Power valve question

[Steelwinky]

In my never ending search for better gas mileage, I called Walker Products today. Several members pointed me in that direction. Although they were didin't seem intersted in selling me parts, the man I talked to was very helpful.

The guy I talked to told me that the MC 2100 carb I put on the jeep was too big for the jeep 258. He aslo told me that if I am getting bad in town mileage, that perhaps jets and power valves wont fix my problems. He said that I couldn't change jets without changing some other components of the carb.

I was told that perhaps I should just plug the power valve, and see if that helps. I was told that all that would do is slow things down, but I'll still reach rpms, just slower.

That being said, I was hoping for some other viewpoints before I do that. I've also read that if you plug the power valve, that often times the jet sizes need to be increased to compensate for a leaner burn. Does that make sense? I have also read that if the power valve is plugged, a lean condition can result, causing damage. Any wisdom on these issues would be appreciated.

[bandhmo]

Don't to it. This guy is way off base. You can change the jets with out changing other parts. And plugging the power valve would be a bad idea.

First, what size MC 2100 do you have. This can be found by looking on the side of the carb (I can't remember which side). There is a circle with a number in it, this is the venturi size. You want the 1.08, other common sizes are 1.21 and 1.5?.

Second, what jets do you currently have. Take the air horn off the carb and look at the jets. They will have a small number on them. I ran #46 and got the same milage as my old Carter BBD, and had good color on the spark plugs. Others have run larger and smaller and reported the same. You may need to remove the jets to read the number, but that is easily done with screw driver. All of this can be done with out removing the carb.

Third, the power valve is not about power. As you vacuum, drops the carb needs to richen the mixture. There are two common ways to do this. One is with stepped or tapered needles that move in the jets. Common carbs that use this method include: Carter BBD, and Rochester G2 and Quadrajet. The second method is to use a Power valve, as is used in the MC2100 and most Holly's. If you plug the power valve you will need to go to larger jets to compensate at lower vacuum, however this will make you too rich at high vacuum and hart mileage.

Now you may wish to go to a different power valve, but to plug it entirely would be pointless and counter productive.

The MC2100 was produced over many years and came with either a two stage or single stage power valve. For best mileage you want a two stage valve. The two stage used a deeper power valve cover which can be seen under the front center of the carb. The single stage valve used a cover that is nearly flat. You will need to remove the carb to pull the cover. All power valves I have seen have a number on them, some of which can be decoded to find the opening points and some can not.

I am tired of typing at the moment so I will sum up for now with this. For the best posible mileage it is very hard to beat a stock non computer controlled Carter BBD (found on 80 to 82 CJ's).

[Steelwinky]

I'm running the 1.08, 47 jets, 7.5 pv. I get 20 mpg hwy, which is fine. But I get 10 in town, which is not fine. I'll try the 2 stage pv, and maybe go down in jet size and try that

[Junk Yard Genius]

Originally Posted by Steelwinky

I'm running the 1.08, 47 jets, 7.5 pv. I get 20 mpg hwy, which is fine. But I get 10 in town, which is not fine. I'll try the 2 stage pv, and maybe go down in jet size and try that

Wilhelm is correct, you SHOULD keep your power valve in the system, but you should TUNE it to work with your current engine/drive train configuration.

First off, tune this like a Holley two barrel, since this carb was designed and built by Holley for Motorcraft, it's a simplified version of a Holley two barrel...

Make sure your throttle linkage has some slack in it when the pedal is at rest.

Make sure when the PEDAL is pressed to the floor, the throttle blades move EXACTLY stright up and down with the throttle bores,
And don't remain slightly closed or go over center when the pedal is pressed.

Make sure you have the Idle mixture screws screwed all the way in, then back them out about 1.5 turns each...
Idle mixture screws ALWAYS move together, the same amount...
If one goes in, the other has to go in that same amount.

Hook a vacuum gauge up to the carb base plate or intake manifold near the carb,
Screw the idle mixture screws in about a 1/4 turn at a time until you get the highest STEAD vacuum reading.
If the idle moves up and down, or the needle moves around, you have probably gone too 'Lean'.

On the first pass, turn the mixture screws in all the way.
The engine should cough, sputter, maybe sneeze through the carb and die.
This is a test of both the Power Valve, and the Accelerator pump.
If either are leaking or siphoning over into the carb throats, the engine will still run.
If it dies, they are both sealed up like they are supposed to be.

