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Sit N' Spin
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Discussion Starter · #1 · (Edited)
Among a couple of threads I've read as of recently, there seems to be a misconception as to what a power valve is and what it does, as well as where it comes into play on a jet boat application. So I thought I'd post up and give a basic understanding of what they are and what they do in hopes that some can figure out what size to run on a jet boat application.

Before reading this article, I would like to mention that this is a basic reresentation of what a power valve is and what it does as well as apply some basic information regarding how a jet boat engine is loaded to gain an understanding of why you would run a power valve in a lake cruiser.

One thing I'd like to make sure that everyone who reads this understands -

Engine manifold vacuum DECREASES as engine load INCREASES. In this way, engine vacuum is a direct indicator of engine load.

On a typical 4150 Holley carburetor, you will have a 6.5 power valve in it stock. What the 6.5 refers to is the value that manifold vacuum has to fall below before it will open. When manifold vacuum falls below this value, the valve opens, allowing fuel to flow through the "Power Valve Channel Restrictions" (i.e. PVCRs). The PVCRs are basically a second set of jets. They basically allow fuel to bypass the main jets and flow into the main wells. This makes it so that when vacuum is above the valve's rated opening point, thereby depicting a minimal load condition, you're running on a set of smaller jets, allowing the motor to achieve max possible cruise fuel economy. Then when vacuum drops below it's rated opening point, thereby depicting a moderate load condition, it allows more fuel to bypass the main jets and flow into the main wells through the PVCRs. In short, the power valve basically makes the carb think you have 8-10 size bigger jets when engine load reaches a point where it wants a bigger jet size.

The size of the PVCRs is fixed...it cannot be changed without some modification (they'd have to be drilled bigger...they don't make different size ones like they do with jets).

To summarize this, think of the power valve as an "auxiliary fuel switch". It's either open or it's closed...there is no in between. When the engine is loaded to the point where it needs extra fuel flow(depicted by how much engine vacuum drops) that the primary jets cannot supply, the valve opens and allows more fuel to bypass the main jets into the main circuit.

When selecting a power valve, you're deciding at what vacuum level the engine will get that extra fuel. They are available in 1" hg increments and are numbered in half sizes (i.e. 6.5, 5.5, 8.5, etc etc). The bigger the number, the sooner in the throttle plate travel it will open (i.e. a 6.5 power valve will require more of a vacuum drop, IOW more engine load, to open than an 8.5 will).

Now for power valve applications in a jet boat...

Most carburetors come out of the box set up for an automotive application. This works great for cars and some prop driven boats. However, jet boats are in a class of their own. The way the drive loads the engine is completely different from the above two. Cars and prop driven boats unload the engine as they pick up speed. However, forward movement of a jet boat has no effect on engine load. The load on the engine is provided by the water flow through the pump and pump RPM determines how much water flows through the pump. More RPM = more water flow through pump = greater engine load. At a constant RPM, this load is a constant load, much like a dyno. Knowing this, it would make it easier to tune a jet boat engine since you're dealing with a constant load that is completely RPM dependant. This can be seen with a vacuum gauge hooked to manifold on both a car and a jet boat. When you dump the throttle in a car, vacuum drops until the engine overcomes the load and the vehicle starts accelerating. As the vehicle accelerates, vacuum will increase with acceleration (the faster the tires spin, the greater the decrease in engine load in a given gear).

However, on a jet boat, vacuum drops as throttle/RPM is increased and stays constant.

I know some carbs come with a power valve in both the primary and secondary metering blocks on some double pumpers. Although having a secondary power valve would allow finer tuning of the fuel map, IMHO on a jet boat you're splitting hairs at this point since most of the time you're in the secondaries in a jet boat you're at WOT anyway. So on these models I'm in favor of plugging the secondary power valve and increasing jets by 8-10 sizes.

