On my '73 AMC Ambassador, I’m still figuring out how to correctly start a carbureted car. One thing I haven’t found a consistent answer to: should I, or shouldn’t I, tap the gas after a few minutes of warm up on fast idle to drop the engine down to low idle? Someone told me I should just put the car in gear and drive away, and not allow the vehicle to idle low first. But when I do that, shifting into drive at that high speed causes a thudding sound and chassis movement. If I tap the gas to low idle first, then shift, it’s perfectly smooth. Is there a real reason I shouldn’t tap to low idle?
When I drove carbureted cars I always tapped the gas to bring the idle down. I can’t see why someone told you otherwise.
You push gas pedal to the floor and let it up.
This injects fuel into the engine and sets the choke.
Then start the engine.
The engine should go to high idle.
Then depending on the ambient temperature, the choke pull-off will open the choke and drop the idle speed when the gas pedal is tapped.
But as the temperatures get colder, this will take longer for the choke system to perform.
Tester
2 Likes
There is absolutely no reason whatsoever not to tap to low idle. It isn’t necessary, but it also does absolutely no harm. Whoever told you not to simply doesn’t understand how carburetors work.
Tester gave an elegant description, to which I would only add that the linkage that sets the choke also sets the “high idle cam” that opens the throttle plate a bit and raises the idle. Then when the engine warms sufficiently, the mechanism that causes that to all happen when the engine is cold allows the choke to open and the idle to drop. The reason tapping helps if the engine is warm enough is that it removes the friction of the high idle set screw from the high idle cam, allowing everything to move more freely, but that would happen when you stepped on the gas to go anyway.
Car Talk Lackey
Great info by Tester and mountainbike–
www.knowhow.napaonline.com/carburetor-vs-fuel-injection-short-history-pros-and-cons
Carburetor vs Fuel Injection: Pros and Cons
Emissions and fuel economy. Fuel injection, because it can be more precisely controlled, results in more efficient use of fuel, reduced fuel consumption and fewer emissions, which is the main reason it began to replace the carburetor in the 1970s.
Power and performance. Again, because fuel injection and modern electronic controls are more accurate, fuel delivery can be tuned to match driver demand. Carburetors are precise, but not accurate, in that they cannot account for changes in air or fuel temperature or atmospheric pressure.
Cost and complexity. Being purely mechanical devices, carburetors have it hands down over fuel injection with regard to cost and complexity. With a can of carburetor cleaner, simple hand tools and maybe a couple of spare parts, you can rebuild a carburetor on your porch or at a rest stop. Whereas with fuel injection, even with years of training and experience and a few thousand dollars in diagnostics gear, you will still need a tow truck to get you off the road should your system burn out on you. Most small engines, such as those on motorcycles, lawn mowers and snow-blowers, are still equipped with carburetors, simply because they are not emissions-regulated, inexpensive, simple and reliable.
While the carburetor may have been around for over a century, fuel injection is a clearly superior alternative, delivering better power, fuel economy and lower emissions.
Now’s the time of year when the choke adjustment is critical. Make sure it’s adjusted exactly right, and that all linkages are clean and able to move smoothly. Do you have a manual on how to set the choke?
Tester’s instructions were likely better than the owners manual.
I meant a repair manual to adjust the choke.
My early 70’s truck is carb equipped, and probably like your car, a little finicky on cold starts. Here’s how I do it: With key in “off”, pump the gas pedal about 1/2 way, just once or twice, more than a tap, but not aggressively, then let foot completely off the pedal. Turn key to “start” and engine pops and runs immediately. For about 20 seconds I just let it idle, don’t attempt to drive away quite yet. If the weather is cold I usually have to give an occasional tap to the gas pedal to prevent it from stalling during these 20 seconds, which shoots some extra gas into the carb throat via the accelerator pump to enrich the cold start mixture. After that initial 20 second period where I have to fiddle with the gas pedal a bit, it then idles ok on its own. Then I just drive away, and it runs perfectly fine. Depending on the driving situation I may find myself about 5 minutes later waiting at a red light and noticing that the engine is idling a little too fast. I’ll put it in neutral then and step on the gas pedal, not much, just a tap, and that will usually cause the idle speed to kick down off the fast idle cam to the normal idle speed.
