There are several charts out there showing pump cam lift curves. AFAIK, the most aggressive cam is the BLUE cam and I think it’s got the number 427 on it, but I’d have to go out and look to say for sure. But it’s definitely blue.

I’m not saying your combo doesn’t need a 45 squirter. It very well may. I tuned a front engine dragster a long time ago and even though it was light, it didn’t have the correct converter so it took a squirter in the high 50’s. It was a foot brake car, and you couldn’t get a big enough nozzle on it.

I your case, I’m still suggesting to get a vacuum gauge on it, and see where you are for manifold vacuum at a cruise. Your 8.5 power valve is better than what most use, but there are several things I’ve learned lately.

One is it’s a bummer you can’t buy a 12.5 power valve any more. Most guys could use it. I know I could. Another is when you test the power valve, the vast majority of them open LATER than what is stamped on them. I tested an 8.5 that didn’t start opening until 6 inches of vacuum. That can definitely affect your tune.

As for the correct method of setting power valve opening, Holley, most authors and the vast majority of end users, because of the aforementioned people, have been incorrectly taught and have over the decades been doing it incorrectly. In fact, the vast majority of end users (and evidently Holley because they still teach it) have no real idea how the power valve (which is really an economizer valve, but that doesn’t market well) circuit functions.

When you are at a cruise, you are running on the primary main jet. Holley (and most very other company producing Holley clone carbs) sends the carb out with very rich jetting. So at a cruise, they are just slobbery rich, they get horrible fuel mileage, kill plugs and are just generally miserable to drive. That’s why many guys won’t use a Holley. Horrible fuel mileage, hideous rich idle and all that. That’s where the power valve comes in.

At your CRUISE, the power valve is shut (or should be) and all the fuel is coming from the primary main jet. And it’s most likely rich. Then, you accelerate and the power valve OPENS and ADDS fuel, and you are now pig rich, wasting fuel and most certainly giving up driveability. The amount of fuel added by the power valve circuit depends on the Power Valve Channel Restriction holes, which are the small holes you see when you remover the power valve. I forgot to mention that as you accelerate, you also add pump shot too, which is more fuel added to an already rich condition.

So, the correct way to tune the carb is to keep reducing the primary main jet until, at a cruise you get a SLIGHT lean stumble or surge. At that point you add a couple of sizes to the primary main jet and move to tuning the power valve, keeping in mind you’ll never touch the primary main jets again, unless you are tuning cruise fuel issues.

The two tuning options you have for the power valve circuit are WHEN it opens, and how much fuel it flows when open. That’s why cruise vacuum is how you set the opening. The later you open it, the later the engine gets the added fuel it needs because of the added demand by the engine. So you need to get the opening time as close to cruise vacuum as you can. Certainly you don’t want the power valve open at a cruise, but it needs to open quicker than what Holley has guys doing. My engine has about 9 inches of idle vacuum so by the de facto setting, I’d need a 4.5 power valve. I have about 15 inches at a cruise. So I use a 10.5 and would use a 12.5 if I could get one.

That later opening requires a bigger pump shot to cover up the lack of fuel because the power valve is opening so late. Running more nozzle than you need just wastes fuel. If you get the power valve/main jet set correctly they run cleaner and make more power.

So now you have the power valve opening at the right time, you have the primary main jet set for best clean cruise but now at WOT you are lean. The first thing guys want to do is grab a screw driver and ADD main jet to the primary side and maybe add some main jet to the secondary side and that is DEAD WRONG. You have now made your cruise air/fuel ratio much richer than it needs to be.

When tuning for WOT after you have the primary main jet correct for cruise and you have the power valve opening as correct as you can get it, you adjust WOT air/fuel ratio by changing the size of the power valve channel restriction (from here on I’ll type out PVCR so I save some strokes). On the “better” carbs, the PVCR’s are threaded holes and either brass set screws or little 6/32 threaded jets are installed in the PVCR holes so you can easily adjust them.

Of your carb doesn’t have the holes threaded for brass, you can get some 6/32 x 3/16 brass set screws from McMaster/Carr. A box of 50 is about 5 bucks. You'll need the correct 6/32 tap and drill, a pin vise and a set of 80-61 number drills and 60-1 number drills. Any decent hardware store will have the 80-61 drill set and pin vise and Harbor Freight has the 60-1 drill set for about 20 bucks or less IIRC.

Then you just drill the PVCR holes to the correct number drill size (I forget what it is off the top of my head) and then thread the holes just enough so the set screw is flush or just below flush and you’re done with that. Now it’s a simple task to drill some brass and tune the PV circuit.

Now at WOT on the primary side you either make the PVCR larger to add fuel or smaller to reduce fuel, never touching the primary main jet. That way, you still have a clean, crisp cruise and the correct amount of fuel at WOT.

The next question is how do you know when to add WOT fuel to the primary side and when to add it to the secondary side. The best answer I have is to disconnect the link from the primary to secondary throttle shaft, wire the secondary throttle shaft SHUT and only tune in the primary side until you get it nailed. Then hook the secondaries back up and tune for WOT only using the secondary main jets. You don’t touch the primary main jet or the PVCR’s, because you already have it nailed.

All this takes time, but the payoff in the end is well worth it. Of course, like everything else Holley, once you get that dialed in, you can start tuning the idle circuit to clean that up. Tuning the idle circuit requires some of the same work...you drill out the idle feed restriction and tap it for a 6/32 set screw,but that’s a discussion for another thread.


Hope this helps clear up some of the bad information out there on how to tune the main jet/power valve circuit on a holley carb. It’s harder to type out than it is to do.