If the engine dies, back the screws out 1.5 turns each, and start over screwing them in 1/4 turn each TOGETHER, until you get the highest STEADY vacuum reading.

Set your curb idle again, and move the vacuum gauge somewhere you can see it while driving...
and go for a test drive.

If the engine stumbles when you try and accelerate or start moving from a stop, then you need to richen up the accelerator pump.

When you are at PART THROTTLE CRUISE, at around 55 MPH in high gear, without moving the throttle like you are in traffic, have a look at the vacuum gauge.

You are looking for a Power Valve designed to open at about 2 InHg BELOW your average Part Throttle Cruise vacuum reading.

Some of the more serious off roaders actually go to 2.5 InHg. below the Part Throttle Cruise setting.

Example, If you Part Throttle Cruise vacuum reading is about 8.5 InHg on the gauge,
You need a 6.5 InHg opening point on the power valve, even as low as 6.0.
This will keep the fuel enrichment from happening too fast and wasting fuel every time the vacuum drops a little on the highway.

Go too low, and you will start getting 'Pinging' or 'Spark Knock' before the vehicle decides to move when you push the throttle pedal...

One you have the correct power valve in place,
You can revisit the accelerator pump adjustments and probably save some fuel there.

Another way to learn to save fuel is to install a vacuum gauge in the line of sight when you are driving.
A vacuum gauge will teach you how to drive an engine economically in a BIG hurry!
The higher the vacuum while under power (no coasting readings allowed!) the better fuel mileage you are getting!

[Steelwinky]

OK, so in an effort to better understand power valves, help me out JYG.

At idle with no load, manifold vacuum is high. 17.5 hg or so. As load increases the manifoild vacuum drops(I'm assuming that pv's operate off manifold vacuum.)

So if my power valve is in the open position while I have it removed and am looking at it, does the high manifold vacuum pull against the diapghram on the outboard side of the pv, side opposite the threads, cuasing it to remain closed? If this is the casem then a pressure drop would cause the pv to open at the set hg.

This is the best I can do at reasoning through how these things work. If this is the case, then the higher the hg, the sooner the pv would open as the manifold vacuum dropped.

If thats the case, why would an off roader want a pv with a lower number?

I'm obviously missing something here, clearly i just don't get it yet. Please help me out.

[Junk Yard Genius]

Originally Posted by Steelwinky

OK, so in an effort to better understand power valves, help me out JYG.

At idle with no load, manifold vacuum is high. 17.5 hg or so. As load increases the manifoild vacuum drops(I'm assuming that pv's operate off manifold vacuum.)

You would be correct, intake/carb base plate vacuum is what keeps the power valve closed when it's not needed.
It's a fully automatic system that adds fuel only when needed.

So if my power valve is in the open position while I have it removed and am looking at it,

If you have it removed and you are looking at it, then the engine isnt' going to run!

does the high manifold vacuum pull against the diapghram on the outboard side of the pv, side opposite the threads, cuasing it to remain closed? If this is the casem then a pressure drop would cause the pv to open at the set hg.

Yes.
You have it, all but the 'Outboard' side...
The valve end will be in the fuel bowl so it can open and let fuel into the enrichment circuit when the vacuum drops.

The vacuum diaphragm will be exposed to what is called 'Base Plate' vacuum...

It differs from 'Manifold' vacuum because the closer you get to the base plate of the carb, the less fluctuation there will be in the vacuum signal from valves opening and closing...

Anyway, you have it figured out!

This is the best I can do at reasoning through how these things work. If this is the case, then the higher the hg, the sooner the pv would open as the manifold vacuum dropped.

Yup! You figured it out!
Kind of simple once you get a grip on the concept isn't it!

If thats the case, why would an off roader want a pv with a lower number?

So when you are idling along, trying to finesse an obstacle, and you give the engine a little throttle, you don't want the power valve opening and making the vehicle 'Surge' or 'Jump' (or DIE!) from the extra fuel all at once.

I'm obviously missing something here, clearly i just don't get it yet. Please help me out.

Nope, you have it, so here is a hypothetical...

The Power Valve is designed to remain closed above it's rated vacuum level.
If you have a 6.5 InHg open point on your power valve, and you exceed that to 7 InHg. the power valve will close and the fuel enrichment will stop.

Try this,
You are tooling down the highway at say 60 MPH,
and you have a constant throttle in traffic.
The baseplate/manifold vacuum is steady at about 8.5 or 9.0 InHg vacuum, the power valve is FIRMLY closed.

