When to drill holes in the throttle plates?

[padam]

Another carb / cam issue I dealt with back then was my vacuum at idle was low enough that the power valve would open, of course causing an over rich condition.
I changed out for the proper valve.

You need a vacuum gauge to check this, can't remember if you said you were using one.

 

[cj's mopar]

Lots of good information here and many different ideas.
I am not 100percent saying this carb was perfect for the application.
More like over carbed slightly.
The fuel requirement is not what the carb is providing i am sure. But I have had the vacuum gauge on it and it was pulling 13 hg .
And will not idle smooth below 1100 rpm.
What I know is if it had a vacuum leak then why would creatng another one give me more adjustment of the mixture screws.
This eng could need more timing but it is all a process. This why some people s cars run better than others. Also why I have not drill the plate years ago wanted to be sure.
It's always this brand sucks and avoid that brand. .
Or do this or that . I am going to try drilling and see if it makes a improvement.
It all personal experience with the dog your working on.

 

[SlinktRR]

yeah even when I had a stock 383 with a "big" purple cam pulling 5" Hg vac I didnt drill the throttle plates. Had to run a decent idle speed though, like 1100, which sucked. I now run a 499 stroker and have had both an edel AVS 800 and a 950 holley DP on it, both run 13" vac at 900 idle rpm, 700 in gear. With that vacuum you should be able to run similar RPM with a nice 4 corner idle modern 4150/4160. I think there is something wrong with the idle circuit on the carb, I would take it off and clean the thing out completely/rebuild.

 

[Bee1971]

383 - Auto

Same Comp Cam XE275 HL - Same Heads - 16 initial / 34 total

AVS - 2 800 cfm - Same 13" vacuum at idle , bounces around a little

Idle 650-700 RPMs all day long - Would idle lower if I wanted

 

[AR67GTX]

Here’s another trick for a Holley if running a big cam and even after drilling the primary plates, the idle mixture response is not as good as you think it should. Reducing the idle bleeds in X-section area increases the vacuum signal strength and improves mixture adjustment response. I didn’t dream this up myself - I have an old 66 Corvette with a 256/266 .579/.600 cam, 12:1 compression, idle vacuum of 8” and several L-88 parts with a 427/425 Holley on it. A Holley guru on the Corvette forums suggested it. I found some wire in my work benches that after calculation was 10% of the bleed office area. So inserting a length of this wire down the bleed reduced it by 10% in area. Of course drilling and tapping the carb body to accept different office bleeds is a better approach but not always one novices warm up to. In any event I did notice an improvement in the idle quality and mixture adjustment response.

 

[cj's mopar]

How much compression are you running ?
I am at 10.2 if I remember correctly.

 

[Wietse]

Instead of drilling holes in the butterfly valves, use a vacuum port.
Install a short section of hose with a plastic or steel plug and create a small orifice by drilling a small hole through the plug.
You can increase the hole size until you are happy with the idle quality.
Or get a Wagner PCV valve and set it up to fixed orifice mode, you can adjust the air flow with an allen key.

 

[kbfallon]

Lots of good information here and many different ideas.
I am not 100percent saying this carb was perfect for the application.
More like over carbed slightly.
The fuel requirement is not what the carb is providing i am sure. But I have had the vacuum gauge on it and it was pulling 13 hg .
And will not idle smooth below 1100 rpm.
What I know is if it had a vacuum leak then why would creatng another one give me more adjustment of the mixture screws.
This eng could need more timing but it is all a process. This why some people s cars run better than others. Also why I have not drill the plate years ago wanted to be sure.
It's always this brand sucks and avoid that brand. .
Or do this or that . I am going to try drilling and see if it makes a improvement.
It all personal experience with the dog your working on.

I would suggest dropping the Holley Tech Dept. an e-mail and giving them your detailed info and they will give you some good advice on what route to take....picking up one of their books on setting/modifying carbs up and making it run as you would like it too. Don't be afraid to try things ....that's how you learn. As far as the timing, I've had some that I've advanced the timing until it just started to ping or the starter would grunt a bit and then back it off until it stopped....old timers called it road timing it.