I should add that how a carb behaves to the driver is very sensitive to how it is set up, how the carb is tuned up in other words. So the procedure above might work fine for my truck, but not for your car. All of the following affect the carb’ed car’s drivability traits
- idle fuel mixture setting
- warm engine idle rpm setting
- cold engine idle rpm setting
- initial choke plate deflection (before starting engine)
- choke pull-off choke plate deflection (after starting engine)
- automatic choke setting (lean to rich index marks)
- fast idle cam setting
- fuel bowl level setting
- accel pump cam setting
How those are supposed to be set up, that info can be difficult to find on a car of your vintage. But if you buy a carb rebuild kit for your car, that info should come with the kit. That’s sometimes the easiest way to determine how to tune up your carb, even if you don’t plan to rebuild it. No worries, you’ll probably want to rebuild it later. If you have to baby your carb a little, to make it work, sometimes that’s the easiest solution, do what it wants you to do. Trying to get it perfect can be more trouble than just babying it as a work-a-round. Best of luck.
Whoever told you not to tap it down to idle was just plain wrong.
1 Like
@oldtimer-11 … probably depends on the carb make/model, but what Tester says about tapping on the gas pedal to move it from fast idle to warm (slow) idle is how it works on my truck. Autolite 2100 carb in my truck’s case. I don’t think it will never move off the fast idle cam unless the accelerator is tapped. It’s often not necessary to consciously tap it tho, b/c as the car is driven around the driver is constantly tapping on the accel pedal, so after the engine is warm it moves off the fast idle cam then, without the driver’s knowledge.
I take it off the highest step of the fast idle cam 10 seconds after starting the engine, no need to let the engine race. The highest step of the fast idle cam is for starting, the remaining steps are synchronized with the position of the choke plate.
1 Like
When you are ready to drive, take the idle speed down. Those old cars were purely mechanical and were not meant to do all things without driver input. If the engine will run, bring the idle speed down even if you are not quite ready to drive. You don’t want to let it fast idle for long. You will get lots of advice from people who simply don’t know the answers.
Excellent post, cyberbabe, and full of good information, but the biggest reason that fuel injection is better than carburetors is missing.
Put simply, fuel injection allows a much finer spray than carburetors.
Carburetors draw fuel from a float bowl using the relatively small pressure difference between the fuel in the bowl, roughly ambient pressure, and the lateral wall pressure of the carburetor throat past the venturi.
Fuel injection blows fuel under pressures of typically 40psi and more. Since only the hydrocarbon molecules on the surface area of each droplet are in contact with oxygen atoms, a much larger droplet from a carburetor must burn through in layers, like an onion. At thousands of revolutions per minute engine speed, the time frame it has to do that in is extremely small. In short, the droplets from carburetors go partially unburned, and what is burned largely burns beyond the best part of the power stroke. Fuel injected droplets, having much greater surface area per volume, burn much faster and much more completely. Put simply, the benefits of fuel injection come largely from the much greater surface area per volume of fuel introduced into the engine… and with multiport injection, directly at the intake ports. That enables the use of less fuel to get the same power and results in much cleaner exhaust out the exhaust ports.
The introduction of port injection further enhances the precision of the fuel metering and allows all of the fuel to stay in extremely fine droplets and to go immediately to exactly where it’s needed. Port injection takes the idea a bit further, although it has proven to be problematic because it eliminates the washing of the intake valves that port injection does.
So has fuel injection. First patented in the 1800’s.
Yup. Engineers have long recognized the benefits of a finer spray. The problem in earlier eras was controlling it. Early FI systems used vacuum, but it wasn’t precise enough and too problematic (finicky) to make FI practical for the mass manufacturing. Until about 1990, when microprocessors evolved to the point where they became cheap to produce and could manage the injector(s) reliably from engine demand inputs (sensors), and then fuel injection rapidly became much less expensive, reliable, and more practical to produce.
Just think, someday our descendants will look back on carburetors the way we look back on the horse and buggy. 
Somehow reminds me of getting a horse drawn carriage to go, maybe rattle the reins, or use the horse whip, One push of the pedal to the floor to set the choke, imagine a millennial looking for a start button to get the horse drawn carriage to go. A friend has a rotary phone just for kicks, amazing how many people do not know how to work it!