt in it. Was a simple mod. It has the correct pair of air bleeds. I went by your book spec for spec. Identified every hole passage slot angle stop opening tube colar and bushed. Sec accel tubes are set to 1 inch. Im using large window n/s with long taper looking through it from bottom. The sec needle fits in it with a little wiggle room For size ref. Ware the fuel enters the main body and turns up into the n/s I rounded and opened the pocket a little.
17056263 is the main body that now my frankstien is. My 6263 main body I opened and removed the hump and venturi. Realizing after getting your book that that was not good for flow. Understanding what a venturi is, And a booster venturi is, I know I screwed that one up. I know it ran strong but could more than likley run better. Their is actually one diffeance that I can see. Thats the center of the smallest booster the brass tube with the hole in it. The tip is shaped a little different. I have a choke pull off with an electric choke. No stove choke. I dont get what you mean by the plastic polloff dosent hsve enough material to unload the choke.
I think you have the 6264, or the 6364?
Using the correct primary pull-off with a straight fitting is WAY better than cobbling in a plastic pull-off and using a secondary pull-off to unload the choke. The late model primary pull-offs have an adjustment screw for infinite control of the choke flap angle on a cold start. The straight fitting designs have easy access to the restriction to modify for best release time. I can even make them adjustable like I do the plastic ones we sell if/as needed.
Slotting the casting and adding APT does NOT help. The 1975 (and some early 1976) units that had the APT system in the right front corner of the carb are HORRIBLE. The models with the auxiliary PP and single jet/rod even worse.
You can block off the APT and single jet hole, then add the later APT, but it's still the early design and the fundamental flaws and shortcomings with it are still there.
Get a 1976 or later casting to replace it, trust me on this, no need to go into great detail to bore the readers with why they are not that great.
I have stacks of them here and will NOT use them for any reason. The better 1976-1980 front inlet main castings are dime a dozen and readily available. Good news for the educated builder as folks to this day look for early designs thinking that they are superior to the later units which they are not.
Other than using a better pull-off and replacing the main casting with a later design things look fine. I pointed out a few things in the first response more to educate readers so no need to cover them again.
Cross-drilled secondary nozzles were only used because they used a long stop on some of the smaller CID engines which limited the flap opening. I guess they didn't want them running lean due to less "pull" on the back side of the divider so provided an alternate path for the fuel down the front side.
On the subject of "nozzles" it amazes me to this day how efficient they are. One would think that having a booster back there positioned in the center and a Venturi shaped casting would be far superior for metering, atomization and distribution of the fuel into the intake. However I've had my 1977 Pontiac carb on the dyno many times against other designs, Holley HP 950's, Holley 4781-2 DPS, Demon 850's, all the Edelbrock "junk" and runs right with or outruns them every single time. We've taken the same carbs tested on the dyno to the track and the ugly Q-jet outruns them there as well. I was even asked by a Rep from Popular Hot Rodding Magazine when we were doing the Engine Masters articles back in 2004 to do some carb testing. When the Q-jet outran the big Holley's on the dyno and at the track for some reason all that didn't make into print........my first lesson in the propaganda of Magazines and "high performance" parts. You are allowed to test against them, but if you win not too much will be said about it because of advertising and sales potential most likely.
I've tested a whole bunch of other parts, including every intake that will fit under my hood, every type of 1" spacer made, even some custom "home-made" ones, and non of them made the grade either. Did some back to back cam swaps as well, and back to backed some "bug zapping" ignition systems. Those are stories for another day on the appropriate topic........Cliff
I looked at the late model sec pull off and did like the adjuster. I thought about cutting tbe nipple before it turns for access to open it a little. That would be easier than what I did. But since my choke pull off seems to do its job well I went plastic. They are cheaper to buy. I block the stove choke port and use the later choke housing. Nothing gets in the way of the choke pulloff . No sec lockout in the way either. Opened wide and tight when warm. No floppy butterfly. Im not using the early apt throtle plate. Im very confident everything is the same now on this main body and the 6263.
I have to at least find out at this point. Either way I am on tbe hunt now for the 76 up style. Good or bad I will post.
So your in Ohio?
