Hesitation when acclerating

[jwarren]

Along with the rear wheel chirp, which I am quite sure is a bad bearing, I cannot completely rid my 66 110hp from stalling when I step on to pull into traffic. It is worse when the engine is cold. When it stalls, it will not re-start easily. Once underway, the engine will accelerate crisply with plenty of power. I have installed new points ,plugs etc. I balanced the cards and it improved slightly. Does anyone have any ideas

[flat6_musik]

Sounds like the accelerator pump (or pumps, both carbs) aren't squirting. If you remove the air filter assy. and look down into the carb with a flashlight (engine off) then move the throttle cross shaft opening and closing the carbs, you should see a small, steady stream of gas squirting down into the carbs. What you describe sounds like a classic case.

You should also check to see if your chokes are working (closing the choke butterfly) when your car is cold.

[bbodie52]

Accelerator pumps function regardless of whether the engine is cold or warm. When you first open the throttle butterfly, air begins being drawn through the throat of the carburetor. However, fuel is not immediately drawn into the increased airflow through the main jets from the float bowl. There is a slight delay in this process until the vacuum draw from the float bowl can be established by the venturi effect. Carburetors that use the effect to suck gasoline into an engine's intake air stream.



The delay in establishing an air flow and then drawing fuel from the float bowl into the venturi causes a momentary lean fuel-air mixture until everything gets moving. The accelerator pump mechanically injects a shot of gasoline into the carburetor to keep the engine happy with a quick shot of fuel until the continuous flow from the float bowl can be established. Every time the driver stabs the gas pedal, another shot of fuel is injected until the fuel flow from the float bowl can "catch up" to the changing demand on the engine.

Once underway, the engine will accelerate crisply with plenty of power.

This statement is a little confusing, because if the accelerator pumps were not functioning it would seem that the engine would stumble any time the throttle is rapidly opened. The impact on the engine might be less because the vehicle forward momentum will help carry it through a temporary lean fuel condition when the carburetor throttle is first opened.

The more you understand about the principles of carburetor operation, the better you will understand what "normal" is supposed to look like. That helps you with the process of fault isolation and troubleshooting. To effectively go through this process you need to be able to properly interpret what is going on with your engine in different operating conditions. Without a good understanding of how the carburetors function it is very difficult to effectively interpret where things are going wrong and why. I have provided some technical information here, but more is available if you read the entire Wikipedia article. In addition the attached Delco Rochester carburetor manual specifically addresses the carburetors and choke systems utilized in the Corvair. When I was a teenager I spent a lot of time reading automotive publications from Petersen Publishing. Those books covered a wide range of automotive engine and powertrain technical theory. I learned about everything from carburetors to camshafts and automatic transmissions and suspension components by reading that material. It helped me a lot when I was learning to rebuild engines and work on my own cars. I can only suggest that you might also benefit from reading some of this material. The more knowledge you gain the more effective you will be at maintaining and repairing your Corvair. The attached Delco Rochester manual does a pretty good job of explaining the operation of the Corvair carburetors as well is how to physically repair them. Hopefully the material in that manual will help you to resolve the issues with your Corvair, but it does sound like your car is suffering from carburetor and fuel control issues, rather than from ignition problems.

In some ways, a carburetor is like a mechanical, rather than electronic, computer. It is "programmed" by the design engineer to do different things at different times, depending on the operational needs of the engine. Varying physical characteristics of airflow and fuel flow are utilized to help the carburetor to adjust itself to a variety of different needs and conditions of the operating engine, and the demands of the vehicle operator. A variety of different fuel circuits, plungers and valves, and passageways come into play depending on the engine temperature, throttle opening, and engine speed. The more the throttle is opened, and the faster the engine is operating, the greater the airflow through the carburetor. The carburetor fuel circuits work to mix the proper amount of gasoline with the air as it passes through the carburetor throat. In modern cars, electronic fuel injection (EFI) systems perform a similar task. But EFI utilizes electronic sensors and electronic computer programming to regulate fuel flow that is sprayed into the injector throttle body. When carburetors were invented no such electronic technology was available. So the mechanical carburetor performs those tasks as effectively as possible considering the given technology that was available when the carburetors were designed and developed. Electronic fuel injection permits a great deal of "fine tuning" in more-carefully programmed increments. So electronic fuel injection can do a better job in most cases the carburetor can.

From Wikipedia...
http://en.wikipedia.org/wiki/Carburetor

Off-idle circuit
As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate blocking air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

Accelerator pump
Liquid gasoline, being denser than air, is slower than air to react to a force applied to it. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which makes the engine misfire (or "stumble")—an effect opposite what was demanded by opening the throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.[9] This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.

The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, most carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts.

Choke
When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up. A richer mixture is also easier to ignite.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls extra fuel through the main metering system to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke can be connected to a cam (the fast idle cam) or other such device which prevents the throttle plate from closing fully while the choke is in operation. This causes the engine to idle at a higher speed. Fast idle serves as a way to help the engine warm up quickly, and give a more stable idle while cold by increasing airflow throughout the intake system which helps to better atomize the cold fuel.

In many carbureted cars, the choke is controlled by a cable connected to a pull-knob on the dashboard operated by the driver. In some carbureted cars it is automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat, or to an electric heating element. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle's accelerator is moved to the end of its travel. This provision allows a "flooded" engine to be cleared out so that it will start.

[jwarren]

Oh yeah I forgot to report that I have checked to see if the pumps are working. Both appear to be functioning fine.

[flat6_musik]

If both pumps are working fine, that confuses me because you say it's hard to start. If they are squirting when you are trying to start, that means it should be getting all the fuel it needs to fire off. See if your chokes are closing when the engine's cold. I also hope you're getting a good hot spark while it cranks.

[bbodie52]

Vacuum leaks at the base of a carburetor or at a vacuum hose can cause poor operation, because even the best, perfectly configured carburetor in the world cannot compensate for such an external flaw that would have the engine operation outside of design parameters. Clogged fuel passages inside a carburetor are difficult to detect and diagnose too, and must be identified by a process of elimination. If all external influences and tuning adjustments have been eliminated as a possibility, then what may be left is dismantling the carburetors, thoroughly cleaning them chemically, and rebuilding them with a couple of rebuild kits to replace parts that can wear out. Carefully adjusting and assembling the carburetor and reinstalling it properly is the final step that should clear things up. All of the other troubleshooting steps, such as external testing and adjustments, checking for vacuum leaks, or bad compression in a cylinder, and verifying the condition of the ignition system, are the easier things to do and should be done first. If those potential proble areas have been eliminated as a cause, then the more complicated, time-consuming and costly step of a carburetor rebuild or replacement is usually the final step, by a process of elimination.

[jwarren]

The car starts fine. Its when it hiccups from a stop. Then the engine is hard to start. Otherwise it performs fine. I am going recheck the chokes and vacuum lines. Maybe the chokes are opening too soon?