Power Valve Question - HELP

[Sport 360]

Anyone know if a mafunctioning power valve in a Holley 750 vac sec would cause the engine to stumble and miss at around 3000-3500 rpm with about 1/3 - 1/2 throttle ?? Sounds and acts like its hitting a rev limiter.
Engine will rev past 3500 with no misses with very slight throttle openning so I'm now ruling out ignition. Idles great, runs great on the highway at 3000 with not miss or stumble.

I've replaced: coil, electronic distributor (checked gap), plugs/wires all good, new fuel filter.

Running out of ideas.

Thanks !!

 

[Southernman]

It could be the power valve, but I've usually seen very poor idle characteristics as well, since a defective one makes the carb run very rich.

The power valve allows you to run lean at part throttle, which sounds like where you're experiencing stumble (1/3-1/2 throttle), and increases fuel flow under power by 8-10 jet sizes. With that being said, your fuel mileage should be hideous if the power valve was shot - to the tune of adding 8-10 jet sizes to the primary side of your carb. Excessively rich symptoms will be an obvious indicator, and your plugs are a good place to start diagnosis.

If you've experienced a recent back-fire, the power valve is probably shot. You can use a Mityvac to test it. Should hold 10-20 In Hg.

I hope this helps, and good luck with the carb.

Southernman

 

[Sport 360]

:banghead: So I used a plug in place of the power valve and it makes no difference !

 

[mullinax95]

:banghead: So I used a plug in place of the power valve and it makes no difference !

Yes... I have plugged the power valve before (tune and testing) and the engine will run without one. It ran rich and the engine loved it.

 

[1968FormulaS340]

The one time I had an issue like this it was dead valve springs in a chebby 350.

 

[Sport 360]

Yes... I have plugged the power valve before (tune and testing) and the engine will run without one. It ran rich and the engine loved it.

Still not right though so its back to ignition I think. Going to upgrade the cap and rotor. Don't like the one on there now, it's just a stock replacement plobably made in China ops:

 

[67Dart273]

Yes... I have plugged the power valve before (tune and testing) and the engine will run without one. It ran rich and the engine loved it.

If it ran rich AFTER you "plugged" the valve, then it wasn't plugged. An engine should run LEANER under hard power settings with the valve plugged.

 

[JoeDust451]

If it ran rich AFTER you "plugged" the valve, then it wasn't plugged. An engine should run LEANER under hard power settings with the valve plugged.

Yes if you don't up the jetting, but it will run a bit richer in the idle curcuit, if you go up on jet size like your suppost to, it'll get about the same amount of fuel for the main metering. The PV is an in-richment valve, it delivers extra fuel into the main jet wells when under load (low vacuum), but leans out the curcuit abit for a cleaner idle, thats why the jets are smaller when running a PV, once you block off those PV channels you need to increase the jet size about 8 to balance out what you took away, the only way it'll run leaner is if you don't increase the jets, in fact it won't run good at all once you get into the main metering, it'll fall flat on its face.

 

[67Dart273]

Yes if you don't up the jetting, but it will run a bit richer in the idle curcuit, if you go up on jet size like your suppost to, it'll get about the same amount of fuel for the main metering. The PV is an in-richment valve, it delivers extra fuel into the main jet wells when under load (low vacuum), but leans out the curcuit abit for a cleaner idle, thats why the jets are smaller when running a PV, once you block off those PV channels you need to increase the jet size about 8 to balance out what you took away, the only way it'll run leaner is if you don't increase the jets, in fact it won't run good at all once you get into the main metering, it'll fall flat on its face.

????? the power valve should not affect the idle circuit. When a properly working power valve with PROPER vacuum no. is installed, it is "same as plugged" at idle, and only opens when vacuum DROPS

Or did I mis-read what you are trying to say?

A PLUG should cause a lean condition at ANY time that the power valve would have been CLOSED normally. Therefore more main jet is required for full power. This should cause a RICH condition AFTER the jets are increased, when running at low throttle settings.

There was a REASON why carb designers incorporated power valves. It was to increase street drivability. They were never intended to be removed for street use.

