Could someone explain the basics of how the accelerator pump and power valve work on a carb? I'm tryin to figure out exactly how this stuff works, just curious. Thanks

 

Here is some General info >>
Accelerator Pump:
Supplies fuel under pressure to compensate for losses in fuel flow when the airflow signal to the booster venturis diminishes when you punch it from a standstill, or when the airflow goes away during changes in engine load.
In Holleys, there are two pump check valves:
The inlet check is above the pump diaphragm, and may consist of a steel check ball held in by a bail, or a rubber umbrella valve. The better of the two is the rubber umbrella valve, since it is normally closed and provides a quicker shot, because it doesn't need to seat itself like the ball does. Just be careful not to put these rubber valves into harsh cleaning chemicals, as some rebuild kits don't come with a new valve.
The outlet check on most Holleys is below the squirter. It consists of either a small ball held down by a steel cylinder, or a sharpened steel cylinder. Either is ok, but a small ball by itself is not enough weight to prevent siphoning by the airflow going through the carb throat. I've seen this on my own carbs, when I didn't realize there was a heavier weight to go on top of the small ball.
Spread bore carbs do not use an outlet check under the nozzle, but rather have one in the metering block. In order to prevent fuel from being siphoned from the passage between the metering block and outlet, they use a special anti-pullover nozzle, which prevents airflow from coming close to the outlet and sucking the fuel out.
Power System:
Activated by the power valve in a Holley, this circuit supplies extra fuel to richen up the main metering system. This is a vacuum signalled valve that simply opens and closes at a preset amount of manifold vacuum. The rating is stamped on the valve: 2.5 up to 10.5 inches of mercury manifold vacuum. The lower the number, the later the valve opens, the higher the number, the earlier the valve opens. The metering for this system is provided by the two little holes underneath the power valve, called "PVCR's" or Power Valve Channel Restrictions.
Replacing the power valve with a plug:
If I could, I'd put a electrical shock device in your mouse right now to deter you from even thinking about doing this on just about any application. Think of the power valve as a switch that richens your mixture for heavy loads and accleration. If you decide to plug that power valve, you will need to increase the size of your main jets for proper full power mixture to the point that your cruise mixture will be so rich that your engine will actually foul spark plugs. Too much gas is just as bad as not enough gas. The engine will be sluggish at low rpms, and just won't run right. If you have problems with power valves blowing out, fix the problem, or install one of the available power valve protectors. Summit sells one for about $8. All 4010 and 4011 Holleys, and many newer 4150 and 4160 series carbs come from the factory with this power valve blowout protector. <<<<



__JOHN__Õ¿ö
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A carb pulls fuel in the engine by airflow going through it. If you are driving slow, and then punch the throttle, there will be little airflow to pull fuel into the motor until it has time to "catch up" in rpms. To compensate for lack of fuel during this throttle transition, the accelerator pump "squirts" fuel into the engine till the engine rpms can start pulling more fuel from the regular sources in the carb(main fuel circuit).

The same applies for the power valve. If you are pulling a hill, the engine goes slower and slower as you push the throttle down. The gradual slowing of the engine, means less and less airflow through the carb which means less and less fuel. It's a viscious circle. So what the power valve does, is add fuel to correct this. As you pull the hill and open the throttle, the vacuum in the engine will go lower and lower till a set point is reach and the spring in the valve overcomes the pull of vacuum and opens a channel to add more fuel.