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Old 09-09-2010, 12:37 PM
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Confused after carb adjustment?

Hey guys, I have a 67 El Camino, with a Holley 4150 (670 cfm) carb, th350, headers and 270 Comp Magnum cam. I recently purchased a vacuum gauge and decided to fine tune the air/fuel on the carb. I want to say before I adjusted the carb, the car was running good and it had a really nice lope to it. The reading was 13'' when I started. I got the best results when I turned the carb screws out to richen it and wound up with 15''. I can tell you the car runs fantastic, but now there is hardly a lope in the idle. My questions are, why did the lope diminish and will it hurt anything to lean out the carb to the original settings to bring the lope back?

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Old 09-09-2010, 12:44 PM
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Lope was likely the rpm being lower due to the idle mixture being incorrect. You can adjust for highest vac OR listen for highest rpm. You could now lower your idle speed to get the lope back assuming the rpm did increase slightly. What's the in-gear rpm?
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Old 09-09-2010, 12:47 PM
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It's about 650-700 rpm.
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Old 09-09-2010, 06:07 PM
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You could try going back to the old settings to compare the rpm and see if there is an actual change (dont forget to change both in gear). My guess is that it was lower rpm. A good tune doesnt always have a thumpy sound though With a difference of only 2hg I cant see much harm in going back lean but according to the holley tuning its not your ideal setting. The current setting should give you overall better performance.

Do you recall how much you turned the mixture screws OUT?
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Old 09-09-2010, 11:05 PM
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Quote:
Originally Posted by bubbahotep
You could try going back to the old settings to compare the rpm and see if there is an actual change (dont forget to change both in gear). My guess is that it was lower rpm. A good tune doesnt always have a thumpy sound though With a difference of only 2hg I cant see much harm in going back lean but according to the holley tuning its not your ideal setting. The current setting should give you overall better performance.

Do you recall how much you turned the mixture screws OUT?
Yes, It wasn't much, only about 1-1 1/2 turns on both sides...
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Old 09-09-2010, 11:29 PM
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It had more lope because it was inefficiently tuned before and was struggling more at idle. I'll trade more power and better response for "lope" any day, even on a full race engine. Lope is actually an undesirable symptom of inefficiency, even though we like the sound.

Listen to a Pro Stock drag engine idle some day, they are not nearly as radical as you would think for the power they make, because they have a very correct tune on the carb, timing, valvelash, etc.
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Old 09-10-2010, 05:37 AM
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Quote:
Originally Posted by ericnova72
It had more lope because it was inefficiently tuned before and was struggling more at idle. I'll trade more power and better response for "lope" any day, even on a full race engine. Lope is actually an undesirable symptom of inefficiency, even though we like the sound.
X2 Well said.
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Old 09-10-2010, 07:26 AM
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1 to 1.5 turns is out of tune. Usually 1/2 turn beyond highest vac is ok, some carbs go 1 turn richer after highest vac but you weren't even at your highest vacuum yet.
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Old 09-10-2010, 02:42 PM
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OK guys, Thanks for all the input. I will keep the carb where I have it now at 15'', I really can't complain, the car is running really good.
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Old 09-10-2010, 03:00 PM
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My 383 used to sound so mean too.....before I tuned it correctly
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Old 09-10-2010, 03:03 PM
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Quote:
Originally Posted by ericnova72
Lope is actually an undesirable symptom of inefficiency, even though we like the sound.
Thanks for that Eric. Exactly what I've been telling newbies for years. The kids in my son's auto shop class in high school used to call me Mr. No-Cam because I tried to make sense to them to match the static compression ratio to the intake closing point and of course they wanted to use a Top Fuel grind in their stock dish-piston 350.

If you fellows want to see something interesting, find someone with a radical rump-rump street car. At night, remove the air cleaner and shine a powerful light at the top of the carb. You'll see a cloud of air/fuel mixture hovering above the carb that is being pushed back up the intake tract by the ascending piston while the intake valve is still open. This monkey-motion confuses the venturi so much that it has no idea whether to shmidt or go blind, so it cannot meter the fuel correctly and you get the rump rump until the revs increase and the efficiency comes up to where the motor smooths out. Cool stuff.

Last edited by techinspector1; 09-10-2010 at 03:08 PM.
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