Hi all, just a simple carb question to the Holley experts out there. Fine tuning my Holley HP double pumper carb (4150 with mechanical secondaries and secondary metering block) and I noticed the secondary metering block has a power valve shaped plug where a power valve would normally be if it were a primary metering block. I’ve always ***umed this plug was something Holley used so they could utilize the same metering block on both ends for simplicity but recently, I’ve found threads and YouTube videos where folks comment about dialing in the secondary power valve. So my question is what advantage there would be to having a secondary power valve. By the time the secondaries are coming into play, your foot is deep into the carb (if not WOT) and the engine is under heavy load. So why not dial in the secondary main jets to achieve the proper a/f mixture in this condition? It seems like having a secondary power valve is not only redundant but also potentially harmful if it is opening and closing during near WOT and likely changing the a/f mixture dramatically. I bought my Holley new so I know parts haven’t been switched on it. And I would never have considered replacing the secondary PV plug with a functional PV yet apparently some do so I’d like to learn more. No, there’s nothing wrong with my carb and there’s no bad symptoms to report. I’m just being curious, trying to learn more.
The secondary side likely has much larger jet settings negating the need for additional enrichment once all the of the ****erflies are open. I would leave it alone
Oh, I plan to leave it be. Just trying to understand why some seem to use it. And you’re right that my secondary jets are 7 sizes larger than my primaries.
In the same way that the primary side needs extra fuel on acceleration the secondary works the same way , some years back folks who had problems tuning the secondary circuit would often plug the PV & go 4-6 sizes up on jets to compensate , that negated somme of the ability to tune fuel requirements under anything but WOT , drag racing , thats ok , street driver or road course , not so much .
That sounds reasonable but isn’t the secondary system really only being used under heavy load? Like beyond what the primary side is providing through its main jets and power valve? And if so, the secondary jets are already several sizes bigger than the primary jets so what would be the point? I guess I’m just missing something. The primary power valve makes perfect sense to me. The main jets need to be a certain size so you can cruise without an over rich condition and when load increases slightly, the power valve opens to provide a certain amount of enrichment. But to me, it seems like you are never driving around with your secondaries open unless you’re under extreme load, like p***ing someone or trying to win the stoplight race. And if the secondaries are only open in rare cir***stances for short durations, then it seems like you’d jet the secondaries where the engine wants and not confuse things by dumping extra fuel with a second power valve. Holley sold me a carb with a secondary metering block that also had its secondary PV port blocked so I guess they had a reason. Perhaps they chose to make it a removable plug for specific racing situations.
I think your initial ****ysis was spot on. IE Holley did it to reduce parts count. There is absolutely no need for a power valve on the secondaries. The internet is flooded with misinformation about every topic under the sun and of course hot rodding and auto mechanics has a long history of nonsense and conspiracy theory like the suppressed 150 MPG carburetor and miracle of the toilet paper oil filter.
OK, instead of getting snarky about it lets try to apply a little logic. Many old carbs did not have a power valve or needles. They were simply jetted for load and the cars ran fine at the expense of some fuel consumption. The real purpose of a power valve is not to increase enrichment for load but to rather to lean it out under low load cruise conditions to improve mileage. The secondaries are never open in cruise conditions and can simply be jetted for power.
That’s kind of how I see it. Secondary jetting is set up for raw power and only tapped into for that reason. It’s just I saw information about it and thought I might be missing something.
Are you sure they just don’t use the same metering block for primary and secondary? That would be the most logical explanation for me.
Front & rear metering blocks are different , different p***ages & metering control systems . Take a few dozen apart , buy some books & learn .
I'm just curious if the secondary metering block has idle mixture screws.... How's about some pictures of the carb and also let us know what the List# is on the choke horn if it has one..... Thanks!..
