Drilling throttle blades - Chevelle Tech
Performance Our High Performance area

 9Likes
  • 1 Post By VinceS427bb
  • 4 Post By Dave Ray
  • 2 Post By hjdca
  • 1 Post By 1968 hot rod
  • 1 Post By Dave Ray
 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 11 (permalink) Old Oct 2nd, 19, 11:07 PM Thread Starter
Lifetime Premium Member
Joe
 
Join Date: Nov 2012
Location: Northern Wisconsin
Posts: 437
Garage
Drilling throttle blades

Hello,
I have a decent sized cam and a 950 pro systems carb. I run the timing locked out at 36* have the throttle blades drilled and primary T slots squared. Adjusted the idle speed with the secondary screw. My question is how do you know when your at the best sized holes (throttle blades) ? I adjusted the idle mixture screws until I got the best vacuum; a shade over 10" but as you know you can't undrill !

Seems to run pretty clean now but just wondering

Thanks Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
NW68SSBBC is offline  
Sponsored Links
Advertisement
 
post #2 of 11 (permalink) Old Oct 3rd, 19, 12:37 AM
Senior Tech Team
Vince
 
Join Date: Sep 2006
Location: Los Lunas, NM
Posts: 8,564
Re: Drilling throttle blades

all 4-drilled?
how big?
i remember a discussion on this site;
there was a range for hole size...
WHT/73 likes this.

68-Malibu 427bbc/t4oo/3.73-12bolt/pro charger f1-r
VinceS427bb is offline  
post #3 of 11 (permalink) Old Oct 3rd, 19, 7:38 PM Thread Starter
Lifetime Premium Member
Joe
 
Join Date: Nov 2012
Location: Northern Wisconsin
Posts: 437
Garage
Re: Drilling throttle blades

Quote:
Originally Posted by VinceS427bb View Post
all 4-drilled?
how big?
i remember a discussion on this site;
there was a range for hole size...
Yes. All four are drilled. I'll have to check the size. But not very big.

I'll update when I have the size

I was wondering if you are supposed to adjust idle speed with the secondary throttle adjustment, or more rely on the extra air coming in. All four mixture screws set for best vacuum. It runs pretty good now but I'd like to be sure I'm doing it correctly

Also T slots squared....primary that is. The secondary is open more

Thanks Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
NW68SSBBC is offline  
 
post #4 of 11 (permalink) Old Oct 3rd, 19, 11:42 PM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 2,308
Re: Drilling throttle blades

No, never use the secs to adjust idle. All butterflies should be set to allow the .020 open idle transfer slots, and if the engine needs more air, then, drill the holes, (NOT YELLING, JUST EMPHATIC), DRILLING THE HOLES IS VERY DELICATE, USE A HAND HELD DRILL HOLDER, GO SLOW, DON'T TAKE BIG CUTS.

Lots of people make the error to adjust idle with the secs "idle screws", but that isn't the way it needs to be done.


One mod I do is to take the base plate off the carb, set the transfer slots for their opening, and then, scribe a line on the throttle bore on the TOPS of the butterflies, and put it back together. Now, I can look down the bores and see if the plates are in the right places if settings for plates opening get changed, without taking the carb off to do so. Just my way of making it easier for me.
Dave Ray is offline  
post #5 of 11 (permalink) Old Oct 4th, 19, 12:18 AM
Tech Team
Harry
 
Join Date: Jan 2011
Location: Los Angeles
Posts: 314
Re: Drilling throttle blades

Quote:
Originally Posted by NW68SSBBC View Post
Yes. All four are drilled. I'll have to check the size. But not very big.

I'll update when I have the size

I was wondering if you are supposed to adjust idle speed with the secondary throttle adjustment, or more rely on the extra air coming in. All four mixture screws set for best vacuum. It runs pretty good now but I'd like to be sure I'm doing it correctly

Also T slots squared....primary that is. The secondary is open more

Thanks Joe
You want to close both the primary and secondary butterflies as much as possible and use the bypass air from the drill holes to get a clean AFR with the mixture screws still turned out around 2 turns. This will insure that you have enough gas for the transition when you open the butterflies - without getting a lean stumble. Tune it with heads and motor hot.
WHT/73 and pannetron like this.