You decide to give your engine a little more throttle, like going up a hill or to keep up with slightly faster traffic, so you tip into the throttle SLIGHTLY and EVENLY, and the vacuum doesn't drop but to about 7.0 InHg...
The power valve isn't needed, so it stays firmly closed!

You pull out in the other lane to pass someone, and you open up the throttle blades a pretty good ways...
Manifold vacuum drops to say 5.0 InHg., the power valve opens, and you get extra fuel to try and make your engine pull that Jeep faster in high gear....

As soon as the manifold pressure comes up to 6.5 or higher, the power valve automatically closes again.

The power valve is OPEN when the engine isn't running, but there isn't any vacuum in the venturi bores to pull any fuel over... So that issue is moot...

As soon as the engine starts, the power valve closes since idle vacuum is MUCH higher than the power valve setting usually is.

[Steelwinky]

So if I'm running a power valve that opens a little later than it should, what will be the end result?

Say that I'm into the throttle far enough that the power valve should be opening at that point, but it isnt because its HG rating is a little too low. Will the fuel efficiency suffer due to that? Will the engine burn more fuel to accelerate that it would have had to if the power valve would have opened sooner?


When the power valve opens, I understand that there is a increase in fuel, but does the extra work load that the engine has to suffer when the power valve is too low of an HG, burn more gas than the extra fuel used when the power valve has a correct hg number?


Now that I understand the power valve concept, how do jets play into this?? I'm assuming that the jets basically just govern the size of the stream of gas as it is shot into the venturi area and air is added to it. So there must really be no relationship between the power valve and jets.

If I run 47 jets, that it seems most guys at sea level are running, and I'm at a higher altitude, I must need smaller jets?? If there is less oxygen at higher elevations, then less fuel must have to go through the jets to maintain the same air/fuel mixture. That sound right at all?

That leads to my final bit of confusion. If the jets are right for the altitude, then do the Idle mixture screws just fine tune the jet size air fuel mixture?

I've been told that I need to worry less about why things work, and just be happy that they do, but for some reason, this stuff really makes me curious.

Again thanks JYG

[bandhmo]

I do not have much to add, and am glad that Junk Yard Genius is here. He knows more about power valves and Holly's then I do (not to mention carb tuning in general), and can do more to help you then I could.

I will just offer this to try to help your understanding of the power valve.

When running your engine is always pulling fuel though the jets. When making power (not at idle but when the throttle opens just a little) the amount of fuel used is controlled by the jets. At idle your fuel usage is controlled by the mixture needles. When the throttle is open some as in general driving the mixture needles have very little effect.

As your vacuum is reduced you need more fuel, and therefore bigger jets. This is the purpose of the power valve, it is to allow fuel to flow though it in addition to the jets, but only when needed. Basically when the power valve opens it effectively increases the size of the jets.

If the power valve has to low of an opening you will not get enough fuel and get a lean condition. This will cause a lack of power and can lead to higher combustion temperatures and pinging in the engine. Engines are much more for giving to running rich then lean, so other then the lack of mileage it is better to be a bit on the rich side.

The idea is that you use the jets to tune the amount of fuel for light throttle driving. Basically for 90+% of the time. And the power valve adds more fuel in the 5% of the time it is needed, too make more power and prevent pinging.

The idea behind the two stage power valve is to add a little more fuel when a little more is needed and a lot more when a lot more is needed. Basically instead of opening all at once it opens some at one vacuum and then some more at a lower vacuum.

Originally Posted by Steelwinky

I've been told that I need to worry less about why things work, and just be happy that they do, but for some reason, this stuff really makes me curious.

I have been told the same thing. I have also been told that curiosity killed the cat. However, cats have 9 lives and I have yet to die even once from curiosity.

Here is that way I look at carbs and fuel injection.

We need to provide fuel to the engine. This fuel needs to be matched to the air flow into the engine and the vacuum the engine pulls (power it needs to make). Now in an electronic fuel injection system the computer measures the vacuum the engine pulls and RPM (Manifold Absolute Pressure (MAP) system) or the volume of air the engine pulls and RPM (Mass Air Flow (MAF) system) and calculates how much fuel to add.

I carb does the same thing it just uses the fluid dynamics with in its many passages to calculate the fuel added. So by changing the size of the jets or power valve you are doing the same thing as burning a new fuel map into a fuel injection computer chip.

Any way sorry for getting off topic and rambling.

