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Old 09-19-2011, 12:50 AM   #1
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mc2100 stumbles at 12-1500 rpm

so, just replaced the carter with the 2100... I had finally got the engine running real nice with the carter... finally had a smooth idle, no vacuum leaks and low idle at around 6 and high at just under 1k. with new carb Idle has improved dramatically the engine purrs and is nearly silent where as the carter would stumble occasionally and was much louder. my main issue with the carter was it ran extremely rich, I get little to no exhaust fumes of unused gas with the 2100. plus the idle is steady at 6 and high idle at 950. I can pull up hill from low idle in first but when i hit the gas revs up to just over a thousand and i can feel it start to stumble and then it farts at 1200 to 1500 rpm. this doesnt happen all the time but it happens often probably most of the time. i was thinking it could be an advance problem but i don't get a wiff of any unspent fuel...
I have to put the clutch in and feather it out.
doesn't ever happen in nuetral.

any ideas?

thanks
zig

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Old 09-19-2011, 08:29 AM   #2
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little bump posted late last night!

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Old 09-19-2011, 09:27 PM   #3
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Could be an accelerator pump issue. It richens the mixture momentarily when you push the go pedal. Did you rebuild the carb? Could either need to be replaced, or need to be adjusted.
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Old 09-19-2011, 09:29 PM   #4
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so, i thought i had sorted out the problem... re fit everything double checked all possible vacuum leaks sprayed around the base with propane and no change in idle... new tubes and connecters. jeep tends to run fine when warm... although i was cruising today and it did it to me in fourth... same run on the tach. anybody out there have this problem before???
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Old 09-19-2011, 09:30 PM   #5
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Could be an accelerator pump issue. It richens the mixture momentarily when you push the go pedal. Did you rebuild the carb? Could either need to be replaced, or need to be adjusted.

I just bought the kit off the guy from ebay. apparently it's rebuilt. I used the adapter and gaskets he sent in this pretty much bolt on kit.
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Old 09-26-2011, 10:44 AM   #6
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still having the same problem anyone out there have any more insight?
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Old 09-26-2011, 11:08 AM   #7
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Mine did this with the MC carb when I first bought my YJ. I cleaned the carb, adjusted the idle, replaced the fuel pump along with a leaky fuel hose, swapped in an HEI with new plugs and wires, and adjusted the timing. This cured the stumble, each fix improved the issue a little, but the biggest improvement came from cleaning the carb and adjusting the idle... After this, it still ran a bit rough and would stumble a bit until the ignition upgrade went in.
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Old 09-26-2011, 11:21 AM   #8
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have hei, new fuel pump and replaced almost every vacuum hose the idle is perfect. I don't want the idle any higher and i've tried it and it doesn't even help the problem a little. should i consider a fuel regulator? I've been running sea foam in my gas too... only one fill up so far though. i can't really drive because that's where most of my driving is done in 1500 - 2000 range.
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Old 09-26-2011, 12:20 PM   #9
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On a 90 I6 you should have the mechanical pump, no need for a regulator. Do you have the inline filter with the fuel return? Have you tried a new filter? Have you checked the rubber sections of your fuel line? These can get soft and collapse.
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Old 09-26-2011, 03:00 PM   #10
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new rubber sections for the fuel line... same filter, but was replaced just before i got it. pretty sure wasn't lying cause it looked nice clean and brand new. which was unusual for everything else under the hood that was coverd in oil.
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Old 09-26-2011, 04:40 PM   #11
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okay, so screwing around with this frikking carb... if i tighten the throttle cable a bit so that it gasses it to 800 rpms the soft spot in the powerband comes in at 1000 - 1300 rpms. which is a little better, but i really like having the idle that low, it sounds super aggressive and it really helps with my mpg's considering i'm doing mostly city driving. what the hell is going on with this thing!?!?!?!?!
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Old 09-26-2011, 04:42 PM   #12
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also, if i tighten the idle screws i can get the idle to drop slightly but pulling such strong vacuum through the butterfly plates that it whistles loudly and annoyingly.
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Old 09-27-2011, 03:26 AM   #13
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I would check a couple things. First check your timing. Start with the factory setting (I think its around 8 degrees)and disconnect your vacuum advance canister and cap off the carb port to it while you check the timing. Set your idle speed low so there is no advance from the centrifugal advance. Once you know the timing is set correctly reconnect the vacuum advance and make sure it is working. Turning your mixture screws clockwise makes the mixture leaner and counter clockwise richens. Turn both screws in all the way until they seat (dont not over tighten) and then back them out one and a half turns which is a good starting point. Leaner mixtures burn slower and need more advance, richer mixtures need less advance. If you have a vacuum gauge it helps set the mixture. Connect it to the port that you hook your vacuum advance to and adjust your idle screws till you find your best vacuum.

