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#1
10-01-2016, 08:02 PM
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Carb question
Ok so I have a 66 :389 (30/30bore), msd ignition, and dt headers. Everything else is pretty much stock. I'm running the original intake and rebuilt carter 4bbl carb. I recently put in 373 gears. I was anticipating more low end torque, but it doesn't seem so. The carb is giving me some probs on start but runs fine once warmed up. Would swapping the carb to say a 600 cfm performer make much of a difference? If I swap the carb which carb would you guys recommend? What about intake?
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#2
10-01-2016, 08:47 PM
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Might try a 67 QJ intake and properly prepared QJ.The factory AFB is around 525 I believe.Tom
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#3
10-01-2016, 09:37 PM
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Quadrajet is good. Tripower will give you the kick-in-the-pants response like no other.
Ask anyone who runs one. Three accelerator pumps and 750 cfm works just fine on a Pontiac!
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BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
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#4
10-01-2016, 09:45 PM
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Keep the AFB.
Perhaps a knowledgeable tuneup shop could help figure out the cold start issue. A Performer carb would be a downgrade, you'd soon find your car is not performing as well as with the stock AFB. I agree a Tri-Power would be a nice way to go as I'm running one myself, just not as cheap as fixing the minor problem you're having with the AFB
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1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/2.56 2002 WS6 Convertible LS1/4L60E/3.23 |
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#5
10-01-2016, 11:11 PM
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What gear ratio are you coming from?
Does sound like your carb needs some attention. A larger cfm carb won't help low end torque.BUT... A proper functioning carb will. I'd look at the accelerator pump discharge as a start, visually, externally. |
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#6
10-02-2016, 12:11 AM
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Ok so the carb was just rebuilt, during rebuild changed stock leather acc pump to an edelbrock equivalent. The issue with the carb is the choke mixture idle and fast idle are good. Just having issues with a cold start and the choke. In regards to gear ratio I'm coming from stock 323.
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#7
10-02-2016, 12:55 AM
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It can take a little time to get the choke correct.
Is the choke blade fully open when the engine is fully warmed up? If it is, that tells us the choke is, at least functioning, another adjustment maybe all that's needed. If you have not already thrown away the old acc pump diaphragm, and only if it was still appearing to be useable, keep it. Leather is one of the few materials resistant to ethanol blend gasoline. Nothing wrong with the AFB in my opinion, engine should perform fine with that carb. You may need more carb refinements. Member here, user name Carbking, his name is Jon Hardgrove, you may want to talk to him about your carb. He is an AFB expert.
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1977 Black Trans Am 180 HP Auto, essentially base model T/A. I'm the original owner, purchased May 7, 1977. Shut it off Shut it off Buddy, I just shut your Prius down... 
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#8
10-02-2016, 01:21 AM
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Ok so the choke issue is at start before turning the key I floor the gas pedal to set the choke. The choke Blade closes, but the fast idle lobe doesn't seem to set to the cold start position
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#9
10-02-2016, 07:29 AM
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The choke needs to be adjusted for just enough closing force with motor cold and the throttle open to close off the choke fully, then when the motor starts the choke counter weight should pull the choke open so that a 1/8" drill bit can fit in, and at that point the motor should be at a 1200 rpm high idle .
If a choke on any Carb in weather below some 65 degrees does not close fully then any motor will have a issue starting without the use of excessive pump shot. Your change in gearing should have provided a 15% torque improvement , if you want to feel more torque install new valve springs if that has not been done , you will not belive the difference it makes on any motor with more than 40k miles on it, even if the rest of the motor is in good shape . I was shocked some 45 years ago when I did this and only this to a motor , and yes this was with a factory Cam! A performer Manifold would provide you with more too end over the factory AFB Manifold , but even this Manifold is done by 5400 rpm, but if your running a stock Cam of less than the 068 grind this would not be a bad combo !
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! Last edited by steve25; 10-02-2016 at 07:40 AM. |
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#10
10-02-2016, 08:51 AM
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The stock original Carter AFB can deliver up to 610 CFM.