Some racers prefer to remove and plug off all power valves and square jet the carb to the secondaries (i.e. if primaries came with 73s and secondaries came with 81s, you'd plug off the primary power valve and up the primary size to 81). For a boat that sees all of its duty at WOT this is fine since the power valve would be open all the time anyway. But for a cruiser, consider this -

My 18' tahiti copy hits a cruise of between 40-45mph at 3500 RPM. This is typically where I cruise. A vacuum reading at this RPM indicated that I was pulling 11-11.5" of vacuum at 3500 RPM, indicating very minimal load. Since my power valve is currently a 6.5 power valve, at this RPM the power valve is factored out of the equation and I'm running just on the main jets, which are the stock 73s. Imagine if I were to plug off the power valve and up my primary jets 8-10 sizes!? How much fuel would I be wasting by dumping it down the motor while it's at minimal load?

There are different methods for selecting a power valve. Some say to maximize idle vacuum, then select a power valve that is half this value, which I think is how you'd do it on a car. Others have said to check vacuum at cruise, then select one that is rated two numbers below cruise vacuum. One method I have not tried but thought of tonight with a little help from a friend (this applies mainly to carbs with progressive linkage...don't think it will work very well for 1:1s) is to get a vacuum reading with the primaries open as far as they'll open without opening the secondaries, then get a power valve rated for this vacuum rating. If the number is a solid number, select a 1/2 size up (i.e. if vacuum at this point is 6", select a 6.5 power valve). By doing this, I would think that you would be running on the economy jets until the secondaries open, then the carb will basically squarejet itself once the secondaries start to open due to the power valve opening EXACTLY when the secondaries open. Not sure how it will work but will be trying it very soon

Anyway, hope this helps to disspell the mystery behind the power valve and its purpose in a Holley carburetor.

EDIT: All vacuum readings obtained for power valve selection MUST be done on the water under load (thanks Tittyman :D)
 

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NW Spectraholic
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565 Posts
Nice read. I've been fighting a lean pop right at 3500, so I'm wondering about the 6.5 PV being a little small too. This could also be my primary bowl running dry, so between a fuel pressure gauge and some vacuum readings it shouldn't be too hard to figure out.
 

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Sit N' Spin
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1,345 Posts
Discussion Starter · #3 ·
Nice read. I've been fighting a lean pop right at 3500, so I'm wondering about the 6.5 PV being a little small too. This could also be my primary bowl running dry, so between a fuel pressure gauge and some vacuum readings it shouldn't be too hard to figure out.
Thanks. Yeah that could be a few things but I wouldn't think the power valve since vacuum is so high at that RPM. My motor is a BBC 468 with a Holley 750dp, Edelbrock RPM Air Gap intake and Performer RPM heads running 10:1. Carb is jetted stock (73 primary, 81 secondary) and pump has an A impeller in it. I also still have the stock 6.5 power valve in it and at 3500 mine runs fine. Pulling 11-11.5" vacuum at 3500 as well. I plan on fattening up the primaries on the next run and checking vacuum again at 3500 to see whether vacuum went up or down.

Could be a stuck float/needle/seat assembly in the primary bowl if the bowl is running dry. You may also be jetted a bit small on the primary side as well. Which carb and what jets do you have installed?
 

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30 over olds 455

my olds has fresh heads with the exaust crossover tubes blocked off, an edlebrock performer intake, 10-1 compression, 280 comp cam, and a holley spred bore 800 cfm dp.
my carb from holley came stock with 62 primaries with an 8.5 pv and 85 seconds with a 6.5 pv.
i trailer tested this @3000 and had white plugs.
so i took the 76 jets out of the secondairys in my 650dp and put them in my primes!
i took it out saturday, did all kind of plug tests, at idle, at 3000, at 4000, and at wot.
all tests were the same looking plug?? i dont know plugs very well?? if you looked at the porcilin and electrode, the plug looked brand new(not altered at all)
(dont know the tech name for-bear with me) if you look at the peice of metal that bends up and around the electrode, if you look straight down on it, it looks light brown.
if you look at the side of it(metal) it has a hint of white
if you look at the base metal that would be flush with my combustion chamber, it is black?????????????

other than being woried about plugs??????

my boat runs way better and faster than it ever has!!
from 0 to 70 in seconds, i have never felt this before in my boat!!
no hesitation, no pops, nothin but thrust!!

p.s. can anyone tell me what cubes i have???? (455 30 over)

any help or thoughts on plugs and or carb would be awesome!!
 

