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Old 09-10-2004, 02:41 PM
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Question Lean pop

I have a lean pop (the carb backfires w/o flames),at least that is what the last mechanic told me when I had my truck in the shop when I try to feather on the accelerator at a stop or when driving between a rpm's under 2500 or so. I also get some intermittent hesitations above 2500.

Here is what I am running:

1971 Chevy C20 2WD
350 V8 w/ stock everything but the RV cam
Edelbrock 600cfm Performer carb w/ elec choke
K&N Air filter
Performance exhaust headers

I've tried to search the forum, but I get an error message that says the site is having problems. Can someone help me to understand what a lean pop is and what causes it.

I appreciate any help I can get.

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Last edited by stheman70; 09-10-2004 at 03:24 PM.
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Old 09-10-2004, 03:58 PM
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As you know, vehicles run on an air/fuel mixture (somewhere around 13:1 I believe). When you mash the accelerator, you are introducing a larger amount of air at a quick pace. Your lean pop comes from the carb (or system alltogether) not being able to supply enough fuel at that instant.

Unfortunetely this seems to be an inherent problem with that carb. I did not stick with it long enough to tear it apart and and reconfigure to fix it however I am sure someone here has a more direct answer for you. I tried changing the metering springs and adjusting the plunge to no effect. I think that the electric choke version (which you have) of Edelbrocks 600cfm had smaller metering jets than its mechanical choke counterpart.

BTW welcome to the forum.
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Old 09-10-2004, 05:17 PM
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Either the accellerator pump in the carb has quit working or you have a vacuum leak. the vacuum leak could be anywhere the vacuum line run not just around the carb. Lean misfire is not an inherent problem with these carbs as they are jetted quite rich from the factory.
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Old 09-10-2004, 07:17 PM
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I just went through the same problem today ,as a matter of fact with my 770 Holley after switching from a performer intake to a Performer RPM. I swapped the #25 squirter to a #32.. its all good now. John
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Old 09-11-2004, 03:41 PM
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Thumbs up Lean pop

Thanks for the info everyone. My truck ran great with this 600 performer carb and the stock jets.

I needed to replace a leaky intake manifold gasket and had a local mechanic do the work for me since I had just been in a motorcycle accident and couldn't do the work myself.

I wonder if during the process of putting everything back together a leak in the fuel line may have developed.

I do notice that The rubber cap plugging one of the vaccum ports on the front of the carb that the installation manual states should be plugged looks cracked and no longer provides a good seal.

I'll check for leaks today and let you all know how things went.

Thanks again for the info.

Last edited by stheman70; 09-11-2004 at 03:47 PM.
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Old 09-11-2004, 11:33 PM
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my engine is also having a poping noise at about 1300rpm to 2000rpms if im just cruising and barley pressing on the gas. it almost sounds like a misfire but since i've seen this thread could it be a vacuum leak in the carb?
i also have an edelbrock carb but with manual choke. on a 350sbc
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Old 09-12-2004, 09:59 AM
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Quote:
Originally posted by Brian Morency
my engine is also having a poping noise at about 1300rpm to 2000rpms if im just cruising and barley pressing on the gas. it almost sounds like a misfire but since i've seen this thread could it be a vacuum leak in the carb?
i also have an edelbrock carb but with manual choke. on a 350sbc
You might need to put in the next step stiffer step up springs.
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Old 09-12-2004, 03:56 PM
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I assume you mean the distributor advance springs?
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Old 09-12-2004, 04:51 PM
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ok i just checked my plugs and wired they are fine. i have msd wires and split fire plugs. and a msd distributor cap and rotor. i didnt really notice this noise until i took off my exhaust and am currently only using an exhaust manifold and pipes no cat.
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Old 09-12-2004, 06:12 PM
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Quote:
Originally posted by club327
I assume you mean the distributor advance springs?
No, The step up springs Push up on the pistons that lift the metering rod up in order to have the rich step in the jet rather than the lean step. Vacuum pulls the piston down against the springs. By installing a stiffer spring the metering rods move to the rich step with less throttle opening.
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Old 09-12-2004, 07:14 PM
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The springs should be color coded and you can pickup a kit of springs for like 8 bucks from most auto stores.
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Old 09-12-2004, 07:46 PM
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ok thanx for the info but i just noticed another strange noise its like a tis tis tis tis tis tis really fast tho when i hit the gas then let off it really quick thats when i hear it starts out kind of loud then get quieter is this a sticking lifter or do i just need to replace the springs to make them stiffer?
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Old 09-12-2004, 08:03 PM
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A broken valve spring or a flat lobe on the cam can both cause a popping back through the carb. Not uncommon on 350 sbc
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Old 09-13-2004, 10:21 AM
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Lean pop

Well, I found one vacuum leak. It was a cracked vacuum port plug on the front side of the carb. But I must have another because I'm still getting the lean pop.

I guess the next step will be to spray some carb clean around the carb while she is running, then listen for the engine to rev up.

Anyone have any other suggestions?
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Old 09-13-2004, 12:21 PM
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Start there and see what you get. Just keep it simple. Most problems aren't that complex. Its the little things that normally get us.
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