Are you selling any carb cores from any of your barrels and barrels and have you finished looking at them all. That was over a yr ago.
The secondary design with the shape of the butter flies and the divider with tbe nozzle right over the end of it, and the emulsion tubes adding some air does work well.
My advice, get away from a secondary pull-off all together, big abortion hanging on the back of the carb and you have to bend the link to adjust the open angle on a cold start. They are called "secondary" pull-offs for a reason, designed to come in slower than the primary pull-off and only there in case the primary one fails.
Upgrade to the primary pull-off with a straight inlet fitting. Makes for a nice clean unit and fully adjustable flap angle on cold starts with the turn of a screw.
I'm confused about the main casting, initial pics show a 1975 early APT unit modified for later APT by a slot made in the casting to use a later PP and APT screw in the Aux PP hole. This does NOT get one away from the design flaws in the casting, far better to get a later APT model as mentioned.
Don't understand the second design/nozzle comments? The only models using drilled nozzles were on tiny CID engines with a huge stop to greatly limit flat open angle and CFM. We had a customer quite a few years back modify a Pontiac 301 carb to the brink of extinction (Max Effort) and he ran into some tuning issues on the dyno. He ignored the issue and just had me custom machine some secondary metering rods with long tiny tips similar to your DX's. The engine was over 700hp, short version of the long story it took solid nozzles to correct the problems. Nice lesson to learn and glad I didn't do the build as he spun a rod bearing from detonation before figuring out it was going lean on the top side.........FWIW.......
Selling carb cores?.........Pretty much everything around here is for sale except my dogs, even the wife. She's the best deal on the property because in a week you will pay me MORE money to take her back!....LOL....
Point taken on the nozzles. I suspect he had other issues though. You almost have to try on purpose to have lean secondaries.
Now I realize the turbo 70 ies 4 barrel was on a v6 or 350. NOT a 455
Good info Cliff. For the love of god please use the 76 and later body!
"Point taken on the nozzles. I suspect he had other issues though. You almost have to try on purpose to have lean secondaries."
Keep in mind that the carb was feeding an engine making nearly 750hp and the car runs well into the 9's in the 1/4 mile. Cars that quick will find issues with things and we can learn from them. When you start renting dyno time and setting up a car to hook hard and run well all the way down the track with deadly consistency, then, and only then will you be able to make accurate comparisons with these things.
Most folks messing around with modifications and swapping in different parts wouldn't really know if they are helping or not as they really don't do any testing beyond "seat of the pants". I've found over the years that seat of the pants assessments when testing parts and modifications are pretty much USELESS. Every single time I install a part that is supposed to make "more power", like a big single plane intake or some fancy/expensive spacer, for example, a test drive and full throttle blast has my butt-meter telling me I'm now making more power. When I get to the track and back to back test, the car hooks hard and the big shift in power to the upper mid-range and top end has the car leaving softer and running WORSE ET even if I picked up some MPH and pulled harder on the top end.
The most important comment I made above was to get rid of the "screen" on top of the airhorn and get a factory baffle or fill that slot with Marine Tex. Venting very quickly becomes a BIG DEAL with these things when you take steps to make your car run quick on the street and at the track..........Cliff
Ive had the baffel for a long time. I have a pile of baffles. Do you think I should angle cut tbe big vent pipe in front? As far as drilling 4 holes in the sec accel tubes. Ive read to do it to smothen the transition. Airates and gets the fuel there quicker. .30in
Or .030 holes 1/4in up from bottom and 1/8 apart. Your book says dont do it.
The POE tubes stick into the bottom of the wells to extract all the fuel from them. If you start putting holes in them higher up on the tubes they start to suck air instead of fuel. There would be no need to add air or emulsify POE fuel, delay it's delivery or reduce how much is delivered on a full throttle hit.
No need to slash cut the big front vent, but you do need to block the huge slot/vent on top of the airhorn at the rear or at a minimum put a baffle behind it. On a really fast car it becomes a big deal in a hurry. For most "street" cars it's not going to matter as they don't leave hard enough for venting back there to become an issue. I don't really know much about your car but I would imagine it's not hooking solid and running 1.5-1.6 second 60' times so probably nothing much to worry about........