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NW Spectraholic
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565 Posts
Yours is the same as mine '75, 461. ;)

Yep, 30 over Olds, iron heads with big valves, 10.25 compression, 280 comp, 750 hp DP with 72/84 front pv 6.5, rear plugged.

I ordered a jet kit, another PV, and a new fuel pressure gauge for testing this weekend. I have a feeling my old glass fuel tanks are the issue...
 

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Jet boat service
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119 Posts
Nice read Jon. I agree with most of what you write. I still prefer secondry power valves because without them you may be setting your self up for a momentary lean condition when you dump the throttle from cruise. Remember, as you said the jet drive is loading the engine continuosly , even at cruise. So when you dump your foot into it from cruise the jets may need some help for a split second. Same as the primary. Especially if you have a tall manifold like a tunnel ram with one carb . Or an air gap with spacers. The 4160 series , vacume secondary Holleys don't have a secondary power valve because the secondarys open on a velocity signal and are slower to respond. I have seen lean conditions before, in the secondary, especially on motors that have a very high volumetric efficiency, with large cams and high flowing heads. Thats why the engineeers at Holley designed in the secondary power valve. (or should I say Smokey Yunick, he invented the Holley in 1956) If your carb is set up right for your application everything should work great. I don't understand why you want to block this off, unless you are running a dragster.. One of the reasons I prefer the Holley double pumper is because it has a secondary power valve. In all of the years I have worked on Jet boats for the lake boater I have never removed a secondary power valve. Never needed to.

Jim :D:D
 

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Sit N' Spin
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1,345 Posts
Discussion Starter · #8 ·
Nice read Jon. I agree with most of what you write. I still prefer secondry power valves because without them you may be setting your self up for a momentary lean condition when you dump the throttle from cruise. Remember, as you said the jet drive is loading the engine continuosly , even at cruise. So when you dump your foot into it from cruise the jets may need some help for a split second. Same as the primary. Especially if you have a tall manifold like a tunnel ram with one carb . Or an air gap with spacers. The 4160 series , vacume secondary Holleys don't have a secondary power valve because the secondarys open on a velocity signal and are slower to respond. I have seen lean conditions before, in the secondary, especially on motors that have a very high volumetric efficiency, with large cams and high flowing heads. Thats why the engineeers at Holley designed in the secondary power valve. (or should I say Smokey Yunick, he invented the Holley in 1956) If your carb is set up right for your application everything should work great. I don't understand why you want to block this off, unless you are running a dragster.. One of the reasons I prefer the Holley double pumper is because it has a secondary power valve. In all of the years I have worked on Jet boats for the lake boater I have never removed a secondary power valve. Never needed to.

Jim :D:D
Actually, not all double pumpers have a secondary power valve. My secondary power valve is not only blocked off, but the metering block does not have any power valve channel restrictions in it so if I were to install one, it would do absolutely nothing unless I get a different metering block.

I think one of the reasons power valves got a bad rap was due to the fact that the older Holleys with no anti-backfire valve would blow them out in the event of an intake backfire. However, it's my understanding that all Holleys made after '92 or so come with this power valve backfire protection valve. For the ones that don't have it, they make an inexpensive kit to add it so that this is no longer an issue.
 

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Sit N' Spin
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1,345 Posts
Discussion Starter · #10 ·
my 650dp does not have secondaries pv capabilities!!
That's probably why you'll notice that the secondary jets are 8-10 sizes bigger than the primary jets. Secondary power valves are a tuning luxury rather than a necessity.

