Holley Carb - Hesitating and Running Rich
- Thread starter Shaggin' Wagon
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Guys, a power valve has nothing to do with idle. Even if it's blown, the idle circuit restrictions will not allow all that fuel to enter the intake.
How it works:
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.
Problems:
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle.
That is from Holley's website. https://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.
Problems:
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle.
That is from Holley's website. https://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning
Greetings Guys; I stand corrected!
But if the idle transfer slots are uncovered by having the throttle plates open (idle speed screw) too far the main circuit starts to feed fuel. Having the plates open (idle speed screw) too far is a common problem with LOTS of Cam & many guys end up idling with the main circuit starting to engage. And if that's the case a blown power valve would affect the fuel going into the engine at idle. I have recently learned that the optimum position for the throttle plates (at idle), primary side is with the minimum exposure of the transfer slot. This helps light throttle transition when you move from a dead stop. So you ask how do I get my idle speed up? By adjusting the secondary plates to desired idle speed. Find secondary idle speed screw under base by secondary throttle shaft, passenger side. Anyway I noticed that OP hasn't even chimed back in, so this conversation is just expanding our education. Thank you Ribbedroof for correcting me, as I wasn't elaborate enough in my explanation. Ole Bob.
Sounds like an expensive way to prove something I already know works. All of my 350s have had a 750dp on top. If you want to buy me one I'd be happy to tell you.
More over, if you want to fund a way to set up two identical cars and engines, with different carbs, you choose 625-650cfm in whatever configuration you think is good and I'll pick a 750 DP, I'll run the same efficiency as you but faster.
I have rebuilt carbs for years and the last one I rebuilt before I installed the wideband I thought the carb was perfect. Once installing the wideband I was surprised what the gauge told me. The wideband has let me know where tweaking was needed and that has improved the performance and saved me gas money.
As for the size of the carb a quadrajet is limited to what the engine demands, a holley provides the amount of fuel based on the size of the carb. The fuel requirements of a 355 does not depend on the compression ratio, it depends on the cubic inch of the engine and the max rpm. For a street use the carb for a 355 with a max rpm of 6000 is under 525 cfm, for racing the recommendation is under 675 rpm. Too much fuel will negatively affect performance. In our sportsman circle track car we used the GM cate 604 engine which is a 350 roller engine, the mandated carb we had to run was a "Holley 500 c.f.m. part # 0-4412. No modifications. Jets and power valve may be changed.
Accelerator pump discharge nozzles may be changed utilizing straight type only." The engine was chipped at 6200 rpm and that engine never ran out of gas.
I don't need to run 2 cars to see what is quicker, that has already been done many times and the engine with the proper cfm carb wins over a holley carb too big for the engine.
I think vanrah hit it on the head. The absolute first thing to do is check the transfer slots.
Also, keep in mind that a vacuum leak will create an artificial need for more fuel.
Also, keep in mind that a vacuum leak will create an artificial need for more fuel.
Bob, you are absolutely right about transfer slots as well as the byproducts of an improperly adjusted Holley. Much Holley tuning is beyond the realm or knowledge base of many people who do not understand the carburetor's design.
My post was simply a reminder to people that the power valve is not the cause of every Holley tuning problem, despite common legend that it is responsible for every ailment. We could burn up a lot of bandwidth on Holley tuning alone.
My post was simply a reminder to people that the power valve is not the cause of every Holley tuning problem, despite common legend that it is responsible for every ailment. We could burn up a lot of bandwidth on Holley tuning alone.
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