[Junk Yard Genius]

Originally Posted by Steelwinky

So if I'm running a power valve that opens a little later than it should, what will be the end result?

You will just have to tip into the throttle a little more before the vacuum drops enough for the power valve to open...

Don't kid yourself, there are lots of guys complaining because their vehicles are 'Sluggish' and 'Slow to take off', it's because they have the power valve blocked, or it's way too small (Low vacuum opening point).

Other guys complain because their jeep smokes and drinks fuel...
That's because the power valve is ruptured, or the engine is old and tired, and doesn't produce enough vacuum when driving to keep the power valve closed.

Say that I'm into the throttle far enough that the power valve should be opening at that point, but it isnt because its HG rating is a little too low. Will the fuel efficiency suffer due to that? Will the engine burn more fuel to accelerate that it would have had to if the power valve would have opened sooner?

No and no.
If the power valve isn't opening, it's not letting fuel in, so that is fuel you aren't burning.

The idea of slightly later opening is to keep it form opening when you make very small throttle corrections.

Once you watch a vacuum gauge on the highway, you will understand...
Every time you make even a very small throttle correction, the vacuum makes a BIG jump...
You don't want this enrichment circuit opening at every little twitch of the throttle pedal...
That's why I told you to DRIVE the vehicle, get an ACCURATE Part Throttle Cruise reading...
Tune the carb for YOUR engine/drive train to the the "Average" engine that might not have your tire size, live at your altitude, drive in your summertime conditions, ect.

Small movements of the throttle are supposed to be supplied by the ACCELERATOR PUMP, not the power valve.
The power valve is there to help you 'Lean Into' Hills, headwinds, Towing loaded weight, ect.
Same thing your Vacuum advance does for your ignition timing the Power Valve does for your carb.

When the power valve opens, I understand that there is a increase in fuel, but does the extra work load that the engine has to suffer when the power valve is too low of an HG, burn more gas than the extra fuel used when the power valve has a correct hg number?

NO, we aren't moving the opening point radically, just tuning it for our purposes and specific engine/drive train.
The factory couldn't anticipate what the vacuum was going going to be at Part Throttle Cruise when that specific engine got 100,000 miles on it,
They couldn't anticipate you would use taller tires and slow the engine speed at Part Throttle Cruise,
They couldn't anticipate you were going to set that carb off camber, and want to negotiate obstacles at barely an idle and then need a quick burst of throttle to jump off or stay on something on the trail...

This is just a way of giving you more precise throttle control off highway...
IF you do mostly On Highway driving, stick with the factory standard 2 InHg. below cruise power valve, but get an ACCURATE READING before you change the power valve!

Now that I understand the power valve concept, how do jets play into this?? I'm assuming that the jets basically just govern the size of the stream of gas as it is shot into the venturi area and air is added to it. So there must really be no relationship between the power valve and jets.

There is a glancing relationship between Main Jets and power valves...
If you block the power valve as the guy suggested, you will have to 'Jet Up', using more fuel all the time to compensate for the loss of the power valve enrichment circuit.

Main Jets are just that the main fuel metering for the carb.

Idle Circuit and idle mixture screws meter the fuel for the very low RPM, where there isn't enough air flow through the venturis to siphon fuel from the main jets yet.

Somewhere around 1,000 to 1,200 RPM, you will overrun the capability of the Idle Mixture circuit to supply fuel to the engine...

That means you are transitioning to the Main Jets.
Once the air flow is up in the Venturi body, there will be enough 'Vacuum' to draw fuel from the Main Jet wells.

As the air flow increases through the Ventruis, the 'Vacuum' increases, and siphons more fuel from the main jet wells.

Main jets should be sized to keep the fuel mixture at around 14:1 as the engine cruises down the road.

If you attempt to accelerate, you MUST add fuel, or the engine will 'LEAN OUT', making for everything from a 'Dead Spot' feeling in the accelerator as you try to accelerate,
Through 'Spark Knock', 'Engine Ping',
All the way up to backfiring and the engine refusing to run.

The Accelerator pump is supposed to deliver extra fuel to the venturis when you move the throttle linkage,
(Called "COMMANDED Fuel Enrichment", since you have to step down on the throttle pedal to get it)

If you DON'T move the throttle,
(like when cruising up a hill or getting hit by a head wind)
The power valve might open since the engine losses vacuum when it's loaded harder, and the power valve will close when the extra load stops.

