Drilling Holley Butterflys
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Melting Slicks
Joined: Mar 2004
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From: Lake Ozark, MO
Drilling Holley Butterflys
I have put a fairly significant cam in my 454 and I read that I should drill a small hole in the primary carb butterflys to help with idle/adjustments. The article didn't say how big the hole should be or where on the butterfly it should be drilled (front/back/side). Carb is a Holley 750 if that makes any difference.
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Joined: Nov 2000
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From: Waterloo ontario Canada
Start with about .100-.125. When you drill if you do not dissasemble the carb and just turn it over you are drilling so the butterflies try to open, you have to hold the throttle shut by hand while drilling, that gives you which side to drill on from the throttle shaft. Drill about center on this half of the throttle.
Another really good adjustment is putting a .015 wire in each of the idle circuit passages that have idle screws.
This leans out the idle circuit and makes the idle screws much more sensivitive.
The Idle screws should be out 1 1/2 turns and very little adjustment from there.
Another really good adjustment is putting a .015 wire in each of the idle circuit passages that have idle screws.
This leans out the idle circuit and makes the idle screws much more sensivitive.
The Idle screws should be out 1 1/2 turns and very little adjustment from there.
Melting Slicks
Joined: May 2002
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From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Hvymtlc5
I have the Holley tuning book, if you would like I can scan the pages on tuning for a large camshaft and e-mail them to you. It would be later tonight or possibly tomorrow before I can get home and do this. According to the book there are other adjustments to make before drilling the plates.
Chevymans 77
I have the Holley tuning book, if you would like I can scan the pages on tuning for a large camshaft and e-mail them to you. It would be later tonight or possibly tomorrow before I can get home and do this. According to the book there are other adjustments to make before drilling the plates.
Chevymans 77
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Joined: Jun 2004
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From: Ventura County ca
depending on how big the cam is you may not have to. I always try if without first. If you can't get enough adjustment out of the idle mixture screws or they don't do anything then you need to drill. Also, make sure your powervalve isn't rated at too high a vacuum. That would be one of the first things I check. You can tell most of the time if the cam is too big by putting your finger over the air bleeds in the horn of the carb while idling and see if it makes a difference indicating that it is working. If not, smaller valve. I forget how to determine it I think it is something like taking your vacuum at idle and dividing it in two and that is the size power valve you need. 14"vac. at idle would be 7" or smaller powervalve. I think. It's been a while. and if you have secondary idle mixture screws your even better off. I like the trick of cracking the secondaries a little bit if you have adjustment. Hope this isn't more confusing..
Last edited by jlch; Oct 25, 2004 at 11:35 AM.
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From: Taylor Michigan
Originally Posted by Hvymtlc5
I have put a fairly significant cam in my 454 and I read that I should drill a small hole in the primary carb butterflys to help with idle/adjustments. The article didn't say how big the hole should be or where on the butterfly it should be drilled (front/back/side). Carb is a Holley 750 if that makes any difference.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
I'll follow the advice of getting her running first....just thought it may be something that was a "must" do with a higher duration cam, and I wanted to do it while the carb was off. From the replies I may not have to drill at all - and that is OK with me : ) As usual, very sound advice from the folks that know!!
