• Mustang Forums
  • 1965 - 1973 Classic Mustangs -General/Talk-

Need tuning help with 4100 Carburetor

  • Thread starter Thread starter Gary’s 67
  • Start date Start date Yesterday at 7:15 AM
G

Gary’s 67

New Member
Aug 18, 2026
2
0
1
Western NC
Yesterday at 7:15 AM
#1
  • Yesterday at 7:15 AM
  • #1
I’m using an Autolite 4100 on my 67 Mustang coupe with a 302 engine and mild 268 cam. I can’t get enough idle rpms to run without cranking on the idle stop screw and getting into the transition circuit of the carburetor that makes the idle mixture screws useless. I’ve got the initial timing set to 13 degrees. I don’t know what the correct fix for this would be. Any help would be appreciated
 

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor
Admin Dude
Jan 4, 1985
43,213
21,398
234
Box behind Walmart
www.stangnet.com
Yesterday at 9:18 AM
#2
  • Yesterday at 9:18 AM
  • #2
Is this a new or used carb?
 
G

Gary’s 67

New Member
Aug 18, 2026
2
0
1
Western NC
Yesterday at 11:34 AM
#3
  • Yesterday at 11:34 AM
  • #3
Well it is used but I recently rebuilt it, everything is adjusted correctly, proper wet float level checked, 1.08 Venturi, proper boosters, checked the power valve, just can’t get enough air flow without opening transition slots
 

Blackhawkxx

Advanced Member
Mar 3, 2023
496
490
73
66
Yesterday at 2:12 PM
#4
  • Yesterday at 2:12 PM
  • #4
Try this if it applies, if you have vacuum advance on the dist. and is connected to ported vacuum, move it to manifold vacuum. It will increase idle rpm. If you get pinging while driving, you can adjust from there.
 
F

fasterthangas

Active Member
Oct 5, 2016
227
66
38
Phoenix
Yesterday at 2:15 PM
#5
  • Yesterday at 2:15 PM
  • #5
Woodchuck's 4100 idle mixture adjustment 101.....

1. Turn each mixture adjustment screw in until VERY LIGHTLY seated.
2. Turn each mixture adjustment screw out exactly 1-1/2 turns.
3. Connect vacuum gauge to intake manifold source (preferably a tee on the PCV hose). DO NOT use a vacuum source tapped into an individual intake runner! If no vacuum gauge is available, connect a tachometer as directed by the instrument's manufacturer.
4. Turn each screw, alternately and no more than 1/4 turn at a time, in whichever direction results in an immediate increase in manifold vacuum and/or rpm.
5. Continue turning in that direction, while maintaining curb idle speed as close to specification as possible, until vacuum and/or idle speed begins to decrease.
6. Return to the last "good" setting.
7. If you have to have more than 1/4 turn difference between the individual mixture screws you have other issues that need to be corrected.

PS: Don't forget about the Secondary Idle Adjustment, which must be made with the carburetor off as it faces up from the base of the carburetor.

https://www.carburetor-parts.com/assets/manuals/autolite_4100_manual.pdf
 
You must log in or register to reply here.

Similar threads

Weber Carburetor tuning help
  • SchwarzStock
  • Jul 6, 2026
  • 1965 - 1973 Classic Mustangs -General/Talk-
Replies
4
Views
117
1965 - 1973 Classic Mustangs -General/Talk- Jul 19, 2026
SchwarzStock
H
Holley Carb Tuning Help
  • hassler
  • Oct 25, 2025
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
6
Views
351
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- Oct 29, 2025
85GTStangGuy
R
1974 Datsun 620 pickup / 5.0L HO
  • RJH46
  • Aug 24, 2025
  • Other Auto Tech
Replies
1
Views
267
Other Auto Tech Aug 24, 2025
Noobz347
Tips for WUE and ASE tuning
  • Mustang5L5
  • Oct 9, 2025
  • Digital Self-tuning Forum
Replies
1
Views
593
Digital Self-tuning Forum Oct 10, 2025
gkomo
1999 Mustang GT - No Start- Need help understanding the fuel pump circuit and how it works
  • 99gtGuy
  • Nov 4, 2025
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
1
Views
541
1996 - 2004 SN95 Mustang -General/Talk- Nov 7, 2025
04newedge-
0
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1965 - 1973 Classic Mustangs -General/Talk-
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?