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| Edge seeking Sharp blade! |
I rebuilt the carb on my 1961 GMC and was having trouble with cold starting post rebuild, but it ran well if I primed it first. I took it apart again and the only problem I found was a small piece of debris in the screw holding down the accelerator pump nozzle. Fuel flows through the screw and into the nozzle. Choke works now, so I must conclude that actuation of the choke causes a pressure decrease in the venturi and fuel is pushed out of accelerator pump circuit and out of the nozzles. I'd never thought about what circuit chokes on carbs pull from, but accelerator pump circuit makes sense since they are full of fuel that is higher than the bowl fuel level. Do you know if accelerator pump circuit is where nearly all carbs pull fuel from? | ||
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| Member |
It could be on that one, but many small engine carbs with chokes have no accelerator pump and pull from the idle circuit. | |||
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Member |
A choke just restricts air and makes the air:fuel ratio rich. Some carbs have an enrichener circuit to add fuel, but don't block air. Rob NRA Life Member | |||
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Raptorman |
Chokes only pull from the main jets from the extremely low air pressure. ____________________________ Eeewwww, don't touch it! Here, poke at it with this stick. | |||
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Nullus Anxietas |
As noted by others: Chokes don't "pull fuel," per se. They restrict air intake, thus making the air/fuel mixture more rich. It sounds to me like what was happening in your case was the accelerator pump wasn't functioning correctly. That's a mechanism apart from the choke. It fills two roles: 1. Under engine-running operating conditions, when you press the accelerator the butterfly valves in the carburetor's throttle body open wider. This results in a sudden "air dump" into the intake manifold that overtakes the fuel rate. Normally this would lean the mixture out badly, resulting in the engine losing power. The accelerator pump squirts a small amount of additional fuel into the carb to compensate for this. The harder/faster you press the accelerator, the quicker/greater the air dump, the more fuel the accelerator pump injects. 2. During starting there is no fuel in the fuel/air intake system at all. Just air. Pumping the accelerator two or three times activates the accelerator pump, dumping raw fuel into the carb--essentially priming it, and intake manifold, making cold starts easier--or even possible. Wow. It's been a long time since I've had to even think about carburetors  Though the engine in the little power boat we sold a few years back did have a Rochester Quadrajet on it."America is at that awkward stage. It's too late to work within the system,,,, but too early to shoot the bastards." -- Claire Wolfe "If we let things terrify us, life will not be worth living." -- Seneca the Younger, Roman Stoic philosopher | |||
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| Page late and a dollar short |
ensigmatic is correct. The accelerator pump feeds fuel to the intake when you pump the accelerator pedal on a cold engine together with setting an automatic choke. With a manual choke you have to set that by pulling the control cable. Accelerator pump also feeds additional fuel under high demand times such as WOT. -------------------------------------—————— ————————--Ignorance is a powerful tool if applied at the right time, even, usually, surpassing knowledge(E.J.Potter, A.K.A. The Michigan Madman) | |||
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| Nullus Anxietas |
As I noted, earlier: The accelerator pump operates in conjunction with the choke during a cold start, but independently of it. Only as the throttle is first advanced. And you don't even have to be going WOT (wide-open throttle) for it to operate. Think of it like a tiny little hand pump--the amount of fuel being metered into the carburetor throat being dependent upon how fast and how far the pump handle is operated. "America is at that awkward stage. It's too late to work within the system,,,, but too early to shoot the bastards." -- Claire Wolfe "If we let things terrify us, life will not be worth living." -- Seneca the Younger, Roman Stoic philosopher | |||
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| Page late and a dollar short |
Yeah, I got simplistic, basically a shot of fuel upon throttle opening to overcome bog. -------------------------------------—————— ————————--Ignorance is a powerful tool if applied at the right time, even, usually, surpassing knowledge(E.J.Potter, A.K.A. The Michigan Madman) | |||
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Member |
Accelerator pump circuits are not related to chokes. Often they are mechanically actuated. The lag in performance noted when the pedal is opened resulted in the accelerator pump circuit being invented. Obviously on your 1961 the solution is a complete engine replacement with a LS1 V8 crate motor, complete with PFI. New fuel tank, fuel pump, lines, a computer, computer wiring harness, sensors for everything, and a Cadillac converter to keep emissions in check. I mean, really, you cannot be serious (John McEnroe) trying to keep the 1961 carburetor functional. Think of the children, the ozone layer, and all the pollution you will prevent by making it modern. If you really get ambitious, convert it to a BEV. ------- Trying to simplify my life... | |||
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| His diet consists of black coffee, and sarcasm.  |
Nothing I can add to this. OP, are you running an automatic choke, or a manual (cable)? Depending on the setup, getting exhaust manifold heat to the choke to open it up as the engine warms can be problematic. Heat risers (manifold heat control valve) can stick or lose spring tension, and intake manifold passages can clog with carbon. If you're having this problem, you can either say the hell with it and run a cable, or get an electric choke heater. With an automatic choke, the pull-off (uses vacuum to pop the choke open slightly right after startup) adjustment is critical. The pull-offs can go bad or the vacuum line clog or break. If it doesn't open enough, the engine will try to flood; if it opens too much at once, the engine will stall after startup and will be fussy until it starts to warm. Typically this is adjusted by bending the linkage. I didn't mind working with carburetors until they started trying to do the job of fuel injection (i.e., have electronic controls on them). Then they sucked. | |||
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| Member |
Shovelhead nailed it. PSA: On a car with a manual choke, you have to floor the accelerator before you pull the manual choke lever (or you'll wreck the manual choke cable and have to replace it). That pumps some gas in, and sets the idle high for warming up. I remember that well, but can't find the book I was reading... === I would like to apologize to anyone I have *not* offended. Please be patient. I will get to you shortly. | |||
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| Page late and a dollar short |
Should have done the LS swap a few years ago, before I retired I sold GM Performance Parts crate engines! -------------------------------------—————— ————————--Ignorance is a powerful tool if applied at the right time, even, usually, surpassing knowledge(E.J.Potter, A.K.A. The Michigan Madman) | |||
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| Edge seeking Sharp blade! |
I know choke plate closing reduces pressure below it, causing atmospheric to move fuel from the bowl to the venturis. I know the choke and accelerator pump are two separate systems, but they are related if the choke application causes fuel flow from the accelerator pump circuit. My truck has a manual choke. The cold start problem is fixed, I was inquiring what circuit mainly flows on automotive carburetors when the choke is applied. I understand that some vehicles have enrichening circuits instead of a choke. I have vehicles with those. Because clearing an obstruction in the accelerator pump circuit on my carburetor cured my cold starting problem, I conclude that choke application on this carburetor causes fuel flow mostly from the accelerator pump circuit. Good tip on depressing accelerator pedal when applying a manual choke. The owners manual for my 1961 GMC says to do that. | |||
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| Member |
Not only does the choke restrict air flow, but it creates higher vacuum/suction to pull in fuel. | |||
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| His diet consists of black coffee, and sarcasm. |
If any fuel comes out of the accelerator pump nozzle(s) (located just below the top of the carb) at any other time besides accelerating, there's a problem. Your cold starting problem was fixed because pushing the go pedal primed the engine. | |||
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Ammoholic |
Forgot about pumping the gas pedal before starting a car.... Jesse Sic Semper Tyrannis | |||
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