As for your spark plug question...

The porcelain should be tan/light brown in color when the mixture is right.
 

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Sit N' Spin
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1,345 Posts
Discussion Starter · #12 ·
I drive thru Monroe all the time. The blended ethanol in the fuels can cause all kinds of problems, especially in a marine environment. Also eats glass tanks, so adds to the other problems in older engines.
Speaking of which...

A friend of mine had a fuel pressure issue last season ever since he had the boat. There were a couple of other issues we corrected (fuel filter, fuel pump wiring, etc etc). At cruise RPM fuel pressure would gradually drop to 0, then run out of gas. After doing everything we could do, we decided to pull the tank.

This tank still had the anti-siphon valve on the tank outlet. Well, what was happening was that the fuel sender had a black foam rubber float. The ethanol in the gas was slowly eating it away (float was coming apart). Pieces of that float ended up stuck in the anti-siphon valve, causing a huge restriction that prevented the fuel pump from pulling adequate fuel from the tank.

We replaced the anti-siphon valve with a plumbing elbow and a threaded hose fitting. Since the fuel gauge didn't work, he cut the float off of the sender to get that foam out of the tank to keep it from happening again. Since then, no more fuel pressure issues.
 

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Speaking of which...

A friend of mine had a fuel pressure issue last season ever since he had the boat. There were a couple of other issues we corrected (fuel filter, fuel pump wiring, etc etc). At cruise RPM fuel pressure would gradually drop to 0, then run out of gas. After doing everything we could do, we decided to pull the tank.

This tank still had the anti-siphon valve on the tank outlet. Well, what was happening was that the fuel sender had a black foam rubber float. The ethanol in the gas was slowly eating it away (float was coming apart). Pieces of that float ended up stuck in the anti-siphon valve, causing a huge restriction that prevented the fuel pump from pulling adequate fuel from the tank.

We replaced the anti-siphon valve with a plumbing elbow and a threaded hose fitting. Since the fuel gauge didn't work, he cut the float off of the sender to get that foam out of the tank to keep it from happening again. Since then, no more fuel pressure issues.
Eth blends suck don't think they work as well with older carb and injection applications set ups.......Also makes your on topic article a must read. Modern blends and using an autmotive set up. Will rob performance! Good tune for jet applications.
 

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Have you checked,and sure you have no vac. leaks. IMO 12 steps up you should see a change in plug readings,allso IMO if you are to rich black is what you will see,wet if way to rich but you will still see black. 455 olds right? are prone to intake leaks.
 

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Sit N' Spin
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1,345 Posts
Discussion Starter · #19 · (Edited)
my 650dp 66 primes 76 seconds
interesting!!!

my 800dp 76 primes and 85 seconds both with pv
so if i took out my seconds pv, then i would need 95 jets??
that seems huge????
What is the part number on that carb?

Did you buy it brand new or used?

Post up the part number and I can look up what it should have stock.

BTW X2 on the intake leak. Carb adjustments can only directly affect the air flowing through them, so any air that's entering the engine through anywhere other than the carburetor can cause an overlean condition that carb adjustments/jetting cannot correct.
 

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AKA OhOneWS6
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2,634 Posts
What is the part number on that carb?

Did you buy it brand new or used?

Most Holley specs that I've seen indicate a dual power valve carb to be square jetted (i.e. both primary and secondary would have 76 jets in them if they both have a power valve).

Post up the part number and I can look up what it should have stock. Sounds to me like someone at some point thought the secondaries should have much bigger jets in it (most single power valve carbs do) while failing to realize that there's a power valve in the secondary.

BTW X2 on the intake leak. Carb adjustments can only directly affect the air flowing through them, so any air that's entering the engine through anywhere other than the carburetor can cause an overlean condition that carb adjustments/jetting cannot correct.
According to his other thread he has a 6213 Spread bore. Not sure if he bought it new or used. (not hijacking 75 Sleek just trying to get you more info before you check back in. ;))
 
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