I can only add some observations from refurbing the original Holley carb on my 57 T-Bird. This is a factory original fitment and the first year of the 4150 type for use on a Ford. It has a secondary metering block and has a power valve in place in the secondary. The secondary jets are smaller than the primaries. 55/48. This is a vacuum secondary carb. There are no idle screws on the secondary block. The casting is the same but they are not drilled. This is on a stock 57 312 Y Block. I have shown the parts in recent videos. I can offer no opinions on what is the best setup but this is what was optimised by Ford and Holley Engineers in 1957.
My secondary block is not drilled and tapped . My carb has 4 circuit mixture screws . There is no need for power valve on secondary side . The actual name and design of the power valve is “ economizer valve “ , not much economy when you drop the hammer .
Normally don't run power valves on my drag car carburetors but did find it was necessary when tuning this Quick Fuel 1050 body ***embled with Quick Fuel metering blocks. During test runs the air fuel ratio gauge had showed lean and never changed even with 95 jets in all 4 corners, seems that the metering blocks jet p***ages could not feed these larger jet sizes Installing power valves front and rear makes up the extra fuel flow allowed to reduce jet size needed in the metering blocks for a good mix.
I have a Holley 800 double pumper that I was thinking of converting too annular venturi boosters..... These boosters were from an old Barry Grant parts book... I'm not sure if Holley still has those in their inventory....
I didn’t check the numbers when I had it apart. They look similar but I didn’t examine the various p***age ways. The carb is a 750 Holley Street HP with mechanical secondaries, double pumper, 4150 with secondary metering block and four corner idle screws, no choke horn. I bought it brand new so no changes other than my own. The secondary metering block is drilled and threaded for a secondary power valve and there’s a PV looking plug in there from the factory.
Interesting! I was wondering if certain racing situations might demand PVs on both sides to help dump more fuel under wot. That I can see since there’s a limit to jet sizes. Out of curiosity, was it also necessary to change the PV restrictors to larger ones to help funnel the large volume of fuel?
Didn't need to change the power valve restrictors, ran them as they came in the metering blocks, ended up putting the 850 cfm back on because it performed better on that 434 small block chevy engine package. But I have successfully run 750 cfm carbs with power valve holes plugged by running 5-8 larger jet sizes
you can buy various booster designs and swap them out of your Holley using parts from Allstate Carburetor Company. I recently built an annular booster Holley 4160 to mess around with: Regarding the secondary power valve- I think it just gives you another tuning feature if so desired. There are lots of ways around a carburetor, usually with trade-offs attached. I could see the secondary power valve come into play when you start messing around with power adders.
I have a couple of those plugs also.... I'm just not sure which one of the double pumper has those.... I know my 850 center squirter (List-4223) has both power valves in them....
Usually the only place you see rear power valves anymore is circle track or road race calibrated mechanical secondary carbs where you might be 75% throttle in a corner and not need full enrichment. Secondary's are still flowing, but not full WOT. Little to no effect on a street/strip car or hot rod cruiser. Otherwise it just some of the vintage Holley's that will still have secondary power valves.
If you're running a direct rather than progressive linkage for the secondaries I can see a benefit of the secondary PV
Interesting you should mention this. I have a Holley 950 HP that runs a bit lean at WOT and it never richens up with jet changes. It also has the rear PV plugged off. It came with both from the factory so I’m going to put the rear back in and retest. I’m up to 92’s and they came with 79’s all around.
The reason you would have a secondary power valve would be if you had 4 corner idle circuit. Some Holleys have 4 corner tuning and those would use the secondary power valve. Same metering block on those carbs as the double pumper.
My Holley is a street HP with four corner idle screws. No choke, double pumper and mechanical secondaries. A performance carb I bought brand new but it did not come with a power valve in the secondary metering block. Just a plug. What you describe might hold true on some of their racing carbs though but wasn’t the case on mine.
I have two sets of boost referenced Holleys, a pair of 750's and a pair of 950's, both 4 corner idle, 750's have no provision for rear power valves but the 950's have a screw in plug. I also have regular 750's with 4 corner that had rears and others that didn't, in this example the rear had them but the front carb didn't, so I blocked the rear on the rear carb to match.
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