Here is a good MC 2100 write up incase you need to make further adjustments.
http://street2mud.com/webfiles/MC-21...%20a%20258.pdf
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Old 09-27-2011, 11:01 AM   #14
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after re setting the throttle, I readjusted timing and set idle back to 600... Should i be able to feel vacuum from ported vac when i hit the throttle? cause i feel like i'm getting little to no vacuum there. although I'm assuming the dizzy is still advancing because i get no knocks or pinging or bogging all the way up to 4k, and I never take it that high... but it seems to be working fine.

This writeup has fewer vacuum nipples than mine. it's missing two that are on the rear of the carb facing the driver. plus, shouldn't i have the advance hooked up to ported so i don't get any advance at idle? when i have it hooked up to manifold vac my idle gets awfully rough.
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Old 09-27-2011, 08:26 PM   #15
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Quote:
Originally Posted by quadrophine View Post
after re setting the throttle, I readjusted timing and set idle back to 600... Should i be able to feel vacuum from ported vac when i hit the throttle? cause i feel like i'm getting little to no vacuum there. although I'm assuming the dizzy is still advancing because i get no knocks or pinging or bogging all the way up to 4k, and I never take it that high... but it seems to be working fine.

This writeup has fewer vacuum nipples than mine. it's missing two that are on the rear of the carb facing the driver. plus, shouldn't i have the advance hooked up to ported so i don't get any advance at idle? when i have it hooked up to manifold vac my idle gets awfully rough.
I prefer to use manifold vacuum when setting up a carb. Ported vacuum is used to keep emissions down. If you set up your carb with ported vacuum you won't have much vacuum advance at idle and with a lean idle mixture you can use the added advance.
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Old 09-27-2011, 10:40 PM   #16
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Did you get the carb from Mike Hermann on Ebay? I got one from him and had the same problems. After my "30 day warrenty" period was up he never returned my emails. I ended up buying a new Holley 500 cfm carb and my Heep has never run better.

On another note, I've got an mc2100 for sale...
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Old 09-28-2011, 09:52 AM   #17
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Does it run rich with such a big carb... That's like twice the CFL of the Carter
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Old 09-29-2011, 10:32 PM   #18
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Does it run rich with such a big carb... That's like twice the CFL of the Carter
The engine will only pull as much air as it needs. It' capable of giving 500 CFM, I jetted it down from 74 to 68 and she runs great!
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Old 09-29-2011, 10:48 PM   #19
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I felt bad at first for leaving him neg feedback but the more ive been googling the more I see people passed off with him so maybe I dont feel so bad anymore. All I know is with the carter I would shift at around 2 k if I do that now the car parts until it dies. I have to make sure that I'm going to shift above that range an that really sucks. I cruise in my heep. Its effing loud and I short shift because the 4.2 Hs plenty of grunt in low rpm and is loud and really loud pst 3k especially if I'm gassing it, I can cruise at 3all day long but why not just shift up and save yourself a litte gas oh yeah because if I do she dies. Crap.
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Old 10-17-2011, 10:21 AM   #20
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okay, I took some advice and tried to run sea foam through a few tanks of gas and still no improvement. i've checked and rechecked vacuum, timing and fitting. I'm so fed up with this carb, i can definitely notice the increase in power but this low end is effin killing me. so i've been told to replace the accelerator pump and the power valve. my question is will these come in a rebuild kit and what rebuild kit should i get? there seem to be several different ones for jeep applications. thanks folks
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Old 10-18-2011, 12:04 AM   #21
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I still say buy a new carb....stop messing with the piece of $h%t Motorcraft, you'll be way happier.
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Old 10-18-2011, 08:22 AM   #22
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I still say buy a new carb....stop messing with the piece of $h%t Motorcraft, you'll be way happier.
how much did you get your holley for? and are there any cars out there i could pull one off of. is it 2 or 4 barrel? i wonder if they make a manifold that i could put six single barrels on. like they do on a lot of hot rodders.
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Old 10-18-2011, 12:13 PM   #23
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Maybe you should consider going with a fuel injection upgrade, better than a carb. Pricey yes, but IMO it's worth it if it makes the engine all that much more reliable and hassle free.
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Old 10-18-2011, 01:38 PM   #24
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to be honest, i like the nature of carburetors theres something organic about them.

