Jet sizes

[mygt350]

Ok. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?

[shelbydoug]

Way too low of idle vacuum. Check the power brake booster. The diaphragm eventually rots out.

[67350#1242]

Ok. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?

Yeah - that's the ticket.   Except that spring tension doesn't decrease, the lower vacuum isn't enough to keep spring from opening valve.

[mygt350]

Would seem engine with somewhat lower vacuum and a "large" PV would have bit too much fuel due to PV spring keeping valve in open position. 
Sorry about the PV discussion but it relates to jet sizing.

[Skidado]

Ok. At idle, engine has highest vacuum reading and power valve is kept closed by the spring and vacuum. When throttle plates are opened under acceleration, vacuum decreases and spring tension is decreased allowing fuel to get sucked thru the "small" end of PV and exit into the two holes in the metering plate that eventually go to the main jets effectually increasing jet size by like 4 numbers?
Am I getting there?

The spring and the vacuum surely work in opposition to each other?  Doesn't the vacuum act on the PV diaphragm to pull the valve closed and the spring tries to force it open?  When the vacuum drops the force of the spring overcomes the vacuum and the valve opens. I don't know that this is right, but it seems logical.

However, counter to this logic is the fact that I swapped a PV with a light spring for one with a heavier spring (suggesting that it should open at higher vacuum) but the mixture seems leaner than it was. I'm beginning to suspect that the PV with the light spring was faulty.

Two other questions remain -

1) while the idle adjustment screws remain effective, why does the idle speed increase when I introduce more air into the inlet manifold, suggesting I'm running rich at idle?

2) why is vacuum so low at idle?  I don't think this is a brake booster issue as that was rebuilt by Jim Cowles just a few years ago and the brakes work as they should. I suspect there's a leak in the rubber pipe I use to connect the vacuum gauge as I'm sure I've seen 14" vacuum in the past.

Lots more investigation to be done when I get a moment to spare!

Onward and upward!

David

[1967 eight barrel]

I just went thought a problem with my dual quad FE. If you have more than 6lb fuel pressure at idle it bumps the needle off the seat. You wind up with a rich condition. Holley had two different size air bleed holes drilled as well as the throttle plates where in two different locations so it couldn't be set correctly. I sent my carbs to Drew Pojentic and they were straighted out and ran OUT OF THE box from him.  I have never seen that happen, ever. I think there are a few that don't understand their operation and a leaking PV can absolutely cause a very rich condition. Ask me how I know.
I can put you in contact if need be.
Here are my corrected carbs running on his  427 Galaxie.
He's on FB.  Tell him Capt Keith sent you.
https://www.facebook.com/AirFuelSParkTech/videos/1231150237067129/

[shelbydoug]

I just went thought a problem with my dual quad FE. If you have more than 6lb fuel pressure at idle it bumps the need off the seat. You wind up with a rich condition. Holley had two different size air bleed holes drilled as well as the throttle plates where in two different locations so it couldn't be set correctly. I sent my carbs to Drew Pojentic and they were straighted out and ran OUT OF THE box from him.  I have never seen that happen, ever. I think there are a few that don't understand their operation and a leaking PV can absolutely cause a very rich condition. Ask me how I know.
I can put you in contact if need be.
Here are my corrected carbs running on his  427 Galaxie.
He's on FB.  Tell him Capt Keith sent you.
https://www.facebook.com/AirFuelSParkTech/videos/1231150237067129/

I agree. It's fuel pressure you may be fighting.

[Skidado]

I managed to get some time with the car yesterday and after some small adjustments to the idle mixture screws and idle speed screw I drove it about 60 miles.

Idle is definitely smoother and throttle response is crisper than it was. I didn't get the chance to pull a plug to see if the colours have changed.

I'm still not clear why a PV with a stiffer spring makes the car run leaner, but while I try to work that one out I will consider this result to be a 'win'!

Thanks for all your help with this.

David

[67350#1242]

Very possible that fuel leaking through pinhole in diaphragm or that the valve itself isn't sealing well.  Holley has had quality control problems with PVs last several years.
An indicator of a leaky diaphragm is evidence of wet fuel in the pocket in the carb body where the diaphragm end of the PV lives.  Fuel is sucked through the diaphragm and directly into the manifold bypassing the boosters.