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When to drill holes in the throttle plates?

cj's mopar

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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.
 
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There have been many articles on how drilled plates can help with a large cam. Usually it's 1/8" as you asked. But you could start with 3/32 first. If you don't like it, you can always solder the holes. :)
08-holley-carb-tune-adjust.jpg


https://www.musclecardiy.com/perfor...dle-and-transition-circuit-calibration-guide/
 
You can also crack open the secondaries a touch. And if you still need more air, close the secondaries in the primaries to make sure you're in the transition slot. But yes, always start small.
 
Before you get out your drill... swap another carb on it and see what happens.
You shouldn't have to modify the carb to work properly.
Did it ever run right before or is this a new build?
 
Its a good trick, opening the secondaries seems to help a little, but very hard to hit the sweet spot.

I did mine years ago and it helped a lot.
I don't remember the size hole, but 1/8 seems about right.
 
Car has been running like this for about ten yrs we .just deal with it .
I just know it could run better .
I don't have another carb to try.
Yes it does have a pcv valve ?
 
This may seem overly simplified but why not drill increasingly larger holes in the plastic port cap? Stop drilling when you are happy with the performance. If you don't get to that happy place just put a new cap on and you haven't done any harm to your carb. Just a thought.
Jerry
 
Drill baby drill.
As mentioned above, you can always plug the holes, even with pop rivets.

I did mine probably back in the 80's when I hung around with a bunch of Mopar guys.
The one we all looked up to as the most knowledgeble built a small block ( big cam man) and could never get it to idle right. Cruising was a big thing back then and his piece would always stall out, then he wouldn't be able to start it. What a mess.
I could never convince him to use this easy fix.
 
Another option would be to add more initial timing and that would increase the idle RPM
allowing you to close the throttle plates more. Of course it may or may not like more timing just a thought before you start drilling.
 
I’ve read blogs by at least one Holley restorer/rebuilder recommending to start off by setting the primary throttle blade so the exposed transition port below is a square. Don’t touch the idle speed screw after that. Mount the carb and start the engine and adjust the mixtures. If the idle speed is good, stop. If the idle speed is too low, pull the carb and slightly open the secondary blade and recheck idle speed. Repeat as necessary to get desired idle speed. Or if the secondary blade has to be opened to where it starts to expose the secondary transition slot, then stop and drill the primary blades to get the idle speed set.

Lot of work but makes sense.
 
We do have a hard time starting when engine is hot as well .
With the vacuum cap off the carb port it does start better when hot .
 
I think a lot of that hot start issue is carb percolation upon shutting off a hot engine which creates a flooded condition. On my GTX I usually press the throttle all the way to the floor - without pumping it - and hold it. Then turn the key. That allows the engine to pull more air in to dilute the fuel and it usually fires right off. Only trick for me is getting off the throttle before the engine revs out of sight. But by the same principal that would make some sense that your engine would start easier with the cap off.
 
It is an old trick...I did mine in the 90s....i think its even In the holly book or mopar book.....still the same carb I run to this day...Holly 780 DP, man secondary . This was with the 509 and a lower compression motor. I have since changed cams and still no problems. I had my buddy hold a vacuum next to it and drilled. Didn't even take it apart.....did his a few works later.
 
I'm looking at drilling my butterflies currently.

Is it better to drill only the primary butterflies or smaller holes in all four butterflies?
 
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Well - new to this forum, but not new to fairly fast cars - mopars in particular.

Not once have I ever had to drill out the butterflies, especially with the quality of carbs out there now. In my opinion it is a band aid for something else happening, and a mistune somewhere along the way. With various cars running the low 8s to mid 11s it was never once a thought.

A 383 with that small of a cam really shouldnt be running out of carb and needs additional drilling. I can only assume you have ensured there is no vacuum leaks and no mechanical issues with the carb.

It could very well be something clogged up in any of the ports within the carb causing issues. Worn out mixture screws, bleeds etc - it may be worthwhile sending it in to a good rebuilt place (there are plenty to choose from) or a new carb. However I understand that either of those options is more expensive than drilling.

So I guess after all that long winded babbling I just did - I would suggest ensuring everything is of working order and as designed before drilling. I know many have and its quite an easy fix to do and no real long lasting issue to replace if needed. Just make sure you get to the root of the problem is all i can suggest.
 
I'm looking at drilling my butterflies currently.

Is it better to drill only the primary butterflies or smaller holes in all four butterflies?
Usually you only drill the primary butterflies. The only carbs I've seen with all four plates drilled had four corner idle screws (idle circuit for the secondary side as well) which most do not have.
 
Well - new to this forum, but not new to fairly fast cars - mopars in particular.

Not once have I ever had to drill out the butterflies, especially with the quality of carbs out there now. In my opinion it is a band aid for something else happening, and a mistune somewhere along the way. With various cars running the low 8s to mid 11s it was never once a thought.

A 383 with that small of a cam really shouldnt be running out of carb and needs additional drilling. I can only assume you have ensured there is no vacuum leaks and no mechanical issues with the carb.

It could very well be something clogged up in any of the ports within the carb causing issues. Worn out mixture screws, bleeds etc - it may be worthwhile sending it in to a good rebuilt place (there are plenty to choose from) or a new carb. However I understand that either of those options is more expensive than drilling.

So I guess after all that long winded babbling I just did - I would suggest ensuring everything is of working order and as designed before drilling. I know many have and its quite an easy fix to do and no real long lasting issue to replace if needed. Just make sure you get to the root of the problem is all i can suggest.
Always good advice to make sure everything is working correctly before doing mods. However, before suggesting that a drilled hole is a 'band aid', remember that it's not just a street hot rodder trick - Holley sells the plates that way as well. https://www.holley.com/products/fue..._body_kits_and_service_components/parts/26-99
In this example, it is for a four corner idle carb and uses 1/8" holes.
 
Always good advice to make sure everything is working correctly before doing mods. However, before suggesting that a drilled hole is a 'band aid', remember that it's not just a street hot rodder trick - Holley sells the plates that way as well. https://www.holley.com/products/fue..._body_kits_and_service_components/parts/26-99
In this example, it is for a four corner idle carb and uses 1/8" holes.

Yes and some Holleys at one point even came with drilled butterflies (I think the HP, or at least some of them did)

But I still feel with the set up he describes its not needed to drill the blades if everything is working properly. Which is why I feel there may be another underlying issues, so drilling the blades in this case could very well be masking another issue. Just my opinion - I have been wrong most of my life in about everything (ask my ex-wife) :)
 
Usually you only drill the primary butterflies. The only carbs I've seen with all four plates drilled had four corner idle screws (idle circuit for the secondary side as well) which most do not have.

Thank you Photon - good information. I had seen some that had drilled all four but the four corner idle adjustment makes sense.
 
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