Questions on carb functionality and vacume
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Joined: Jan 2000
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From: Colorado Springs, CO
Questions on carb functionality and vacume
I have a couple of carb questions:
1. When the power valve opens up under load, where is that exra gas delivered through ?. Is it through the squirter (same as acc pump), the main jet nozzels, or some other orifis.
2. When I install my vacume gauge (on manfold) to adjust the 4-corner idle mix screws, the idle drops down. After I am finished adjusting the idle mix, I plug back on the accessory vacume lines and the idle goes up by about 500 rpms. I then have to reset the idle back down.
My question is, should the idle be the same with only the gauge connected as well as when the accessory vacume lines are connected ? (vacume leak?)
Or maybe should I be teeing off the accessory vacume lines with my gauge to best duplicate the vacume the engine will be seeing, then adjust the mix srews ?
1. When the power valve opens up under load, where is that exra gas delivered through ?. Is it through the squirter (same as acc pump), the main jet nozzels, or some other orifis.
2. When I install my vacume gauge (on manfold) to adjust the 4-corner idle mix screws, the idle drops down. After I am finished adjusting the idle mix, I plug back on the accessory vacume lines and the idle goes up by about 500 rpms. I then have to reset the idle back down.
My question is, should the idle be the same with only the gauge connected as well as when the accessory vacume lines are connected ? (vacume leak?)
Or maybe should I be teeing off the accessory vacume lines with my gauge to best duplicate the vacume the engine will be seeing, then adjust the mix srews ?
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Questions on carb functionality and vacume (jeffs)
Jeff -
The PV discharges fuel through an independent discharge oriface located in the carb baseplate (throttle plate) between the primary throttle bores.
You should not get any change in idle speed whether you have a vac gauge or a vac accerssory plugged into the carb/manifold. If your idle goes up when the accessory is plugged in, your accessory is leaking.
The Demon has two side-by-side distributor advance vacuum port fittings on the passenger side of the carb. You should be using the ported source for your distributor vacuum, leaving you with an unused manifold vacuum port. Use this port to hook up your vacuum gauge.
Good luck, and contact me if you have any questions.
The PV discharges fuel through an independent discharge oriface located in the carb baseplate (throttle plate) between the primary throttle bores.
You should not get any change in idle speed whether you have a vac gauge or a vac accerssory plugged into the carb/manifold. If your idle goes up when the accessory is plugged in, your accessory is leaking.
The Demon has two side-by-side distributor advance vacuum port fittings on the passenger side of the carb. You should be using the ported source for your distributor vacuum, leaving you with an unused manifold vacuum port. Use this port to hook up your vacuum gauge.
Good luck, and contact me if you have any questions.
Safety Car
Joined: Feb 1999
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From: Fountain Hills AZ
Re: Questions on carb functionality and vacume (jeffs)
Sounds like you might be using manifold vacuum instead of ported for your advance. If you want to use manifold vacuum instead of ported for some reason, make your idle mixture and speed adjustments with it connected.
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Joined: Jan 2000
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From: Colorado Springs, CO
Re: Questions on carb functionality and vacume (Vetterodder)
vetterodder,
my advance is controlled by springs. I have no vacuum to the dist advance. The only vacuume connections I have are off the back of the carb for the brakes, and off the intake manifold for the lights. The two ports on the carb are capped.
I was setting idle mixture with my gauge (apparently by mistake) using the fitting on the intake manifold and not the manifold vacume port on the carb. This could make a difference if I keep the intake manifold vacume connected to the lights, and use the port on the carb.
But it also sounds like I might have a vacuum leak because of what I stated in #2 above. It makes sense that the engine rpm should not change when connected to either the vacuum gauge, or, the vacuum plumming to the lights.
Does this sound right?
my advance is controlled by springs. I have no vacuum to the dist advance. The only vacuume connections I have are off the back of the carb for the brakes, and off the intake manifold for the lights. The two ports on the carb are capped.
I was setting idle mixture with my gauge (apparently by mistake) using the fitting on the intake manifold and not the manifold vacume port on the carb. This could make a difference if I keep the intake manifold vacume connected to the lights, and use the port on the carb.
But it also sounds like I might have a vacuum leak because of what I stated in #2 above. It makes sense that the engine rpm should not change when connected to either the vacuum gauge, or, the vacuum plumming to the lights.
Does this sound right?
Instructor
Joined: Aug 2001
Posts: 215
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From: Concord Cal
Re: Questions on carb functionality and vacume (jeffs)
On a Holley the power valve adds fuel to the main circuit. The hole in the base plate if for the signal to the power valve there should be no fuel there.
