Holley Carb Tuning Debate

« **on:** November 05, 2007, 08:38:33 AM »

So I've been hangin' with Propless lately and him and I have been really analyzing Holley carburetors and the need for a power valve.

It got those wheels turning in my brain. As I'm thinkin' about things, I start realizing the type of load that different vehicles/vessels place on a marine built automotive engine.

Lots of jet guys try to apply their automotive knowledge to a jet boat to tune them. Some of them end up chasing their tail...always tuning at the lake...they can never seem to get it quite right.

So the question remains...is a power valve really needed in a jet boat?

Before that question is answered, let's look at the type of load placed on the engine.

A car...from a dead stop places a certain load on the engine. As the vehicle picks up speed, the load on the engine decreases, allowing the motor to pick up RPM as the load on the engine drops. At a certain RPM, the transmission shifts to the next taller gear, reloading the engine, allowing the vehicle to pick up more speed, and unloading the engine again. This continues until the transmission locks up and becomes a 1:1 drive straight to the pumpkin. Your differential gear ratio will determine how many turns of the engine/drivetrain it takes to produce 1 turn of the wheel.

A prop boat...from a dead stop places a certain load on the engine. As the boat picks up speed, the front of the prop unloads, allowing the motor to pick up RPM as the load on the engine drops.

To summarize...a car/prop boat is a load that decreases with vehicle/vessel speed and is independant of RPM.

Now let's look at a jet. A jet is a constant load. From a dead stop, you hammer down to WOT and immediately go to max RPM. This max RPM is dependant on the size impeller and how much HP the motor is capable of making. The load placed on the motor is completely independant of the vessel's forward speed...you can tie the boat to a dock and still get your max RPM at WOT. However, the load placed on the engine increases with RPM...the faster you spin the pump, the more water you move through it, and the more it loads the engine.

Now back to the power valve...the reason for this debate is that we're trying to see if we can keep the power valve from opening and just dumping raw fuel through the motor when that extra fuel is not needed. Why burn more gas if you really don't need to?

Now my way of thinking is that on a jet, you would only need the power valve once you hit a certain RPM. At a certain RPM, a certain constant load will be placed on the engine that will cause the vacuum to drop to a certain point where more fuel than what the jets can supply is needed...you would need the power valve to open at that point. The power valve would remain open as long as the engine RPM is held above the RPM at which the power valve opens at. The reason for this is that load increases with engine RPM...which causes vacuum to drop even further...keeping the power valve open until you fall below the RPM at which the power valve opens.

Knowing this...it would make sense to find out first at what RPM the stock power valve opens at. On a stock Holley 750DP 4150 carb, the stock power valve is a 6.5Hg. I would then use a vacuum gauge to find out at what RPM the motor drops to 6.5Hg of vacuum to find the RPM that the power valve opens at. I would then want to check my mixture at that RPM...if it's too rich I would want to change to a power valve that opens at a lower vacuum setting.

What are your thoughts? Am I correct in my way of thinking of how the power valve works in a jet?

« **Reply #1 on:** November 05, 2007, 02:15:38 PM »

If you take the PV out, you will have jet up the carb anywhere from 5-10 jet sizes to make up for it and have the curve correct. Even though the engine sees little to no load going up the RPM from a stop, there are times when you may just put around at part throttle, but high RPM. It is in those cases that you would want the engine to be leaner so you get more out of a tank.

When you hammer it, you want the fuel to make power, and that is when the PV will open and bump the jetting up.

If you were steady state or always idle or WOT like a drag car/boat, then you could ditch the PV and be good.

The PV works on load only, so if you are heavy load (like pulling a skiier out of the water) it will open and keep a lean condition from happening. It really has nothing to due with RPM, so you can't base it on this.

If you want to think of it in car tearms, judge it like a auto with a 5500 or 6K stall. There are still times you are just loafing around, and do not need a fat mixture.

« **Reply #2 on:** November 05, 2007, 05:30:26 PM »

Good answer !!

« **Reply #3 on:** November 05, 2007, 06:38:05 PM »

It's been my experiance from that other site that speedy knows his shit. Isn't the deal take your vac and divide it by 2 and there is your starting point for pv. I'd like to mount a vac gauge permanent preferably right on the motor, but where would be a place to test for vacumn, pcv hose?

« **Reply #4 on:** November 05, 2007, 09:27:45 PM »

Quote from: SLCKREG on November 05, 2007, 06:38:05 PM

It's been my experiance from that other site that speedy knows his shit.

Hmmm I dunno...I'm not exactly too sure about that...

I mean...after all, he is a Mopar guy...

Nah, in all seriousness, that was a great response. I think I finally understand it now.

« **Reply #5 on:** November 06, 2007, 06:54:58 PM »

Thanks Guys :-)

In my new boat config - I have the 2 most important gauges where I can see them.

#1 Vaccum and #2 Oil pressure. Yes. Those two. In that order. I can live without everything else.