 

[mullinax95]

You're right... I forgot about the increased jetting that I did. I can't remember how much I stepped it up because it was a long time ago. A friend of mine was telling me to block the PV and so forth. He said it would "run like a scaled dog". I didn't even back it out the shop doors because it ran so rich. But damn the engine sounded good... it loved that extra fuel. I installed the PV back in along with the same jets that was in it the same day.

 

[JoeDust451]

????? the power valve should not affect the idle circuit. When a properly working power valve with PROPER vacuum no. is installed, it is "same as plugged" at idle, and only opens when vacuum DROPS

Or did I mis-read what you are trying to say?

A PLUG should cause a lean condition at ANY time that the power valve would have been CLOSED normally. Therefore more main jet is required for full power. This should cause a RICH condition AFTER the jets are increased, when running at low throttle settings.

There was a REASON why carb designers incorporated power valves. It was to increase street drivability. They were never intended to be removed for street use.

1st. off lol, listen to what you are saying, you qouted, a plug should cause a lean condition at ANY time the PV would be in the closed position, but then you say, Increasing the jets will cause a RICH condition at low throttle settings LOL, But you qoute, the PV should not affect the idle curcuit, but its the same as plugged & the reason they incorperated one is to increase driveability. All your doing is confirming what i said. You need to do more research on carbs before making statements, The PV is there for reason, If it "did not" have any bareing on idle, there would be no reason for installing one, just run the jet you need & be done "because" like you stated it has no bareing on idle quality, I won't say no more, you abviously no ALL the ins/outs & carb building hum, I'll just sit back & learn LOL!!

 

[Mr Hemi]

The power valve restrictor channel determines the amount of fuel enrichment. When the vacuum drops and overcomes the spring on the power valve It's the timing of when it happens. Check the engine with a vacuum gauge. It will determine the mechanical condition of the engine.

 

[67Dart273]

1st. off lol, listen to what you are saying, you qouted, a plug should cause a lean condition at ANY time the PV would be in the closed position, but then you say, Increasing the jets will cause a RICH condition at low throttle settings LOL, But you qoute, the PV should not affect the idle curcuit, but its the same as plugged & the reason they incorperated one is to increase driveability. All your doing is confirming what i said. You need to do more research on carbs before making statements, The PV is there for reason, If it "did not" have any bareing on idle, there would be no reason for installing one, just run the jet you need & be done "because" like you stated it has no bareing on idle quality, I won't say no more, you abviously no ALL the ins/outs & carb building hum, I'll just sit back & learn LOL!!

You are not making any sense at all, Please try to answer my points, you said:

you qouted, a plug should cause a lean condition at ANY time the PV would be in the closed position, but then you say, Increasing the jets will cause a RICH condition at low throttle settings LOL,

Of course it will. If you take a carb that was operating CORRECTLY, plug the PV, and jet for full power, you have to go richer IN THE LOWER POWER/ RPM ranges because NORMALLY the PV would have been CLOSED during this time, and running with the stock LEANER jets. Now, when cruising at high vacuum/ low power settings, the carb MUST go rich because of the richer jets. Normally, you'd have leaner jets and a closed PV

But you qoute, the PV should not affect the idle curcuit, but its the same as plugged & the reason they incorperated one is to increase driveability.

You are confusing two points. A plugged PV should idle EXACTLY the same as a properly operating PV which is CLOSED at idle.

The drivability point was in reference to rejetting the carb. You can NOT plug a power valve and then rejet the carb so it acts the same through all power settings --the very reason the PV was put there to start with

All your doing is confirming what i said. You need to do more research on carbs before making statements, I won't say no more, you abviously no ALL the ins/outs & carb building hum, I'll just sit back & learn LOL!!

If you really want to start another ridiculous name calling thread, go ahead. I am NOT confirming what you said. If you can point to ACTUAL research that refutes anything I've said, I'll be happy to read it

The PV is there for reason, If it "did not" have any bareing on idle, there would be no reason for installing one, just run the jet you need & be done "because" like you stated it has no bareing on idle quality,

A PROPERLY operating PV should NOT have a bearing on idle quality because IT SHOULD BE CLOSED at idle speeds. This is a well documented fact, in any book written by and four Holley or any other carb.

