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Unread 12-28-2009, 05:59 PM   #1
EngChase
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1980 CJ7 
 
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MC 2100 mixture screws??

I know I'm a noob, so just bear with me here. I have a 1980 CJ7 w/ a 258 with a MC 2100 1.08 carb. It runs a bit rich right now, particularly at start up, so I tried adjusting the mixture screws. I have turned the screws all the way in (clockwise) and it doesn't affect the idle, still runs like it has 2 turns out at 600 rpm. Shouldn't it begin to starve the engine? Is there another issue? I haven't opened up the carb to see what size jets I have yet. Any help would be appreciated.

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Unread 12-28-2009, 07:07 PM   #2
JeepHammer
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Blown power valve and vacuum leaks.

Blown power valve will supply fuel to the engine when the idle mixture screws are closed off.
Screwing the idle mixture screws 'IN' is one of the ways to test the power valve in an MC 2100 that is pretty darned effective!

Power valve is located UNDER the float bowl, so taking the carb off is already in your stars...

Consider a rebuild,
About $13 to $18 for the rebuild kit,
Another $20 or so for the cleaning chemicals.

GET SAFETY GLASSES WITH LOTS OF PROTECTION!
YOU WILL GET CARB/BRAKE CLEANER IN YOUR FACE AND THEY WILL TAKE YOUR EYES IN A HEART BEAT!


You DO NOT have to remove the throttle valve (butterfly) linkage to rebuild the carb!
Just clean it in place and leave it alone.

Make sure you take the internal passage restrictions out so you can clean the passages!
Idle mixture screws, jets, accelerator pump hardware, inlet valve, ect. all need to come OUT so you can clean the passages with pressure cleaner!

Takes about an hour for an experienced rebuilder,
Should take a 'First timer' about 4 hours.

Just remember to take the parts off and lay them out IN ORDER!
Clean towels on clean work surface where the parts can lay undisturbed... (NO CATS OR KIDS!)

By laying things out in the order they came off, you aren't as likely to loose or forget anthing when you reassemble,

And take your gaskets off intact!
Most kits have gaskets & parts for several different versions, so you wind up with 'Extra Parts' left over.

If you take the gaskets off INTACT, then you can compare your old gasket with the verity of gaskets in the kit, and pick the CORRECT one!

DO NOT take the external linkages off if you don't have to, just clean in place!
All those set screws, shaft springs, ect. will be a NIGHTMARE to get back together if you don't know EXACTLY what you are doing!
And most guys don't know you need some specialized training to get things like throttle blades back in the bores correctly...
So it's better to clean them in place than to disassemble them.

Clean your parts one at a time, then replace in order to your lineup.
When you get better at identifying all the little screws and stuff, then you can throw things in a basket and 'Dunk' or 'Dip' them to clean them.
Better off with a single carb, or an inexperienced rebuilder to clean the parts one at a time, inspect them, figure out how they work and what they do,
And you will have a MUCH better understanding of the carb when you are through!

If you still have the carb tag on top of the carb, take that along with you when you order a rebuild kit!
Take my word for it, you will live and die by that tag if it's still on the carb!

Put it back on the carb when you are done!
KEEP THAT TAG!
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Unread 12-28-2009, 08:52 PM   #3
EngChase
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What would cause the power valve to go? This is relatively new reman carb.
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Unread 12-28-2009, 09:52 PM   #4
h2ojeep
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Quote:
Originally Posted by engchase View Post
What would cause the power valve to go? This is relatively new reman carb.
First off, X2 on what Hammer wrote

To answer your question, my "wiseguy" answer is who cares?, just replace it.

I would not want to spend a lot of time trying to track it down, it is cheep enough to just do a rebuild and for the most part elimate a bunch of problems. kinda like a bad valve stem, you will spend more time on the WHY? than just taking care of the problem. (trying to get away from the wiseguy image and on to the practical approach.)