 

[Geoff 2]

When cam duration/overlap increases, idle rpm needs to be increased for that cam to idle. That means the engine needs needs more air [ & fuel ] to idle. Hence the reason to drill t/blades as one method of providing extra air.

The description in post #1 sounds like t/blades are open too far, exposing too much t/slot. The fix is to provide more air by drilling the blades. However before doing this, I would advance the dist to provide about 30-32* at idle [ yes, 30-32*!!] . Idle rpm will increase significantly, maybe enough to back off the idle speed screw, get a great idle & have the t/slots in the correct position. The permanent fix for the ign is then to reset initial timing & use an ADJUSTABLE vac adv unit connected to MANIFOLD vacuum [ MVA ] to provide the wanted timing.

More on MVA, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

 

[Sweet5ltr]

I don't know if this is a common folk question about Holley carb secrets or not but here goes.
My dad has a nicely warmed up 383 with Eddy rpm heads and intake .
The cam is a 275 Comp hi lift with 525!lift.
I know it has always run good with 750!hp Holley vac secondary carb.
But I believe it needs more air into the engine to run even better.
The timing is set at 18 deg with about 14 to 16 on the plate all in by 2800 and has vacuum advance hooked up and working fine.
I noticed that when I pull off manifold vacuum cap below the carb I can lower the idle speed down and get better adjustable idle.screws adjustmemt. With the manifold vacuum port plugged I loose adjustability and need to open throttle to get idle . Idles perfect with vacuum leak 900 rpm.
With no vacume leak idle is rough at 1100 rpm.
Believe getting into the transition slot to get idle .
I think it needs air in intake how do you know where to start on the hole size if you drill the blades? is 1/8 on the primary a good starting size or should I go smaller?
Sorry about the long winded question.

Holley has this in their tuning guide. If you drill out the primary throttle plates, start small, 3/32" drill bit. We've done this with all of our carburetors, actually, one had both the primary & secondary throttle plates drilled. The Ultra XP series has an idle bypass circuit that's adjustable, which doesn't require drilling the throttle plates for idle tuning with larger camshafts.

There is a another method posted above, by AR67GTX that I would recommend first before drilling.

Just don't adjust the idle speed with the.. idle speed screw, it doesn't make sense, but it often uncovers the transfer slot and you'll end up with more issues than you began with.

Proper method to set the primary & secondary throttle plates, in relation to the transfer slot.

 

[383man]

Holley has this in their tuning guide. If you drill out the primary throttle plates, start small, 3/32" drill bit. We've done this with all of our carburetors, actually, one had both the primary & secondary throttle plates drilled. The Ultra XP series has an idle bypass circuit that's adjustable, which doesn't require drilling the throttle plates for idle tuning with larger camshafts.

There is a another method posted above, by AR67GTX that I would recommend first before drilling.

Just don't adjust the idle speed with the.. idle speed screw, it doesn't make sense, but it often uncovers the transfer slot and you'll end up with more issues than you began with.

Proper method to set the primary & secondary throttle plates, in relation to the transfer slot.

