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Old 07-16-2012, 07:27 PM   #1
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Brake pedel push higher idle

Really need some help here. I have a 89 YJ, 4.2L, MC 2100, all emissions are gone. I have adjusted the carb so when I initially start the Jeep, she idles at about 550 rpm. But when I push the brake pedal 2 - 3 times, my idle increases to 1000 Rpm. Can someone point me in the right direction?

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Old 07-16-2012, 07:35 PM   #2
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Vacuum, its got something to do with that i'm about 99% positive. Brake booster has alot to do with vacuum as well and could be the culprit.

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Old 07-16-2012, 08:54 PM   #3
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Normal operation, Sorry, your Jeep is not broken, but dont get pulled over with all that emissions stuff missing, they CAN write you a fix-it ticket, no matter where you are in the USA.
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Old 07-17-2012, 06:54 AM   #4
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Thanks guys for responding. I'm stubborn in some things and want to get my idle rpm's down. Shouldn't the rpm's go down when at a stop light?
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Old 07-17-2012, 10:19 AM   #5
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Quote:
Originally Posted by 2xs View Post
Normal operation, Sorry, your Jeep is not broken, but dont get pulled over with all that emissions stuff missing, they CAN write you a fix-it ticket, no matter where you are in the USA.
Trying to learn something here. Is that because he has the MC 2100? My stock carb on my 89 4.2 5spd. doesn't do that. After a few seconds at a stop the rpm's go down to normal and also after letting out the clutch a bit. My initial thought was vacuum.
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Old 07-17-2012, 12:35 PM   #6
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I am not certain if it matters or not, but I did not do the Nutter Bypass. I removed the old Carter carb, all emissions components and then mounted the MC2100 along with timing adjusted to 9 degrees brdc.
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Old 07-17-2012, 12:43 PM   #7
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mine does it also but i can hear a vacuum leak when i do it so i know the diaphragm in my booster is leaking
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Old 07-17-2012, 12:45 PM   #8
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that is just one of the characteristics of the MC carb. Mine did the same thing...although having your idle way down at 550 seems mighty low to me...700-800 is better.
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Old 07-17-2012, 01:21 PM   #9
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Thanks so much for this'd feedback guys. Maybe I'll rebuild or replace the booster. Is is viable to rebuild or should I plan on replacing?
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Old 07-17-2012, 03:20 PM   #10
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While your foot is on the brake, it takes a certain amount of vacuum to boost the pressure your foot is putting on the brake to keep it easier to hold the vehicle stopped.

basically the brake Vacuum booster is using vacuum from your engine to "Boost" your braking ability, the amount of vacuum being used causes a measured vacuum leak Causing your idle to raise, its a normal operation of vacuum brake booster systems.

Similar to a power Steering Hydroboost system that uses power steering hydrolic pressure to hold the brake, in vehicles like a Chevy Astro, or one ton truck, you push the brake and hear your Power Steering pump working....

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http://www.wranglerforum.com/f330/2x...ad-490865.html
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http://www.svrider.com/forum/showthread.php?t=280873
Old Jeep that was totaled (and SOLD!):
http://www.wranglerforum.com/f330/2x...ad-101879.html
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