Off Idle stumble?
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Lead Driver
Joined: Apr 2001
Posts: 6,175
Likes: 21
From: Texas
Off Idle stumble?
I went a few steps leaner on my edelbrock 750 Performer carb, in hopes of losing this stumble just off idle. It is now harder to start and the stumble is possibly worse.
My thinking was that the 750 was probably a bit large for my 390, and leaning it out would help. Is that backwards thinking, maybe it needs to go richer?
It didn't seem so bad a few weeks ago, just after I made the adjustment, but tonight it is running pretty rough at low RPMS. It is also colder tonight, would that help or hinder a lean running condition?
My thinking was that the 750 was probably a bit large for my 390, and leaning it out would help. Is that backwards thinking, maybe it needs to go richer?
It didn't seem so bad a few weeks ago, just after I made the adjustment, but tonight it is running pretty rough at low RPMS. It is also colder tonight, would that help or hinder a lean running condition?
Posting Guru
Joined: Aug 2002
Posts: 1,214
Likes: 2
From: Minneapolis, MN
Off Idle stumble?
Off idle stumble would probably indicate a lean condition or late timing. Colder air is denser and needs more fuel, not less. You got a double whammy when you leaned it and the weather got cold. Richen up your idle mixture 'til it smooths out. Also check your timing,making sure the vacuum and centrifugal advance both work. Try advancing it a little. I'm sure others will chip in with what the total advance should be and at what rpm. Or check previous posts. It's been discussed before.
Just my $.02
Greg
'77 F-250 Camper Special 400ci driver
'76 F-250 Supercab 360ci current project
'71 Mach I 429CJ in storage
'79 F-150 for parts
huct on foniks wurkt fer me
Just my $.02
Greg
'77 F-250 Camper Special 400ci driver
'76 F-250 Supercab 360ci current project
'71 Mach I 429CJ in storage
'79 F-150 for parts
huct on foniks wurkt fer me
Post Fiend
Joined: Feb 2001
Posts: 6,624
Likes: 8
From: Missouri
Off Idle stumble?
He's got it right. A 750 might be a little on the large side but not enough to have a major effect on low end response. Rich conditions won't casue popping either. They will cause a bog and lack of power along with fouled plugs etc... I know you've messed with the timing before so hopefully this fixes it.
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Lead Driver
Joined: Apr 2001
Posts: 6,175
Likes: 21
From: Texas
Off Idle stumble?
I doubt it's timing. I have played with that to no end. I have the MSD Dizzy, no vacuum voodoo. I will go a few stages richer next. I've got one of the Edelbrock tuning kits, so it just costs me time.
You all have a Merry christmas.
You all have a Merry christmas.
Laughing Gas
Joined: Jul 1999
Posts: 1,048
Likes: 22
From: Lacey, WA
Off Idle stumble?
Gtex,
If I am not mistaken a faulty Accl pump can cause an off idle stumble also. Is the carb a new or replacement one? I don't know much about Edle carbs but Holley's have an Accl pump linkage adjustment. Check the linkage for correct alignment. Or maybe the pump diaphram is torn. Check and see if there is a healthy shot of fuel in the primaries.
Just a few un-educated guesses, hope they help.
Lee
If I am not mistaken a faulty Accl pump can cause an off idle stumble also. Is the carb a new or replacement one? I don't know much about Edle carbs but Holley's have an Accl pump linkage adjustment. Check the linkage for correct alignment. Or maybe the pump diaphram is torn. Check and see if there is a healthy shot of fuel in the primaries.
Just a few un-educated guesses, hope they help.
Lee
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Lead Driver
Joined: Apr 2001
Posts: 6,175
Likes: 21
From: Texas
Off Idle stumble?
The carb was brand new a year ago.
The timimng has been checked and rechecked and tuned until I was blue the face!
I've about decided that Sean is right, my cam is not the best choice. But, I thought I might be able to tune the carb to run better. I have adjusted the lever arm to get more pump shot and it did not help.
I've also adjusted the step up springs as suggested by the Carb manual for long duration cams. No improvement.