View attachment 1262417

Hence the reason to drill the throttle plates. Thanks for the good pics "Sweet5ltr" The setup is that you want the throttle transition slot as shown in the pic at about a square. Once its set there if you change the idle speed by the idle speed screw it changes the transition slot. And if you have to much of the transfer slot showing it starts pulling fuel out of the transfer slot at an idle. Then you get the idle mixture screws turned all the way in and the eng still runs since its running to rich with more of the transfer slots showing and dripping fuel from the transfer slots and now when you step on the gas you dont get the extra fuel from the transfer slot since its already dripping that fuel so the eng may stumble on light acceleration also. Bottom line is when you get the transfer slot right and the idle speed is to low then you need to give the eng more air without touching the idle speed screw on the primaries. So Holley says you can drill the throttle plates for more air and idle speed. And anyone who has been around modified engines knows many will idle to low and will need to have additional air let into the eng to set the idle. You can have everything right and many times the eng still needs more air and it has to get the air some how without using the idle speed screw. So drilling the throttle plates works good. Many modern carbs have and air adjusting screw under the carb stud to adjust and let more air in to change idle speed.
Also the wire that AR67GTX talks about wont fix the idle air problem as when you make the idle air bleed smaller sure the vacuum signal may be stronger but the idle circuit will now be richer since you are cutting air off when you want more idle air. He may also be thinking of the old trick to lean out the idle circuit when racers used to put wire into the idle feed restriction to lean out the idle and off idle. The problem is when the transfer slots are set right many engines will idle to low and you need to raise the idle without moving the primary idle speed screw. So the eng needs more air and fuel (mostly air to raise the idle speed a little) but when you make the idle air bleeds smaller it wont raise the idle speed since its not giving the eng more air as its giving the eng less air and making the idle richer.
There is nothing wrong with drilling the throttle plates if it comes down to needing more idle air to raise the idle speed and keep the transfer slots right. Any way you can get more air into the eng can work and also make sure it dont need more timing. Ron

 

[Wietse]

Just be sure about the holes you are about to drill.
Replacing throttle blades can sometimes be a challenge to get them to sea properly and even then you will find an uneven gap between the blade and the bore.
For that reason I suggested a simple bypass to feed it with air, even as a trial to ensure it works properly and no chances of modifying things that requires replacing things/buying new parts.

 

[AR67GTX]

Hence the reason to drill the throttle plates. Thanks for the good pics "Sweet5ltr" The setup is that you want the throttle transition slot as shown in the pic at about a square. Once its set there if you change the idle speed by the idle speed screw it changes the transition slot. And if you have to much of the transfer slot showing it starts pulling fuel out of the transfer slot at an idle. Then you get the idle mixture screws turned all the way in and the eng still runs since its running to rich with more of the transfer slots showing and dripping fuel from the transfer slots and now when you step on the gas you dont get the extra fuel from the transfer slot since its already dripping that fuel so the eng may stumble on light acceleration also. Bottom line is when you get the transfer slot right and the idle speed is to low then you need to give the eng more air without touching the idle speed screw on the primaries. So Holley says you can drill the throttle plates for more air and idle speed. And anyone who has been around modified engines knows many will idle to low and will need to have additional air let into the eng to set the idle. You can have everything right and many times the eng still needs more air and it has to get the air some how without using the idle speed screw. So drilling the throttle plates works good. Many modern carbs have and air adjusting screw under the carb stud to adjust and let more air in to change idle speed.
Also the wire that AR67GTX talks about wont fix the idle air problem as when you make the idle air bleed smaller sure the vacuum signal may be stronger but the idle circuit will now be richer since you are cutting air off when you want more idle air. He may also be thinking of the old trick to lean out the idle circuit when racers used to put wire into the idle feed restriction to lean out the idle and off idle. The problem is when the transfer slots are set right many engines will idle to low and you need to raise the idle without moving the primary idle speed screw. So the eng needs more air and fuel (mostly air to raise the idle speed a little) but when you make the idle air bleeds smaller it wont raise the idle speed since its not giving the eng more air as its giving the eng less air and making the idle richer.
There is nothing wrong with drilling the throttle plates if it comes down to needing more idle air to raise the idle speed and keep the transfer slots right. Any way you can get more air into the eng can work and also make sure it dont need more timing. Ron

I didn’t mention in that particular post that restricting the idle bleeds was done after drilling my throttle plates in increments to .135” dia. Yes, the bleed restriction was to richen the idle mixture and reduce the amount I had to back the idle screws out to optimize idle rpm and close my primary blades down. This was all in conjunction with opening the secondary plates up to where the transfer slots were still barely covered. It made a somewhat wild engine noticeably more civilized for the street.

 

[Geoff 2]

Not a good idea to alter IABs. They do more than just add air. They control when the idle system drops off. The carb makers are not fools, passages are sized to give a clean A/F ratio. Restricting the IAB will make the idle system hang on longer & could give a rich overlap as the main system starts.