Holley carb. gurus needed!

[Dark Knight]

BTW, whats the idle vac. and what power valve do you have?

[82 GT]

Idle vacuum is between 14-15hg(Park) and I'm running a 6.5 power valve.
I'm beginning to wonder if maybe my PV is closing before the end of the run and shutting off the power circuit of the carb.
I have a vacuum gauge I'm going to hook up so I can watch it during a pass down the track.

[Rev]

Originally Posted by 82 GT

Idle vacuum is between 14-15hg(Park) and I'm running a 6.5 power valve.
I'm beginning to wonder if maybe my PV is closing before the end of the run and shutting off the power circuit of the carb.
I have a vacuum gauge I'm going to hook up so I can watch it during a pass down the track.

Remember that it's when the manifold vacuum is below the rating that the power valve is open. At WOT, the vacuum is right at 0 or a hair more. No matter what pwer valve # you use, it will always be open at WOT.

Rev

[~The Jester~]

There is absolutely NO WAY I would start playing with transfer slots, and drilling holes in the butterflies until I got the idle screw issue figured out.

I'm with Brian on this one, that motor isn't going to want to idle much below 950 rpm at best, if that's where she's happy, then that's where she needs to be. You may however find that changes once you get your issue figured out.

Exactly which carb are we talking about? Some models, that screw adjusts the air/fuel mixture for the idle circuit, on other it does not. TRUST ME, read on.

On MY carb, those screws did squat for air/fuel ratio. That is determined internally, all those screws did was adjust the AMOUNT of air/fuel that got fed through the idle circuit. Yes, I learned that in the book too. heehehe

Brian, we need to make you a "book on tape" so you can listen to it on your way to work! hehehehehe

[82 GT]

Originally Posted by Rev

Remember that it's when the manifold vacuum is below the rating that the power valve is open. At WOT, the vacuum is right at 0 or a hair more. No matter what pwer valve # you use, it will always be open at WOT.

Rev

Not exactly true. Manifold vacuum WILL raise as rpms increase. I DID read that in my Holley book....LOL
According to the book, you're supposed to find a PV rating that will remain closed at idle yet high enough to stay open during acceleration periods(drag racing)
My Holley is a 750dp(4150).
Only a vacuum gauge will tell you exactly what PV you really need without the "guess" work.

[Rev]

Originally Posted by 82 GT

Not exactly true. Manifold vacuum WILL raise as rpms increase. I DID read that in my Holley book....LOL

Yup, it's true. Reread the book or just check what happens with a gauge when you stand on it. The vacuum will fall to near 0 and stay there while at WOT. If it doesn't, it means you're using a carb that's too restrictive.

Rev

[Rev]

Originally Posted by ~The Jester~

On MY carb, those screws did squat for air/fuel ratio. That is determined internally, all those screws did was adjust the AMOUNT of air/fuel that got fed through the idle circuit. Yes, I learned that in the book too. heehehe

That's the way it is on my Road Demon also. You can make that air/fuel mixture leaner by opening up the low speed air bleeds slightly. Some carbs have removable air bleeds allowing easy changes. That will require also opening the idle mixture screws a little allowing a little more of that leaned out mixture through the idle circuit. This assumes that the idle air/fuel mixture is a little too rich to begin with (as was my case).

Rev

[82 GT]

Originally Posted by ~The Jester~

There is absolutely NO WAY I would start playing with transfer slots, and drilling holes in the butterflies until I got the idle screw issue figured out.

Well, the book says if your idle/air screws don't work then that's what you have to do in order for them to work again.
It works with them all the way in. I know it's not supposed to be that way but unless it's hurting performance, I'll just keep them there because I KNOW if I start screwing around with this carb and doing things I'm unsure about then I'll end up with a nice paper weight and buying ANOTHER carb.
I'm lucky I was able to swap jets and have it still function afterwards.

[Dark Knight]

How much timing are you running? Advancing the initial timing will increase idle speed and allow you to back off the idle speed screw on the carb.

[82 GT]

I have 36* total timing but 1000rpms is 1000rpms no matter how you arrive at it. If the engine wants to idle at 1K, it's not going to care if you got it by advancing the initial or turning the idle speed screw.

[Dark Knight]

You need to read the holley book... It DOES matter. If the throttle blades are opened up enough to be into the main or transition circuit, then the idle mixture screws wont do crap. If you can keep the idle speed up with a timing change and lower the idle speed screw so that the blades arent open as far, the idle mixture screws will work better.

Maybe you should take it to someone and have THEM tune it

[82 GT]

Originally Posted by Dark Knight

Maybe you should take it to someone and have THEM tune it

Yep, just as predicted....the sarcasim trian has arrived! All aboard!

[Dark_5.0]

LMAO!!!

And people say they run carbs cause they like things simple.

[Rev]

Carbs are pretty simple if you take them circuit by circuit and undertstand how a change in one circuit might affect the other circuits. They all have to work together for the engine to run well.

Rev