The MSD Dizzy lets me do all kinds of things with the timing, nothing seems to completely get rid of this stumble just off idle. With very high inital advance, it runs better but then the high RPM advance is too high. Even with the lowest setting for max advance on the Dizzy. It's also very hard to start with high initial advance when it's warm.
This thing realy runs at higher RPM, but is pretty weak at low RPM.
The timimng has been checked and rechecked and tuned until I was blue the face!
I've about decided that Sean is right, my cam is not the best choice. But, I thought I might be able to tune the carb to run better. I have adjusted the lever arm to get more pump shot and it did not help.
I've also adjusted the step up springs as suggested by the Carb manual for long duration cams. No improvement.
The MSD Dizzy lets me do all kinds of things with the timing, nothing seems to completely get rid of this stumble just off idle. With very high inital advance, it runs better but then the high RPM advance is too high. Even with the lowest setting for max advance on the Dizzy. It's also very hard to start with high initial advance when it's warm.
This thing realy runs at higher RPM, but is pretty weak at low RPM.
Post Fiend
Joined: Feb 2001
Posts: 6,624
Likes: 8
From: Missouri
Off Idle stumble?
Well whatcha waiting for. Get to tunin'! You may never get low end with that cam but it should still quit stumbling when you get it right.
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Elder User
Joined: Jun 2001
Posts: 775
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From: Northern Va
Off Idle stumble?
What about checking the power valve. Wouldn’t the wrong one cause a stumble off idle?
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Off Idle stumble?
Hey GTEX
i have an edelbrock 600cfm performer and i was wondering, how the heck do you lean those things out other than the mixture screws? I know all about holleys but these edelbrocks look like they have NO provisions for swapping in leaner jets! am i missing something here?
how did you lean yours out?
thanks!
~~NUT~~
i have an edelbrock 600cfm performer and i was wondering, how the heck do you lean those things out other than the mixture screws? I know all about holleys but these edelbrocks look like they have NO provisions for swapping in leaner jets! am i missing something here?
how did you lean yours out?
thanks!
~~NUT~~
Thread Starter
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Lead Driver
Joined: Apr 2001
Posts: 6,175
Likes: 21
From: Texas
Off Idle stumble?
[updated:LAST EDITED ON 27-Dec-02 AT 09:57 AM (EST)]Changing metering rods and step up springs is very easy (5-10 minutes) and changing jets is about a 45 minute deal. There are good instructions in the Performer series carb manuals. You should buy one of the kits with a variety of metering rods ,jets and step-up springs made by Edelbrock. They run about $60.
http://www.edelbrock.com/automotive/index.html
follow the carb link on the left and then the accessories link. they also have a link to the online owners manual!
your carb is probably number 1405. If so, you should get the following kit #1479.
Calibration Kit for #1405......#1479
Metering Rods (Pair) Metering Jets (Pair) Step-Up Springs (Pair)
.068 x .042 #1446 .089 #1424 3-inch Hg Blue
.068 x .052 #1448 .092 #1425 4-inch Hg Yellow
.070 x .052 #1452 .098 #1427 5-inch Hg Orange
.073 x .047 #1456 .101 #1429 7-inch Hg Pink
.073 x .052 #1457 .104 #1430 8-inch Hg Silver
.075 x .047 #1459
http://www.edelbrock.com/automotive/index.html
follow the carb link on the left and then the accessories link. they also have a link to the online owners manual!
your carb is probably number 1405. If so, you should get the following kit #1479.
Calibration Kit for #1405......#1479
Metering Rods (Pair) Metering Jets (Pair) Step-Up Springs (Pair)
.068 x .042 #1446 .089 #1424 3-inch Hg Blue
.068 x .052 #1448 .092 #1425 4-inch Hg Yellow
.070 x .052 #1452 .098 #1427 5-inch Hg Orange
.073 x .047 #1456 .101 #1429 7-inch Hg Pink
.073 x .052 #1457 .104 #1430 8-inch Hg Silver
.075 x .047 #1459
