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

Dead spot on tip-in regardless of throttle speed; Holley 4150

  • Thread starter Thread starter 65ShelbyClone
  • Start date Start date Dec 8, 2007

65ShelbyClone

Founding Member
Sep 9, 2000
4,675
38
119
Antelope Valley, SoCal
Dec 8, 2007
#1
  • Dec 8, 2007
  • #1
This is something I have searched and read quite a lot about, but none of the 100+ articles, books, and threads I scoured directly addressed this. I even have the Haynes Holley rebuild/tuning book and it doesn't either.

Anyway, my mechanical 650dp wasn't holding a steady idle; it would slowly climb from ~600rpm cold and hang around 1000-1100 once it was fully warm. I baselined the butterflies and idle screws according to a post on here by IIRC D.Hearne because I thought maybe the transfer slots were open. The idle cleaned up a bit, but now there is dead spot right off idle. It's totally dead, especially on slow and medium tip-in speed. This leads me to think that it is not the accelerator system. If I slap the pedal down, it shoots past the dead spot and the car responds immediately.

Any recommendations?
 
1

10secgoal

Active Member
Dec 1, 2003
2,801
3
49
46
San Diego
Dec 8, 2007
#2
  • Dec 8, 2007
  • #2
Take compressed air with the idle mixture screws out and shoot it in the holes. The idle circuit runs at idle, and ends right about tip in. Sometimes junk gets lodged in there that can't be removed with carb spray, causing it to go lean on one bank. Could explain the pump shot covering it.
See if the timing chagnes during warm up. Seen springs beome weak after warm up and advance the timing at idle. Could lock it out for a quick test.
 

65ShelbyClone

Founding Member
Sep 9, 2000
4,675
38
119
Antelope Valley, SoCal
Dec 8, 2007
#3
  • Dec 8, 2007
  • #3
Thanks. I'll try clearing out the idle circuits like you say. The carb was used and I put a rebuild kit in it, but I've always been suspicious that it could have problems later on. Fortunately I have another 650 and two 600dps to borrow parts from for troubleshooting.

The distributor is a rebuilt Duraspark that might have all of 5000 miles on it, so I kind of doubt that the springs are sacked out(unless they get reused[?]).
 
D

D.Hearne

New Member
Sep 29, 2000
11,730
6
0
south louisiana
Dec 8, 2007
#4
  • Dec 8, 2007
  • #4
Check for slack in the accellerator pump linkage. If there's too much, that'll cause your problem.
 

65ShelbyClone

Founding Member
Sep 9, 2000
4,675
38
119
Antelope Valley, SoCal
Dec 8, 2007
#5
  • Dec 8, 2007
  • #5
Yeah, I did that too. There again, I can very, very slowly apply the throttle and it will hit that dead spot and start stumbling. It even does it on engine braking. I can be on a slight downhill with my foot completely off the pedal and it's fine, but when I just breathe on the pedal I can hear it missing in the exhaust(Shelby-style side exits....under my seat).
 
D

D.Hearne

New Member
Sep 29, 2000
11,730
6
0
south louisiana
Dec 8, 2007
#6
  • Dec 8, 2007
  • #6
What jets and powervalve are you running?
 
1

10secgoal

Active Member
Dec 1, 2003
2,801
3
49
46
San Diego
Dec 8, 2007
#7
  • Dec 8, 2007
  • #7
Vac leak? Know you probably checked, but tip in can have a gap if the floats are low.
 

65ShelbyClone

Founding Member
Sep 9, 2000
4,675
38
119
Antelope Valley, SoCal
Dec 9, 2007
#8
  • Dec 9, 2007
  • #8
Jets are 65p/76s and PV is 6.5.

I pulled the idle screws and blew out the passages. That alone seemed to help, but I also leaned out the idle and adjusted the butterflies and it seems to have cleared up quite a bit.
 

pabear89

Active Member
Apr 15, 2003
2,126
0
46
High in the Hills of So Ca with the Voices in My H
Dec 9, 2007
#9
  • Dec 9, 2007
  • #9
Use some carb cleaner on the air bleed holes.
when they get plugged it will have this effect on idle and off idle responce.
 
You must log in or register to reply here.

Similar threads

S
Carb tuning and suggestions plz
  • stang387
  • May 18, 2008
  • Fox 5.0 Mustang Tech
Replies
7
Views
764
Fox 5.0 Mustang Tech May 18, 2008
stang387
S
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1965 - 1973 Classic Mustangs -General/Talk-
  • Classic Mustang Specific Tech
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?