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Discussion Starter #1
Is anyone familiar with this mod?? Usually it's done to promote a fair idle quality on long duration camshafts. I've noticed the new HP series holley carbs are already drilled, I'm wondering about performing the same mod on my 351C with a 780 holley. It seems to load up at idle, and I'm thinking I have to set the idle so high to get her to idle (I've done all the tuning, vac gauge, etc)that the butterflies are open to much and uncovering the transition slot and causing her to load up. I'm thinking if I drill a .060" hole in each plate (start small) and see if the idle quality improves, it's a step in the right direction. Any thoughts? Anyone performed this mod? What size hole to start with? I'm not quite ready to buy that Bigs 950 yet


Thanks for all the help, sorry it's a long post
 

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Try opening your secondaries a little with the set screw in the bottom of the baseplate. This is a better deal than the hole drilling if your cam is not super-big.

Plus, it's reversible!

It works on my 351c with 230 deg @ .050 and single plane intake with 700 DP. Idles pretty good now.

Greg
 

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That is what I did. On my tunnel ram with 2 600 holleys. I had a problem with idle quality. So I went with Holley's tech tips and adjusted the set screw for the secondaries. The screw in the base plate. It worked great.
 

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I have been around and around on this with my setup. My idle is so delicate that a 2 degree change in timing will mess it up and require me to go back and readjust it. I'm right on the edge of the primaries wanting to crash in with the motor at idle.

The trick is to experiment until you get a clean idle that still can take advantage of your primary idle screw for a very small adjustment. In my case(every case is different) this involved some secondary throttle blade stop screw and holes in the secondary throttle plates. I started with all 4 holes but the primary holes were too much. I plugged them back up with a little JB weld(gotta love JB).

My book says to drill 3/32 holes(approx .09) but I started with something around .06's. It didn't do anything until I enlarged the holes a little so I guess there is just a minimum hole size before it has any effect.

Since you are adding more air to the idle you will also have to fatten up the idle mixture screws. If you add too much air you will not be able to give it enough idle gas unless you also have secondary idle mixture adjustments. You want to try and get it to idle at as low an rpm as it will without trying to die when you put it in gear.

If your converter is too tight for your motor you will never reach a happy state with this situation. You want the idle to sit so that when you pull it down in gear the idle doesn't drop any at all. If it wants to drop down on you and you have the idle as low as you can get it to run clean the converter is just too tight for your setup.

I found out later after much frustration timing is a critical part of this . If your idle is raised up your timing advance may be trying to come in too soon so the motor will hunt up and down continually with the primaries coming in and out. You will want to set your distributor up so that the advance doesn't start to come in until 2 hundred rpms or so above whereever you motor wants to idle. I had to buy a new MSD distributor with the bushings just because of this because my old one wouldn't cooperate.

If you get you timing adjustment right and use the secondary throttle stop screw a little it could prevent you having to drill any holes in the throttle plates at all or you may still have to have 2 or possibly all 4. I found that some idle screw and some secondary throttle plate would get it up to just about 900 rpm idle. Secondary holes will get it on up to close to 1000. LArger secondary holes with primary holes will get it up between 1000 and 1200. Beyond that and your idling on the primary jets, LOL.
 

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Discussion Starter #6
Yes, I'm running a 3.5" power valve in the front, and blocked in the rear. I'm pulling around 6-7" vacuum at about 1200 rpm idle. She's pretty choppy, sounds nice, the cam is solid flat tappet, 250°int/260° exhaust duration with 108 lobe centers, so I don't expect her to idle at 800-900 rpm. Just bought the Holley modifying book, so I'll figure something out. Thanks again all!!!
 
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