The vaccum gauage will tell you the load the engine is under, the condition, the mixture, if a spark plug or valve is going bad, if the rings are OK, if the idle mixture is correct - etc etc etc. The best place for the vac gauge hose connection is in the intake manifold plenum. If you hook it to the PVC connection, it tends to be a hair lower. Not saying it's bad, just lower. In reality, you can hook it to ANYTHING EXCEPT the vacum advance port on the carb - you will quickly learn that you have 0 in HG at idle...

The oil pressure tells me if I need to shut the engine off or when it's time to change the bearings.

Back to the PV
When I am at part throttle, I like my A/F to be around 14.5:1
At full throttle, 12:1.

So, with enrichment on (basically when the vaccum is lower than the rated PV installed), I can jet my main jets to be perfect for full throttle and max HP. When I am at a higher in HG (and a lighter load) that same jet will be effectively smaller, and give me better mileage. Without the PV, the jet size goes up to get the correct A/F at full throttle, but there is nothing to back it off. Works fine if you are sponsored by texaco....

There are things you can play with, std / high flow PV's, primary and secondary PV's. I like my race carbs with dual powervalves, this way I can make it lean for the street, but add 20 jet sizes as soon as I tip into it. The trick is making sure that they do not open it idle, that is why you take your loaded idle reading, and drop it by 2 sizes (in a jet, you can make it like a manual because there isn't a drop in gear).

How I set up my personal 850 DP on my boat - dual 5.5 PV's, 80 primary jets, 83 secondaries, 35 pri squirter , 25 sec squirter (on a car this would be 35-37).

« **Reply #6 on:** November 06, 2007, 07:33:47 PM »

I haven't figured out how to hook up a 02 sensor and a gauge(water injected headers) so i'm flyin in the dark.

« **Reply #7 on:** November 06, 2007, 11:33:05 PM »

Quote from: speedymopars on November 06, 2007, 06:54:58 PM

Thanks Guys :-)

In my new boat config - I have the 2 most important gauges where I can see them.

#1 Vaccum and #2 Oil pressure. Yes. Those two. In that order. I can live without everything else.

The vaccum gauage will tell you the load the engine is under, the condition, the mixture, if a spark plug or valve is going bad, if the rings are OK, if the idle mixture is correct - etc etc etc. The best place for the vac gauge hose connection is in the intake manifold plenum. If you hook it to the PVC connection, it tends to be a hair lower. Not saying it's bad, just lower. In reality, you can hook it to ANYTHING EXCEPT the vacum advance port on the carb - you will quickly learn that you have 0 in HG at idle...

The oil pressure tells me if I need to shut the engine off or when it's time to change the bearings.

Back to the PV
When I am at part throttle, I like my A/F to be around 14.5:1
At full throttle, 12:1.

So, with enrichment on (basically when the vaccum is lower than the rated PV installed), I can jet my main jets to be perfect for full throttle and max HP. When I am at a higher in HG (and a lighter load) that same jet will be effectively smaller, and give me better mileage. Without the PV, the jet size goes up to get the correct A/F at full throttle, but there is nothing to back it off. Works fine if you are sponsored by texaco....

There are things you can play with, std / high flow PV's, primary and secondary PV's. I like my race carbs with dual powervalves, this way I can make it lean for the street, but add 20 jet sizes as soon as I tip into it. The trick is making sure that they do not open it idle, that is why you take your loaded idle reading, and drop it by 2 sizes (in a jet, you can make it like a manual because there isn't a drop in gear).

How I set up my personal 850 DP on my boat - dual 5.5 PV's, 80 primary jets, 83 secondaries, 35 pri squirter , 25 sec squirter (on a car this would be 35-37).

I am droppjng my boat off at your house!!!!

« **Reply #8 on:** November 07, 2007, 09:19:02 AM »

Quote

I am droppjng my boat off at your house!!!!

ya no kidding...line them up...im on the way w/ mine too

« **Reply #9 on:** November 07, 2007, 06:01:40 PM »

Quote from: SLCKREG on November 06, 2007, 07:33:47 PM

I haven't figured out how to hook up a 02 sensor and a gauge(water injected headers) so i'm flyin in the dark.

O2 sensors have only been around for a little while....read the plugs (WOT) and the vac gauge (PT), it will tell you all you need to know. For fine tuning, a GPS for top MPH (jetting) will always help.

The other way is to put an O2 sensor it, and kill off the water to that header when testing - assuming of course you are over transom, and nothing will catch on fire without the water....Once you have your mixture set, you don't need the O2 anymore.

And chromey - I've been to your house, you aren't to far away from me :-)

« **Reply #10 on:** November 07, 2007, 06:30:59 PM »

THANKS SPEEDY I was thinkin a set of dry headers for like a temp tune run with 02's. Also do i need 2 gauges for 2 sensors (left and right?) BTW Anyone have a set of cheap/ disposable headers

« **Reply #11 on:** November 08, 2007, 08:34:41 AM »

Unless you have some kind of individual runner manifold, a single O2 will work. The O2 sits in the header in the collector, so it gets 4 of the cylinders. Assuming you don't have a messed up carb (like one barrel doesn't flow any fuel at all), the results will be the same across the cylinders.