New Power valves are standard in ALL carb rebuild kits (for the MC2100/2150) and a rebuild every 3-4 years just needs to be done. In the rebuild process you are going to eliminate some of the possible vacuum leaks and at least get to know your carb better.

Knowing you carb better: the MC 2100/2150 are easy carbs to rebuild.

The other thing you will need to know is adjusting the needle valve after the rebuild.

Hammer does it different than me (but he does it more often so what does that tell you, but you get to the same place) Screw your needle screws in all the way GENTLY, DO NOT FORCE, JUST LET THEM TOUCH AT THE BOTTOM. then back out EACH one 1.5 turns, prime the carb and start your jeep. Adjust your idle to about 750 RPM (NO CHOKE engaged) Attach a vacuum gauge to the Carb and adjust each screw OUT evenly until you get the max vacuum reading on the gauge. count your turns, both needles should be the same number out. Then re-adjust your idle back to 750 RPM.

Vacuum Lines:
Vacuum lines get old (like heater hoses and fan belts), and the cost of new lines is minimal, so I would consider just replacing ALL the lines, as opposed to trying to chase the leaks down.
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Unread 12-28-2009, 09:57 PM   #5
John Strenk
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Old age, poor rebuild parts, bad assembly and back fire have been known to damage the power valve.

Just so you know, the idle mixture screws really adjust the flow of premixed fuel and air into the engine under the throttle plates so they can be closed or almost closed. A blow power valve will also let in a mixture of air and fuel

Last edited by John Strenk; 12-28-2009 at 10:11 PM..
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Unread 12-28-2009, 11:35 PM   #6
EngChase
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Thanks guys, I'm just trying to get a grip on the "Just Empty Every Pocket" syndrome. Sounds like I'm about to get close and personal with a 2100.
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Unread 01-03-2010, 03:21 PM   #7
CJFF29
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Sounds like I am in the same situation. Will be workin on my carb soon also. Are there any books out there specific to the MC2100 to help identify parts throughout the rebuild process? You guys on here are awesome !!! Tons of usefull knowledge
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Unread 01-03-2010, 04:01 PM   #8
_Kyle_
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Quote:
Originally Posted by CJFF29 View Post
Sounds like I am in the same situation. Will be workin on my carb soon also. Are there any books out there specific to the MC2100 to help identify parts throughout the rebuild process? You guys on here are awesome !!! Tons of usefull knowledge
Any good rebuild kit should include an exploded view diagram that is labeled. However, they may not have "everything" on them as I ran into. So if you remove linkages, take multiple photos from every angle!
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Unread 01-03-2010, 04:18 PM   #9
thich
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the past few rebuild kits ive gotten from Advenced Auto ($20) had an instruction manual with a few different exploded views for different model carbs. its not the best in the world, but it got me through my first rebuild. i would buy the rebuild kit, look at it, and determine if you are over your head. i think you guys can handle it. dont forget protection like JeepHammer said. those cleaners are wicked nasty, and i would hate to hear of some one losing an eye because it wasnt mentioned. wear crappy clothes, gloves, and goggles. good luck.
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Unread 01-03-2010, 04:30 PM   #10
Mike Romain
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The Haynes manual has great carb blow up drawings and the exact directions to tune them that the good carb kits give.
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Unread 01-03-2010, 05:32 PM   #11
VACJ7
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Make sure you know the power valve rating. I think I run a 8.5 Hg PV in my 2100. And some have a single stage PV and others a 2-stage PV. The 2-stage has a deeper cover. You can use a holley PV on the MC2100, just not holley jets.

And download this PDF: http://street2mud.com/webfiles/MC-21...%20a%20258.pdf
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Unread 01-03-2010, 06:09 PM   #12
CJFF29
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Thanks for all the great info I will keep you posted on my progress. One or two guys on my shift at the firehouse have offered some help for beer, we will see what happens. Good luck Engchase.
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Unread 01-03-2010, 08:08 PM   #13
EngChase
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Thanks for the follow up guys. I'm going to get into to this week. I'll let you know how it goes.
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