Previous Thread
Next Thread
Print Thread
Page 1 of 2 1 2
Can't get it to idle. #3043348
05/18/22 04:30 PM
05/18/22 04:30 PM
Joined: Sep 2017
Posts: 301
Red Deer, Alberta
G
Greenwood Offline OP
enthusiast
Greenwood  Offline OP
enthusiast
G

Joined: Sep 2017
Posts: 301
Red Deer, Alberta
After disassembling my carb, blowing out the air bleeds, and re-assembling, I finally got it running sort of properly. I can get it to idle down to about 1200-1300, and it sounds and smells nice and crisp. However, even after getting up to temp, it will suddenly drop off and not recover. It should actually idle about 900, and I've been sneaking the timing back closer to a proper initial, but the only way I can keep it from randomly dying is to keep the idle up past 1400. The carb is an older BG 750 DP, with no choke or air horn, and downleg boosters. I set the idle mixture screws at 1 1/2 turns out, as a baseline. It starts great. Half a shot of gas, hit the button, and it's running. Just can't get the idle sorted. ??

Re: Can't get it to idle. [Re: Greenwood] #3043380
05/18/22 06:00 PM
05/18/22 06:00 PM
Joined: Jan 2003
Posts: 2,709
A shed in England
Tig Offline
master
Tig  Offline
master

Joined: Jan 2003
Posts: 2,709
A shed in England
Was it OK prior to the carb re-assemble ?


'74 Challenger..9.46 and 145.9 so far. 4023lb!!!# N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @142.4 with our old mule 580 wedge
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
Re: Can't get it to idle. [Re: Greenwood] #3043405
05/18/22 07:17 PM
05/18/22 07:17 PM
Joined: Aug 2003
Posts: 40,823
Bend,OR USA
C
Cab_Burge Offline
I Win
Cab_Burge  Offline
I Win
C

Joined: Aug 2003
Posts: 40,823
Bend,OR USA
Sounds like the timing is retarding causing the lower idle speed and dying scope wrench up
I have had the same deal, problems, on a motor with a big cam and a not real loose converter whiney
Changing the advance springs and shorting up the mechanical advance slot fix it by being able to set the idle timing in gear at 14 to 18 BTDC and have right at 34 BTDC full advance over 1800 RPM shruggy scope twocents


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Can't get it to idle. [Re: Cab_Burge] #3043425
05/18/22 08:50 PM
05/18/22 08:50 PM
Joined: May 2005
Posts: 1,616
Apollo, PA.
B1MAXX Offline
top fuel
B1MAXX  Offline
top fuel

Joined: May 2005
Posts: 1,616
Apollo, PA.
Usually the reason that it will idle turned up on the idle rpm is that you getting extra idle fuel from the transfer slots that it wants. You probably need to back the mixture screws out more.

Re: Can't get it to idle. [Re: B1MAXX] #3043454
05/18/22 10:20 PM
05/18/22 10:20 PM
Joined: Aug 2003
Posts: 40,823
Bend,OR USA
C
Cab_Burge Offline
I Win
Cab_Burge  Offline
I Win
C

Joined: Aug 2003
Posts: 40,823
Bend,OR USA
All the cars I've work on that had the idle speed screw up far enough for the throttle plates to expose the transfer slots ran rich and stunk like unburnt gas out the tail pipes.
I can remember seeing a post about intentionally exposing the transfer slots on here several years back, really bad suggestion down
The transfer slots are their for adding additional fuel at light part throttle changes from under 1200 RP up to around 2100 RPM twocents
The idle circuit is tunable by using the idle air bleeds as well as the idle mixture screws wrench up Don't make it idle on the transfer slots tsk


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Can't get it to idle. [Re: Cab_Burge] #3043463
05/18/22 11:31 PM
05/18/22 11:31 PM
Joined: Sep 2017
Posts: 301
Red Deer, Alberta
G
Greenwood Offline OP
enthusiast
Greenwood  Offline OP
enthusiast
G

Joined: Sep 2017
Posts: 301
Red Deer, Alberta
Okay. I had some self-induced carb issues last summer. I did solve them, but I also had the carb apart for new gaskets. I kind of overthought my first fire-up issue, solved it (crossed wires. Doh!), and then it was all over the place on the next fire up. Thought for a bit, and pulled the carb apart, blew out the bleeds on a hunch. I set the throttles at about .040 of transfer slot visible, and set the idle screws at 1 1/2 turns out. Right now, the timing's at 32 at 1500. Can't get a shot any lower, but in the past it has idled nicely at 10 initial, with 36 all in. Cab, am I reading it correct that I should close the throttle blades up until the transfer slots are NOT exposed? I also suspect I could bring the idle screws out a bit more, so I will try that tomorrow night when the neighbor's at work. smile