Changing to a performer 600 carb will make a noticable difference! But not in the direction I believe you want to go!!!  A 50 year-old original leather pump is probably superior to the e-clone pump. If you kept it, soak it in a light oil overnight (Neetsfoot oil, 3 n 1 oil, sewing machine oil, etc.) and reinstall. Here is a link on the choke: http://www.thecarburetorshop.com/Automaticchokes.htm Here is a general link on Carter AFB carbs: http://www.thecarburetorshop.com/AFBtuningtips.htm Here is a link specifically on Carter AFB carbs used on the GTO: http://www.thecarburetorshop.com/GTO_AFB_carbs.pdf You might be interested to know that I have been saying for years that the older updraft carburetors used in the 1920's were much more reliable than modern downdraft carbs! The reason: the updraft was so difficult to remove, most enthusiasts fixed the REAL problem BEFORE they tackled the carburetor! Take another look at the post above by Steve about valve springs. Have you done a compression test? Have you had the ignition system tested on a 'scope? 50 year old valve springs are much more likely to give problems than a 50 year old AFB, especially if you rebuilt the AFB with a quality rebuilding kit.Jon.
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"Good carburetion is fuelish hot air". "The most expensive carburetor is the wrong one given to you by your neighbor". If you truly believe that "one size fits all" try walking a mile in your spouse's shoes! Owner of The Carburetor Shop, LLC (of Missouri). Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings. Last edited by carbking; 10-02-2016 at 08:59 AM. |
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#11
10-02-2016, 11:13 AM
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Jon, thank you for all the links. Also the motor was rebuilt in 95.
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#12
10-02-2016, 11:18 AM
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So I've been doing a lot of surfing and I came across fitech efi. Opinions ?
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#13
10-02-2016, 11:37 AM
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The original Carter AFB on these engines is very driveable and performs well, as Jon pointed out.
There is a separate adjusting screw on the throttle shaft driver's side that is only accessible when you hold the throttle open. You need to adjust this screw so the fast idle is about 1200 rpm at startup as Steve25 pointed out. Most people don't know the location of this adjustment. The idle speed on these Pontiac AFB's is controlled by the large screw on the front of the carb, not by a throttle stop like most carbs. Before you give up on this setup, at least tweak it so it works as intended. You won't be disappointed.
__________________
BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
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#14
10-02-2016, 12:20 PM
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Db, yes I have adjusted the fast idle screw. It's set a bit high right now. Well good news on choke. I was adjusting it yesterday, but had to wait till this morning for the engine to be cold. Seems better, fired right up. Sadly fast idle is set a bit high. Gonna adjust it to db's suggested 1200
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#15
10-02-2016, 12:52 PM
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According to BOTH the Carter Master Service Manual, and the 1966 Pontiac Chassis Manual, initial fast idle should be set at 2500 RPM. It will only stay there a few seconds, and then it will begin to drop as the cam begins to turn. If the fast idle on the first step is set too low, then the possibility of stalling exists after the cam begins to rotate to lower steps. Of course, you can set the fast idle too low, and then go back in the house and drink a second cup of coffee before driving the car, thus eliminating the possible stalling.
(Opinion) Money spent with our host on a reprint factory chassis manual would be much more wisely spent than the purchase of the efi. Nothing (including the efi) will work properly if not adjusted properly. Jon.
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"Good carburetion is fuelish hot air". "The most expensive carburetor is the wrong one given to you by your neighbor". If you truly believe that "one size fits all" try walking a mile in your spouse's shoes! Owner of The Carburetor Shop, LLC (of Missouri). Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings. |
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#16
10-02-2016, 02:03 PM
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#17
10-02-2016, 02:04 PM
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Where did you find the fast idle set point?
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#18
10-02-2016, 03:26 PM
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Side 6B-66.
And, the fast idle setting must be made after the idle speed and mixture adjustment has been made. With the engine completely warmed up and fast idle screw on highest step of fast idle cam, set fast idle screw to give an engine speed of 2500 rpmīs. |
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#19
10-02-2016, 06:04 PM
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I'm a little in clear how to set the fast idle once engine is warm? Do I manually put fast idle screw on the lobe at the spot for cold start?
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#20
10-02-2016, 06:56 PM
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Ideally you would have the choke linkage go to the high cam position on the fast idle cam when you depress the accelerator linkage in the morning.
You can verify that the linkage is working correctly, the choke cover is adjusted properly and the high idle screw is resting on the high cam step. If not then, you need to get that working every time and then set the high cam engine rpm to the 2500 rpm setting Jon mentioned. The FACTORY AFB Carb is a great driving carb and is exceptionally smooth when installed with factory specs. Tom V.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
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