HOW DOES "running any jet" change idle quality?

 

[JoeDust451]

You are not making any sense at all, Please try to answer my points, you said:



Of course it will. If you take a carb that was operating CORRECTLY, plug the PV, and jet for full power, you have to go richer IN THE LOWER POWER/ RPM ranges because NORMALLY the PV would have been CLOSED during this time, and running with the stock LEANER jets. Now, when cruising at high vacuum/ low power settings, the carb MUST go rich because of the richer jets. Normally, you'd have leaner jets and a closed PV



You are confusing two points. A plugged PV should idle EXACTLY the same as a properly operating PV which is CLOSED at idle.

The drivability point was in reference to rejetting the carb. You can NOT plug a power valve and then rejet the carb so it acts the same through all power settings --the very reason the PV was put there to start with



If you really want to start another ridiculous name calling thread, go ahead. I am NOT confirming what you said. If you can point to ACTUAL research that refutes anything I've said, I'll be happy to read it



A PROPERLY operating PV should NOT have a bearing on idle quality because IT SHOULD BE CLOSED at idle speeds. This is a well documented fact, in any book written by and four Holley or any other carb.

HOW DOES "running any jet" change idle quality?

!!!!!!

 

[mullinax95]

!!!!!!

He'll keep going to the end of time if you let him.

 

[Southernman]

Still not right though so its back to ignition I think. Going to upgrade the cap and rotor. Don't like the one on there now, it's just a stock replacement plobably made in China ops:

Hey Sport, here's a simple check you can use to verify whether or not the power valve is actually blown:

Turn the idle mixture screws all the way IN. If the engine dies, the power valve is good.

One other thing, every Holley carb mfg after 1992 has the check-ball modification done to them and that has virtually eliminated blowing power valves as a result of a back-fire.

Also, once you know how much vacuum your engine is making (warmed up and at idle - neutral with a stick or in-gear idling with an automatic), you can divide your vacuum value by 2 (1/2 the vac reading) to verify your power valve matches your cam. If you're running a long duration cam you may need to change the PV to match your true idle vacuum value. Any reading above 13 in hg should work with the standard PV. If you land on an even number, drop DOWN to the next size PV to get you where you need to be.

I hope this helps.

Southernman

 

[Sport 360]

Hey Sport, here's a simple check you can use to verify whether or not the power valve is actually blown:

Turn the idle mixture screws all the way IN. If the engine dies, the power valve is good.

One other thing, every Holley carb mfg after 1992 has the check-ball modification done to them and that has virtually eliminated blowing power valves as a result of a back-fire.

Also, once you know how much vacuum your engine is making (warmed up and at idle - neutral with a stick or in-gear idling with an automatic), you can divide your vacuum value by 2 (1/2 the vac reading) to verify your power valve matches your cam. If you're running a long duration cam you may need to change the PV to match your true idle vacuum value. Any reading above 13 in hg should work with the standard PV. If you land on an even number, drop DOWN to the next size PV to get you where you need to be.

I hope this helps.

Southernman

Just pulled out the distributor (a Mopar perf. one) and while spinning it with my fingers I notice the shaft is bent !! the gap is from aboy .002-.045 if you can believe it :wack: I didn't think it would run like that.

The rotor tip looks fried and half of the contact points inside the cap are all gouged up. The rotor would have been really wobbling inside there.
I'll be putting the stock dist. back in and a new cap and rotor tomorrow.

Sure hope this is the problem. It all makes sence cause it seems to be getting worse (right after I changed it) and I never had any issues with the stock dist. It was just a little weak with the advance curve. Got the Mopar Dist. here on FABO from a member. I sure hope he didn't know it was bent :violent1:

 

[dart star]

take a look at davis unified ignitions (D.U.I) Tri power set up simple to wire and you can remove the stock ignition box and wiring