I would really like to get this one working, i'd like to spend my money on a lift, as opposed to another carb. does anyone know which rebuild kit i should get for this? i got the carb off the ebay guy
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Old 10-19-2011, 09:08 PM   #25
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how much did you get your holley for? and are there any cars out there i could pull one off of. is it 2 or 4 barrel? i wonder if they make a manifold that i could put six single barrels on. like they do on a lot of hot rodders.
2 barrle, not sure if they come stock on any vehicle, paid around $300. 6 carbs sounds like too much adjusting to me
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Old 10-19-2011, 11:36 PM   #26
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Well poop same problem with my yj. I just rebuilt the motor and messed with the accelerator pump setting and it doesn't helP. Mine bucks pretty bad too hm
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Old 10-19-2011, 11:54 PM   #27
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Ive been using an mc2100 for about a year now and have had no issues at all. I got mine from the guy on ebay as well. My only complaint is the electric choke is annoying down here in the south. Choke is so rarely needed down here, but i cannot find a manual choke kit to fit the mc2100 at all. The only thing i can say is i get a similar sputtering when the choke is engaged unnecessarily, you may want to check that.
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Old 10-20-2011, 08:30 AM   #28
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i thought that may have been it as well... I do have a manual choke, and i keep it pinned open all the time. I usually only use it when it's really cold in the morning and just enough to smooth out the idle so it doesn't rumble itself out. I'm pretty sure it's either the accelerator pump or the power valve. it seems like it's just losing fuel, at about a quarter throttle.

Anyone out there know what rebuild kit i should go with? there are an awful lot of options but i have no idea which version of the 2100 i have. there is no tag anywhere on it. or what comparable truck i could say it was pulled from to get the rebuild kit.
I'm so insanely frustrated with this carb. the worst part is, i can tell that if it was running right it would be a nice upgrade from the carter.
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Old 10-20-2011, 08:57 AM   #29
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Originally Posted by tslroper
Ive been using an mc2100 for about a year now and have had no issues at all. I got mine from the guy on ebay as well. My only complaint is the electric choke is annoying down here in the south. Choke is so rarely needed down here, but i cannot find a manual choke kit to fit the mc2100 at all. The only thing i can say is i get a similar sputtering when the choke is engaged unnecessarily, you may want to check that.
they sell universal manual conversions that work for all carbs at autozone

Quote:
Originally Posted by quadrophine
i thought that may have been it as well... I do have a manual choke, and i keep it pinned open all the time. I usually only use it when it's really cold in the morning and just enough to smooth out the idle so it doesn't rumble itself out. I'm pretty sure it's either the accelerator pump or the power valve. it seems like it's just losing fuel, at about a quarter throttle.

Anyone out there know what rebuild kit i should go with? there are an awful lot of options but i have no idea which version of the 2100 i have. there is no tag anywhere on it. or what comparable truck i could say it was pulled from to get the rebuild kit.
I'm so insanely frustrated with this carb. the worst part is, i can tell that if it was running right it would be a nice upgrade from the carter.
I may need to go this route too darn.
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Old 10-20-2011, 09:04 AM   #30
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anyone on here rebuild there 2100 from ebay guy?
i really would like to sort this out

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