Modern day cars have 2 O2's, because they can adjust each cylinder on the fly and read the result. With a carb, there is no on they fly adjustment, so you get a measurement, change the carb, and go again.

I would use the exhaust you are going to run - the exhaust will change the jetting needs. Tune it for the throw away headers and it will run great - with the throw away headers.

Why can't you just put a valve inline to the header and cut out the water when tuning?

« **Reply #12 on:** November 08, 2007, 09:09:52 AM »

I was told the weep holes in the headers would cause a false lean.

« **Reply #13 on:** November 08, 2007, 12:20:48 PM »

Didn't realize there were weep holes. yes, that would throw it off. Read your plugs :-)

« **Reply #14 on:** November 08, 2007, 02:33:31 PM »

My plugs always look lean to me. I was told with all the spec additives in todays fuel it makes plug reading a little harder. Earlier this season I pulled my headers and a few of the tubes where white, I thought that was too lean, my machineist said perfect. Part of my problem is by pos single tr it burns 5+7 way lean. I've been rejettin all season richer and losing #'s.

« **Reply #15 on:** November 08, 2007, 03:42:39 PM »

Have you tried to stagger jet the secondaries so the barrel over 5/7 is richer?

« **Reply #16 on:** November 08, 2007, 03:51:48 PM »

I'd never thought of that

About 20 min ago i just called jegs an ordered a vic jr.(BY BY BLING) when it says it has nitrous bosses that means the holes are there right?

« **Reply #17 on:** November 08, 2007, 06:07:18 PM »

...nope, ya gotta drill 'em. The boss is just a place to drill 'em

« **Reply #18 on:** May 29, 2008, 08:14:12 AM »

BTW - I ran my boat yesterday - Tunnel ram, twin 450's, 8.5 PV's (looks like I need 7.5's) 3 adults,2 kids coolers with drinks, lunch etc. 50 gal of gas on board, crusing the channel, driving to steamboat, pulling the kids on the raft, idling for long periods of time. Averaged 5.5MPG (GPS based Miles).

There is still a ton of tune left to go also....I haven't optimized the jetting, PV, squirters,or timing.

« **Reply #19 on:** May 31, 2008, 10:38:50 PM »

8.5, 7.5? How much vacuum do you have?

I was running the stock 6.5 PV in my dual Holleys and dropped them down to 3.5 because I felt that they were causing my rich condition because of low vacuum due to the cam.

« **Reply #20 on:** June 02, 2008, 11:27:21 AM »

I have to lower my PV's because when I get off of it after a run until the wake passes, the engine wants to be at around 7 in HG. Which of course tells the carb to bump the jets up 10 sizes, loads the engine, and causes it to stall until the wake passes.

I currently have 8.5's in there now. I am going to swap in some 5.5's, and that should eliminate that issue.

Idle is 10-11 in HG @ 800 RPM, cruise at 3500-4000 is sitting about 10.

« **Reply #21 on:** June 07, 2008, 08:15:00 PM »

an a bad powervalve cause my boat to cut out at my top rev? Like if its stuck closed would it do this? I am dumbfounded

« **Reply #22 on:** June 08, 2008, 09:21:10 PM »

Quote from: countryboyaustin on June 07, 2008, 08:15:00 PM

an a bad powervalve cause my boat to cut out at my top rev? Like if its stuck closed would it do this? I am dumbfounded

Usually the PV's go bad from a tear resulting from a backfire. This causes gas to flow directly into the plenum from the float bowl bypassing the carb. At that point, the thing usually won't idle as it's choking on excessive fuel (or if it's small, it just runs rich), but at upper RPM's it only adds a very slight percentage to the fuel ratio.

If it's stuck closed, then at the the upper RPM you are running very lean, so it will kill HP and possibly lead to pinging / preignition and a definate HP loss.

« **Reply #23 on:** June 09, 2008, 11:20:46 AM »

I am having some issues with my 4150 750 DP. The engine is on a stand, so I can't test under load yet. The carb has a 6.5 pv in the primary, no pv in the rear. The engine is pulling 10in hg at 900 rpm. The carb has 73's primary and 81's in the rear with 30 cc acc pump.

When the engine is at idle and I roll the throttle it sounds fine, but if it is at idle and I stab it, it falls on its face and then back fires.

Any suggestions as to what is going on and what to possibly do??

Thanks

« **Reply #24 on:** June 09, 2008, 01:18:12 PM »

Quote from: KonaCole on June 09, 2008, 11:20:46 AM

I am having some issues with my 4150 750 DP. The engine is on a stand, so I can't test under load yet. The carb has a 6.5 pv in the primary, no pv in the rear. The engine is pulling 10in hg at 900 rpm. The carb has 73's primary and 81's in the rear with 30 cc acc pump.

When the engine is at idle and I roll the throttle it sounds fine, but if it is at idle and I stab it, it falls on its face and then back fires.

Any suggestions as to what is going on and what to possibly do??

Thanks

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