If you block the power valve, you will have to add some extra jetting to keep the engine from 'Pinging' when you encounter a little hill, get hit by a head wind, ect.
Remember, the power valve delivers the extra fuel ONLY when needed...
Jetting up will waste fuel anytime the engine isn't heavily loaded.

You will also have to richen up the accelerator pump to cover for the loss in the power valve enrichment circuit...
Since the power valve opens every time the vacuum drops a couple of inches, that meas the power valve helps add enrichment fuel most of the time when the accelerator pump is active, so you will have to compensate for that fuel loss somewhere, either a lot of jet, or some extra jet and some extra accelerator pump.

If I run 47 jets, that it seems most guys at sea level are running, and I'm at a higher altitude, I must need smaller jets?? If there is less oxygen at higher elevations, then less fuel must have to go through the jets to maintain the same air/fuel mixture. That sound right at all?

Yup, Exactly correct.
Good thing the 2100 is easy to change jets in if you are going to the mountains!

That leads to my final bit of confusion. If the jets are right for the altitude, then do the Idle mixture screws just fine tune the jet size air fuel mixture?

Idle mixture has NOTHING to do with main jets.
Idle mixture is a very fine, light weight, fuel deliver circuit.
Very sensitive to fuel flow.
It has to be because there is so little air flow at idle, the main jet circuits would be WAY too big to fine tune.

I've been told that I need to worry less about why things work, and just be happy that they do, but for some reason, this stuff really makes me curious.

There are all kinds of 'How To Tune A Holley' books out there, grab one and it will explain a bunch of how and why things work the way they do...

[Steelwinky]

OK, I followed all of your advice to the best of my abiltiy.

The end results are that I adjusted the rod that hooks to the accelerator pump. As per the wiite up by coas, the lever that controls the accel pump was pushing in on the pump while the engine was shut off. I bent the shaft so its causing the lever to just touch the mating surface on the pump.

I adjusted the idle mixture screws, and it acted just as you said is should under normal conditions.

At idle, my manifold vacuum was about 17.5 hg. At Part Throttle Cruise, 55-60 mph on flat surface in 5th gear, I was runnning about 10.5 hg. Since i have a 7.5 power valve, I'll change it to 8.5.

That just leaves jets, and I'm a little leary of changing them for fear of hurting the engine. I'm out for mileage, so perhaps I should try swapping out the 47's for 45 or 46. I'm not exactly in the mountains, at about 2500-3000 feet, but from what I've read 47 might be too big.

[Junk Yard Genius]

Originally Posted by Steelwinky

OK, I followed all of your advice to the best of my abiltiy.

The end results are that I adjusted the rod that hooks to the accelerator pump. As per the wiite up by coas, the lever that controls the accel pump was pushing in on the pump while the engine was shut off. I bent the shaft so its causing the lever to just touch the mating surface on the pump.

I adjusted the idle mixture screws, and it acted just as you said is should under normal conditions.

At idle, my manifold vacuum was about 17.5 hg. At Part Throttle Cruise, 55-60 mph on flat surface in 5th gear, I was runnning about 10.5 hg. Since i have a 7.5 power valve, I'll change it to 8.5.

That just leaves jets, and I'm a little leary of changing them for fear of hurting the engine. I'm out for mileage, so perhaps I should try swapping out the 47's for 45 or 46. I'm not exactly in the mountains, at about 2500-3000 feet, but from what I've read 47 might be too big.

One or two jet sizes aren't going to cuase your engine to 'Melt'...

Use new plugs, or get a 'Sand Blaster' type plug cleaner so you can keep them bright white...
And check your plug A LOT when trying to figure out if your Jetting is too rich or too leal...

You are looking for a Light tan on the plug insulator.
Darker, and you are too rich,
Lighter, and you are too lean.

Something else that might help you is a 'Rich/Lean' indicator...
It uses an Oxygen sensor (02) in the head pipe to figure out if the fuel mixture is too rich or too lean.

They are a little on the expensive side, but well worth the money in the long run if overall fuel mileage is your goal...
http://store.summitracing.com/partde...5&autoview=sku
http://store.summitracing.com/partde...5&autoview=sku
http://store.summitracing.com/partde...5&autoview=sku

So is a Vacuum gauge to teach you how to drive for best economy.
The higher the vacuum, the better fuel mileage you are getting.
http://store.summitracing.com/partde...5&autoview=sku
http://store.summitracing.com/partde...5&autoview=sku