Re: Can't get it to idle. [Re: Greenwood] #3043471
05/19/22 12:41 AM
05/19/22 12:41 AM
Joined: Aug 2003
Posts: 40,823
Bend,OR USA
C
Cab_Burge Offline
I Win
Cab_Burge  Offline
I Win
C

Joined: Aug 2003
Posts: 40,823
Bend,OR USA
the carb is design to idle on the idle circuits, not on the transfer slots tsk work
Unscrew the idle speed screw so there is no throttle plate opening and then adjust so it has around 1 to 3 turns in against the idle stop on the throttle shaft and see how that does, most of the Holley after market non stock OEM carbs I've used and tuned like between 1/2 to 1 1/2 turns out from gently bottomed out on the idle mixture screws scope up
I also make sure if it is not a double pumper to adjust the rear throttle plates so they are open a little tiny bit from being full close to help clean up the idle mixture some up scope Maybe try opening them from .005 to .015 visible wrench


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Can't get it to idle. [Re: Cab_Burge] #3043521
05/19/22 08:39 AM
05/19/22 08:39 AM
Joined: Mar 2003
Posts: 2,288
PA
moparacer Offline
top fuel
moparacer  Offline
top fuel

Joined: Mar 2003
Posts: 2,288
PA
Quote
I've been sneaking the timing back closer to a proper initial


How much cam? Might need to go the other way to get the idle where you want it.. Crank the initial up and take some centrifugal out to get your total.

Sometimes if the cam is big enough I have had to locked the distributor out, set the total and roll with it.


67 Barracuda street/bracket car 11.27-119
68 Dart 502 BB 8.70s-152
414 cid SB Dragster 7.65-174
Re: Can't get it to idle. [Re: moparacer] #3043525
05/19/22 09:19 AM
05/19/22 09:19 AM
Joined: Sep 2017
Posts: 301
Red Deer, Alberta
G
Greenwood Offline OP
enthusiast
Greenwood  Offline OP
enthusiast
G

Joined: Sep 2017
Posts: 301
Red Deer, Alberta
The cam is a Comp 308 solid roller. 262 @ 50. It's the same cam as I've been running before. I'm pretty sure what I'm dealing with right now is a bit of carryover from last years little carb tuning adventure. smile I do thank everyone for their input, as it will get me back on track.

Re: Can't get it to idle. [Re: Greenwood] #3043612
05/19/22 01:29 PM
05/19/22 01:29 PM
Joined: Jan 2003
Posts: 4,366
Moved to N.E. Tennessee
GomangoCuda Offline
master
GomangoCuda  Offline
master

Joined: Jan 2003
Posts: 4,366
Moved to N.E. Tennessee
That cam will like more timing at idle. Try 20° or more. Then adjust the Carb.


In theory, there is no difference between theory and practice. In practice, there is.
Re: Can't get it to idle. [Re: GomangoCuda] #3043618
05/19/22 01:56 PM
05/19/22 01:56 PM
Joined: Mar 2003
Posts: 2,288
PA
moparacer Offline
top fuel
moparacer  Offline
top fuel

Joined: Mar 2003
Posts: 2,288
PA
iagree

What Cuda said. Crank that initial up!


67 Barracuda street/bracket car 11.27-119
68 Dart 502 BB 8.70s-152
414 cid SB Dragster 7.65-174
Re: Can't get it to idle. [Re: GomangoCuda] #3043639
05/19/22 02:45 PM
05/19/22 02:45 PM
Joined: Jan 2003
Posts: 14,171
So. Burlington, Vt.
F
fast68plymouth Offline
I Live Here
fast68plymouth  Offline
I Live Here
F

Joined: Jan 2003
Posts: 14,171
So. Burlington, Vt.
Originally Posted by GomangoCuda
That cam will like more timing at idle. Try 20° or more. Then adjust the Carb.


Agreed.
Fundamentally, I like to have the basic ignition curve sorted out before trying to dial in the carb.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: Can't get it to idle. [Re: fast68plymouth] #3043658
05/19/22 03:54 PM
05/19/22 03:54 PM
Joined: Jun 2008
Posts: 3,844
usa
L
lewtot184 Offline
master
lewtot184  Offline
master
L

Joined: Jun 2008
Posts: 3,844
usa
for what it's worth i use a whole lot less cam than that 308 and idle about 25 degrees before TDC. my initial is 18 with one light primary spring that bumps the timing up and idle speed in neutral is 850rpm. this works very well for me. you're doing over 40 degrees of overlap at .050"; i do about 18. trying to make that work with 10 degrees initial would be a big challenge. like said, fix the ignition first and the carb will be easier to manage.