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
hjdca is offline  
post #6 of 11 (permalink) Old Oct 4th, 19, 7:18 AM
Senior Tech Team
Bob
 
Join Date: Aug 2002
Location: Pa
Posts: 1,362
When you open the secondary's too far they push fuel from the transition circuit easily and really richen the mixture too much
pannetron likes this.
1968 hot rod is offline  
post #7 of 11 (permalink) Old Oct 4th, 19, 9:07 PM Thread Starter
Lifetime Premium Member
Joe
 
Join Date: Nov 2012
Location: Northern Wisconsin
Posts: 437
Garage
Re: Drilling throttle blades

Quote:
Originally Posted by Dave Ray View Post
No, never use the secs to adjust idle. All butterflies should be set to allow the .020 open idle transfer slots, and if the engine needs more air, then, drill the holes, (NOT YELLING, JUST EMPHATIC), DRILLING THE HOLES IS VERY DELICATE, USE A HAND HELD DRILL HOLDER, GO SLOW, DON'T TAKE BIG CUTS.

Lots of people make the error to adjust idle with the secs "idle screws", but that isn't the way it needs to be done.


One mod I do is to take the base plate off the carb, set the transfer slots for their opening, and then, scribe a line on the throttle bore on the TOPS of the butterflies, and put it back together. Now, I can look down the bores and see if the plates are in the right places if settings for plates opening get changed, without taking the carb off to do so. Just my way of making it easier for me.
Great ideal with scribbing a line to check to make sure the throttle blades are in the right place
Thanks
Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
NW68SSBBC is offline  
post #8 of 11 (permalink) Old Oct 4th, 19, 9:09 PM Thread Starter
Lifetime Premium Member
Joe
 
Join Date: Nov 2012
Location: Northern Wisconsin
Posts: 437
Garage
Re: Drilling throttle blades

Quote:
Originally Posted by hjdca View Post
You want to close both the primary and secondary butterflies as much as possible and use the bypass air from the drill holes to get a clean AFR with the mixture screws still turned out around 2 turns. This will insure that you have enough gas for the transition when you open the butterflies - without getting a lean stumble. Tune it with heads and motor hot.
Perfect. That's exactly what I wanted to know

Thanks everyone
Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
NW68SSBBC is offline  
post #9 of 11 (permalink) Old Oct 5th, 19, 11:42 AM
Tech Team
Harry
 
Join Date: Jan 2011
Location: Los Angeles
Posts: 314
Re: Drilling throttle blades

Quote:
Originally Posted by NW68SSBBC View Post
Perfect. That's exactly what I wanted to know

Thanks everyone
Joe
Note: I use the Idle-ease from the Holley Ultra XP body to get my idle air bypass for my big cam. It is the same as drilling holes in the butterflies. fyi, it is my experience that the holes will give you a higher idle and leaner mixture when it is 100 degrees outside. When it is 50 degrees outside, with the same settings, the idle will be lower and the AFR will be richer, ie. you may have to open your primary butterflies a little and lean out your mixture screws a little. Keep that in mind when tuning... If you drill the holes too big, then, in 100 degree weather, no matter how much you close the butterflies, you may not be able to get your idle low enough..

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
hjdca is offline  
post #10 of 11 (permalink) Old Oct 5th, 19, 11:44 AM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 2,308
Re: Drilling throttle blades

NO, don't just close the butterflies, they need to be open enough to "square off" the idle transfer slots, NOT close them off.

The idle fuel screws do not have a specific setting, they need to be adjusted to the proper IDLE setting, and let the idle transfer slots do the transition from closed throttle to first abbeleration circuit.

If you don't get this right, there will be an off idle leanness, because the ITC is the FIRST fuel circuit that feeds fuel to the engine off idle, way before any other jetting, and acceleration pump squirt reaches the combustion chambers.

The drilled air bypasses are there to give the engine the increased air it wants to idle correctly WITHOUT having the throttle blades sit too far open to destroy the function of the ITC fuel at idle.
1775SS likes this.
Dave Ray is offline  
post #11 of 11 (permalink) Old Oct 5th, 19, 12:25 PM Thread Starter
Lifetime Premium Member
Joe
 
Join Date: Nov 2012
Location: Northern Wisconsin
Posts: 437
Garage
Re: Drilling throttle blades

Quote:
Originally Posted by Dave Ray View Post
NO, don't just close the butterflies, they need to be open enough to "square off" the idle transfer slots, NOT close them off.

The idle fuel screws do not have a specific setting, they need to be adjusted to the proper IDLE setting, and let the idle transfer slots do the transition from closed throttle to first abbeleration circuit.

If you don't get this right, there will be an off idle leanness, because the ITC is the FIRST fuel circuit that feeds fuel to the engine off idle, way before any other jetting, and acceleration pump squirt reaches the combustion chambers.

The drilled air bypasses are there to give the engine the increased air it wants to idle correctly WITHOUT having the throttle blades sit too far open to destroy the function of the ITC fuel at idle.
I knew not to close the butterflies completely! The primary T slots were/are squared! I was wondering about the secondary adjustment. Thanks to everyone , I now know how to proceed

Thanks again
Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
NW68SSBBC is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in










Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome