Hello,

I read the Holley tuning sticky but I have a question regarding the power valve tuning. After I decrease the primary jet size 3 sizes ant a time and get the car to surge while cruising do I go up 2 jet sizes at that point? The sticky says to increase the power valve until the flat spot is gone but I’m not sure what that means because once I find the point to where it surges and then go up 2 sizes then it shouldn’t be flat I would assume. I have a 8.5 in the carb currently. Thanks.

 

Yes, at that point go up 2 jet sizes. If you don't have a flat spot then don't worry about the power valve.

 

I didn't exactly "write" that; it's an aggregation of pieces of various posts I had posted previously, put together by one of the mods. As such the wording is a bit garbled.

Tuning a carb is a bit like working out at the gym. You do a particular exercise to "isolate" a muscle or a group of them, to work on those specifically; even if the motion you perform to do this, isn't natural or correct or easy. With a carb you "isolate" each of the subsytems in the carb and work on them one by one, in the order they affect each other in. You start with the one that affects ALL others (fuel level), then the next one that affects ALL the rest (jets), then the next one that affects only a couple of things (PV), then the fine setting that will get disturbed by ALL the others on the primary side (transition slot), then the last one of all (secondaries).

It says, lean the jets out until you find the size where it starts to lean surge, then go back up 2 sizes. Ignore any and all other misbehavior of any and all kinds; it will probably not really drive very well at this point.

Once you get it to that point it will probably have a lllloooonnnnggggg flat spot under cruise conditions when it's idling (foot off the gas) and you very slowly give it throttle. Then and only then, work on the PV; keep increasing its "number" (the vacuum above which it's closed) until the flat spot that will probably be happening on throttle tip-in goes away.

The goal is, get the jets to where the engine runs at stoichiometric (14.7:1 air:fuel ratio), more or less, when cruising, but then begins to richen up when the vacuum drops, which is a consequence of opening the throttles. Without the PV opening up soon enough you get the flat spot. What you DO NOT want is the way they usually come "out of the box": typical factory Holley calibration is WWWWAAAAAAAYYYYYYY rich at cruise and idle (too much jet), and the PV doesn't open until your foot is almost on the floor (PV # too low). Most of them come stock with a 65 PV. That number is the vacuum below which they open (richen the mixture by adding fuel to what's being fed through the jets) and above which they're closed (running on jets only), without the decimal point. If you ever drive around with a vac gauge, you'll see just how ridiculously far you have to open the throttle to get it down to 6.5" of vacuum, or 8.5" for that matter. The less cam your car has, the more ridiculous it becomes. A typical one of these cars with the stock cam will do around 15 - 18" of vacuum cruising, which is why you'll often end up needing a 125 (12.5") PV to match it. When you lean the jets down properly, the carb needs the PV to open AS SOON AS there is ANY power demand at all to speak of.

If the transition slot is REALLY messed up, you may find yourself having to go back and re-do the jets after you get that right. Reason for that is, if too much of that is exposed, it will be feeding gas 100% of the time, at idle; and the passage will be getting sucked dry just idling. There won't be any fuel sitting in the passage waiting for the need to make the "transition" from idle to acceleration. Having too much slot showing creates the flat spot you get from a stop (which leaning down the jets can make ALOT worse until you get everything else tuned in afterwards). That's ANOTHER reason why Holley sets them up that way: if they try to more nearly optimize the jet size (smaller i.e. leaner), it'll be too lean at that other time, so having the stuuupid fat jets in there covers up yet another n00b tuning shortcoming.

 

Holley's rule-of-thumb for power valves is to install one rated at half of the vacuum reading at idle (in gear if automatic transmission).
My F*rd idles at 11 inches of vacuum in gear, so I run a power valve rated at 5.5 .
Supposedly, they are jetted from the factory for sea level (of course they have no idea what size engine the carb will be installed on) and their rule-of-thumb is to decrease 1 jet size for every 2,000 feet above sea level...

 

Right... that's exactly what they say... but it works ONLY if the jets are already about 5 sizes too large. IOW, it's WRONG. It's part of the reason why Holleys have a reputation for drinking gas like a broken hose. If the jets are set up so that the engine runs at or near stoich at cruise, a PV at half idle vacuum will produce the LLLLLLLLLOOOOOOOOOOOONNNNNNNNNNGGGGGGGGG flat spot we all know and love when stepping on the gas. The mixture on the jets alone has to be at least as low as 13½:1, if not lower, to avoid the flat spot if using a PV set that low. And since the jets also affect the idle mixture in a Holley, excessively large jets also contribute to the stinky fat blubbering burn-your-eyes smoky idle a Holley often gives.

The weird thing about it is, there's no real reason why a Holley can't get pretty decent mileage, in most cases. Even what looks like a terrible mismatch on paper, such as a 780 CFM 3310 on a 327 (yes that was the GM FACTORY choice) can be tuned to work well. But their tuning advice is GUARANTEED to make their products a gas-sucking pig. I've never figured out why they tell people to do such bizarrely inappropriate things. You'd think they'd know their own product better than that.

 

Ok, so I’m at the point where I have the transfer slot exposure set and I closed the primary idle speed screw and opened the secondary idle speed screw to keep the same idle speed. I leaned the primary main jets until it surged in 4th gear going up a slight hill and cruising on the highway. I went up 2 jet sizes. So I don’t really feel a flat spot but I’m just trying to see where it will be. My cam rpm range starts at 2500 so should I be in the rpm range and give it throttle to determine if it has a flat spot because under the rpm range it isn’t that responsive regardless especially in high gear. When you increase power valves do you go up one size at a time or do I go to a 10.5 since I’m at a 8.5 currently? Thank you for all of your help by the way.