Re: Can't get it to idle. [Re: Greenwood] #3043664
05/19/22 04:02 PM
05/19/22 04:02 PM
Joined: Nov 2015
Posts: 735
Missouri
J
jwb123 Offline
super stock
jwb123  Offline
super stock
J

Joined: Nov 2015
Posts: 735
Missouri
If my memory serves me they want a .020 feeler gauge in the throttle plates so the transfer plate has the correct amount of vacuum to pull fuel in at idle. If you need more air you can drill small holes in the throttle plate. Some of the BG carbs have an air valve you access down the center hole. smaller idle air bleeds may help. Good advice on advancing the timing as well.

https://www.motortrend.com/how-to/ctrp-0707-carburetor-tuning/

Re: Can't get it to idle. [Re: fast68plymouth] #3043731
05/19/22 07:25 PM
05/19/22 07:25 PM
Joined: Oct 2003
Posts: 11,499
W. Kentucky
justinp61 Offline
I Live Here
justinp61  Offline
I Live Here

Joined: Oct 2003
Posts: 11,499
W. Kentucky
Many carb problems have been fixed by correcting the timing.

Re: Can't get it to idle. [Re: justinp61] #3043774
05/19/22 10:04 PM
05/19/22 10:04 PM
Joined: Apr 2006
Posts: 4,130
West Plains, MO
DrCharles Offline
master
DrCharles  Offline
master

Joined: Apr 2006
Posts: 4,130
West Plains, MO
Originally Posted by justinp61
Many carb problems have been fixed by correcting the timing.

"90% of carburetor problems are in the ignition" - Smokey Yunick up

Re: Can't get it to idle. [Re: justinp61] #3043784
05/20/22 12:05 AM
05/20/22 12:05 AM
Joined: Jul 2005
Posts: 670
California
BigDaddy440 Offline
mopar
BigDaddy440  Offline
mopar

Joined: Jul 2005
Posts: 670
California
Originally Posted by justinp61
Many carb problems have been fixed by correcting the timing.


This has been true in my experience once or twice. If you don't run enough timing, you'll have to turn the idle screw beyond the idle circuit of the carb -causing problems. By advancing the timing which increases RPM, or making sure you're running the proper initial timing for your engine, you should be able to idle within the idle circuit the way the carb was designed to run. This is the first thing I'd confirm before moving forward.

Last edited by BigDaddy440; 05/20/22 12:05 AM.

1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
Re: Can't get it to idle. [Re: BigDaddy440] #3043798
05/20/22 05:47 AM
05/20/22 05:47 AM
Joined: Jun 2008
Posts: 3,844
usa
L
lewtot184 Offline
master
lewtot184  Offline
master
L

Joined: Jun 2008
Posts: 3,844
usa
knowing what distributor is being used would help.

Re: Can't get it to idle. [Re: lewtot184] #3043849
05/20/22 09:25 AM
05/20/22 09:25 AM
Joined: Jan 2003
Posts: 14,171
So. Burlington, Vt.
F
fast68plymouth Offline
I Live Here
fast68plymouth  Offline
I Live Here
F

Joined: Jan 2003
Posts: 14,171
So. Burlington, Vt.
If the timing situation seemed to be close to what I thought I’d end up with(20-25 initial/32-38 total), then “my” next step is to just grab another carb off the shelf to see if a different carb solves the problem, or at the least changes the idle characteristics.

I realize a lot of people don’t have that option........ but I haven’t had only one carb on hand for many many years.

I find that swapping to “known good” parts is a pretty effective way to troubleshoot.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
Re: Can't get it to idle. [Re: fast68plymouth] #3043900
05/20/22 11:42 AM
05/20/22 11:42 AM
Joined: Jan 2003
Posts: 2,094
central texas
K
krautrock Offline
top fuel
krautrock  Offline
top fuel
K

Joined: Jan 2003
Posts: 2,094
central texas
also a 750 carb has smallish throttle blades, with a cam that large (after timing is figured out) it might need holes drilled in the blades to allow enough air at idle and to keep the transition slot exposure small enough.

Page 1 of 2 1 2






Powered by UBB.threads™ PHP Forum Software 7.7.1