OK I'm never too old to learn something so someone please explain how a power valve opening up at idle in a Holley carb changes the idle mixture.
When the power valve opens it adds fuel to the main fuel well after the main jets. This would give more fuel to the discharge nozzles in the main metering system only. The idle system pulls fuel from the main well through the idle restrictions (jets). It has to be pulled above the fuel level in the float bowl and then out the idle discharge nozzles.
The only way to increase idle fuel mixture would be to:
Raise the fuel level in the main fuel well (this determined by float level only)
Open the idle fuel screws
Increase the size of the idle jets.
Increase engine vacuum
Decrease idle air bleed size
I can't see a power valve opening doing any of these.
My Holleys are very sesitive to butterfly position at idle. I would assume that any additional fuel in the main metering system would find it's way into the idle circuit through the butterflies being slightly open?
On a side note, when we were trying to tune my carbs, float bowl level did make a difference in idle characteristics.
Just my worthless 2 cents.
Well HH only if the main well fuel level was increased. And it will not increase above the level in the float chamber. The power valve will only let more fuel into the main system. Will not change the level. The idle system only flows a fraction of what the main system can supply. So additional fuel available should not matter.
I am trying to find a holley carb schematic. Anyone have one to post?
The idle system gets it's fuel from the main well through the main jet. Fuel is then bled off from the main well into an idle well where it's mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle xfer syst. It seems to me that introducing more fuel down stream of the main jet into the main well, which is what a power valve does, (higher in the main well above the idle tube) is what will richen the idle mixture. Just a thought.
OK I'm never too old to learn something so someone please explain how a power valve opening up at idle in a Holley carb changes the idle mixture.
When the power valve opens it adds fuel to the main fuel well after the main jets. This would give more fuel to the discharge nozzles in the main metering system only. The idle system pulls fuel from the main well through the idle restrictions (jets). It has to be pulled above the fuel level in the float bowl and then out the idle discharge nozzles.
The only way to increase idle fuel mixture would be to:
Raise the fuel level in the main fuel well (this determined by float level only)
Open the idle fuel screws
Increase the size of the idle jets.
Increase engine vacuum
Decrease idle air bleed size
I can't see a power valve opening doing any of these.
Ron
Mine seems to drip from the booster venturi's when the power valves are above 5.5 and are staying open at low speed idling.
If you want to come to Chetopa, KS this weekend(90 min north of Tulsa) I can throw the 8.5 powervalves back in the carbs and you can try to make it idle. Bring your own tools though, enough of mine were thrown into the woods near my house in Colorado while trying to make the boat idle at 10,000 ft with no air for either it or me to breathe.
If the butterflys are open too far the idle circuit won't work at all. There must be a vaccume pull on the idle bleed slots in the wall of the throttle bores. If the idle speed screw is holding the butterflys open, the vacuume isn't there for the idle circuit. Have you actually got the idle to change by adjusting the idle mixture screws? A sure sign of throttle opening is when the mixture screws make no difference, like all the way in and the motor doesn't change. MP
The idle system gets it's fuel from the main well through the main jet. Fuel is then bled off from the main well into an idle well where it's mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle xfer syst. It seems to me that introducing more fuel down stream of the main jet into the main well, which is what a power valve does, (higher in the main well above the idle tube) is what will richen the idle mixture. Just a thought.
Interesting discussion guys. I would agree that introducing more fuel would richen it if the idle system were using enough fuel. The idle system has a tiny little jet in the metering block as compared to the main jet. If you introduce more fuel when the system is only using a fraction of what is available should not matter. Only if that additional fuel raised the level of the main and idle wells should it change the mixture.
A little carb FYI for those that do not really know how they work. If you do know then disregard.
What makes a carb flow fuel is pressure differential. The higher the differential the more "force" there is to flow fuel. A mechanical restriction (jet) is what works along with the pressure differential to help meter the fuel.
The float chamber has atmospheric pressure and it requires a lower pressure at the other end of the metering circuit somewhere to pull fuel above the level in the float chamber and out the discharge nozzle. The higher the float level the easier it is to flow fuel. The lower the float level the harder it is to pull fuel.
The idle discharge nozzle is below the fuel level so it has to be raised above the level of the main system in order to flow. This is not a problem because there is more pressure differential at idle than any other time. Engine vacuum at idle is always more than a venturi pressure drop. A venturi pressure drop is what flows fuel above idle. The main jets meter the fuel.
The idle system is in series with the main system. Idle fuel has to flow through the main system before reaching the idle system. The smallest orifice in series is what will meter the fuel. The idle jet or adjusting screws will do this. The main jets are too big to meter the fuel at idle and opening the power valve only allows more fuel to be available. The engine has to be able to pull the additional fuel. And the idle jets and mixture screws are after the power valve.
Lets say you have two chambers filled up half way with a liquid and they are connected beneath the liquid level with a sizeable connection.
Start draining liquid from one container from a hole only a fraction the size of the connection between the containers. Then add another connection between the containers below the liquid level. It will not drain out any faster.
This is basically what is happening when you open a power valve during idle.
Ron
Mine seems to drip from the booster venturi's when the power valves are above 5.5 and are staying open at low speed idling.
That may be true, but fuel dripping from the booster area is a classic sign of the needle/seat assembly sticking open due to dirt, debris, etc. I'd check pull a needle/seat assembly and make sure they are free of crap...
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