 

Try going up a step and see what happens. Your current cruising vacuum would be a clue: if it's much above 8.5" you'll probably feel a significant improvement in responsiveness.

Out of curiosity, how far did you have to change the jets to get the mixture to stoich (beginning of noticeable surge)?

 

6 steps down from the original 74 primary jets. So I’m using 70’s now.

 

That's fairly typical. Let us know before & after gas mileage and dyno numbers, if they're available.

Noticeable surging usually starts somewhere around 15.2 to 15.5:1.

For a perspective, from 74s to 70s is about a 12% reduction in cross-sectional area, and somewhat more than that in fuel, since the center of the jet flows better than where the fuel is up against its "walls" (a reduction in CSA cause somewhat more than a proportional reduction in fuel flow); so, if you're at or near stoich now, you were at about 13:1 before, or worse. That's right near the usual "max power enrichment" mixture that most engines like, which is usually around 12.7:1.

This is what's wrong with Holley's out-of-the-box tunes and their "advice" in a nutshell. They put such YYYYUUUUUUUUUUUUUUJJJJJJJJJJJJJE jets in them that the PV almost isn't needed at all. I don't know why they do what they do, but it's bats*** crazy.

 

Yeah I can’t wait to take it back out but the weather this week isn’t looking good. I’ll definitely post the results when I get it together.

 

I should also mention at some point, you'll probably want to advance the timing; probably ALOT. One of the consequences of running too rich is a tendency to ping, which is nearly always the limit to how much timing you can run, even before it reaches optimum.

Best way to go about that is to put down "light", forget about "mark" and "tab", close "book" on "spec", and just SEE WHAT THE ENGINE WANTS. To do that, simply advance it until it starts to ping, then back it off until it quits. Probably won't be "right" like that, but should be somewhat close, and maybe give you an idea how far off it used to be. Tweeeek it from there until it's the best it can be. The process looks like:

Make an adjustment.
Is it better or worse?
Better: adjust it a little more the same direction.
Worse: adjust it back to where it was, and a little bit more.

Better or worse?
Better: adjust it a little more the same direction.
Worse: adjust it back to where it was, and a little bit more.

And so on, until ANY adjustment you make in ANY direction of ANY amount, makes it worse. When you arrive there, you know it is now THE BEST IT CAN BE. Lock it down and move on to something else.

Then if you're REALLY addicted to "light" and "mark" and all that, go see what all that says AFTER the ENGINE has had its say, so you can put it back if it's ever disturbed from there. THE ENGINE does not whup out "light" and "book" and compare "mark" and "tab" to all that, and decide how it's going to run; all it cares about, is when the spark occurs relative to piston motion. Never forget a piece of wisdom that came from one of the GOATs in a totally unrelated field, which if he had been working on a car at the moment he would have said, "If it RUNS good, it IS good."

Not the least reasons for this include, "mark" and "tab" are NOTORIOUSLY inaccurate; and "spec" in "book" was developed by the factory for a factory motor, with the priorities being emissions, ability to run on any crappy 198x fuel they happened to need to, emissions, CAFE, emissions, altitude tolerance, emissions, tolerance for every climate condition that ever occurs from northern Manitoba to Death Valley, emissions, ... oh and in case I forgot to mention, the SINGLE MOST IMPORTANT FACTOR OF ALL was, EMISSIONS. If none of those factors are important to you, or if your motor has changed (which of course it obviously has, if in no other way than having a Holley on it), then YOU get to decide how to set it so it runs the best TO YOU, the way YOU drive it, in the place YOU drive it in, on the fuel YOU can get, with the priorities YOU find important. "Spec" becomes irrelevant. The wrinkles in the sheet metal, as in "spec for a Camaro", are even less relevant.

You'll most likely find that changing the timing will also affect the transition slot setting. Maybe drastically. You'll almost certainly have to go back and re-do that.

 

Something I find interesting is that decades ago, Holley's rule-of-thumb was to install a power valve rated at 2 inches below the idle vacuum, but sometime in the 21st century, they changed their recommendation to "half".
My carb runs great on my SBF. No flat spots or anything. But I sometimes wonder if I'm leaving some power on the table with the current tune. I've never used an AFR tester to dial it in...

 

Wow, I never knew they changed their information like that. It’s weird that they’re providing this weird tuning information because it’s almost like they don’t know their own product.

 

Sorry for the delayed response but I forgot to respond back. I ended up using 70 primary jets and 80 secondary. 8.5” power valve. I originally had a 4.5” valve which was completely wrong. In 4th with 3.36 gears and cruising at 60 I’m at about 2500 rpm which is the start of my cams rpm range. I have a pretty healthy cam and if I drop to 3rd and floor it it’ll respond but if I floor it in 4th at that rpm it’ll take a second to start to really pull but it won’t stumble or surge like its lean. If I slowly give it throttle let’s say to pass someone it doesn’t stumble so I figured the power valve was good. If I floor it at 3,000 rpm in 4th it’ll take off so I know it’s just because of the gearing. With the 4.5 it was more of a bog perhaps. I didn’t try the 10.5. I kept increasing power valve sizes until the engine stopped stumbling on tip in at cruise and called it a day.

My other engine liked the 8.5 as well but could maybe use a 10.5 in the fall time when the temperature really changes and it gets colder. I would get a slight stumble when trying to accelerate at cruise but it only did it one day and that was the last car show of the year. I don’t drive the car in the winter so it’s fine for the spring/summer and I left it alone for that reason.