Scrambler with MC2100 and HEI having issues - JeepForum.com
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Unread 12-23-2014, 01:41 PM   #1
2k2wranglerx
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2003 TJ Wrangler 
 
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Scrambler with MC2100 and HEI having issues

Hey guys,

I am driving this around diagnosing some issues for a buddy of mine:



anyway, it has an HEI and MC2100 that came on it. the carb was rebuilt in 2012 (or remaned as per the sticker on it) and it came with the HEI already installed.

Stock 258 motor but it has a CAI to accommodate the carb.

the throttle will stick for a second at WOT and the e-choke sticks sometimes.


but the biggest issue, is when you try and get on the gas, it falls on it's face. My guess is it's when the secondaries kick in it's falling on it's face. It always smells rich. And it runs great at 1/3rd throttle. you can drive around all day not lugging the engine. But as soon as you hammer into it, it just won't go. It'll keep running... sometimes it'll chug, spit, go flat, but it'll keep running.

Im thinking about running through the carb again, but i wanted opinions. My 85 with a MC2100 ran like a raped ape compared to this thing.

It does have the vacume advance hooked up to the distributor. I haven't tried ditching that yet. And it has the EGR valve on the manifold (also not hooked up to anything).

it's been tuned up more than once with plugs and wires trying to diagnose (before me).


anyway, it runs rich. a lot. it falls on it's face when you get on it. And it sometimes will sputter and die when lugging the motor and have to sit before it'll restart. It all sounds like over fuel to me.

Any thoughts before i go through the carb? Should i look at a fuel pressure regulator and gauge to see where i'm at?

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Unread 12-23-2014, 08:43 PM   #2
Mortgage-payer
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Check vacuum first.
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I call it a distributor, not a dizzy.
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Unread 12-23-2014, 09:13 PM   #3
Hbryant123
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I would say check the timing and make sure it is where it needs to be, for my 304 its 8 degrees before top dead center. I have had alot of mc2100s apart and messed with them alot. Running rich i believe can be the Power valve being busted from a back fire, which may have been caused by incorrect timing. On the front their is the two idle mixture screws, if you can screw them in without the engine dieing that is an indication the power valve is blown. I would also check the pump shot to make sure it is working well and is adjusted in. I dont think a mc2100 has secondaries, i think you may be talking about a quadrajet.
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Unread 12-23-2014, 10:21 PM   #4
CSP
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Yes, the 2100 won't have secondaries. A secondary is on a carb with multiple venturies that one (or two) open before the other(s). A non-double pumper 4bbl carb for example. Q-jet is one of many of this type.


Sounds very much like a vacuum leak or bad power valve to me.
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Unread Today, 10:09 AM   #5
2k2wranglerx
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Quote:
Originally Posted by Hbryant123 View Post
I would say check the timing and make sure it is where it needs to be, for my 304 its 8 degrees before top dead center. I have had alot of mc2100s apart and messed with them alot. Running rich i believe can be the Power valve being busted from a back fire, which may have been caused by incorrect timing. On the front their is the two idle mixture screws, if you can screw them in without the engine dieing that is an indication the power valve is blown. I would also check the pump shot to make sure it is working well and is adjusted in. I dont think a mc2100 has secondaries, i think you may be talking about a quadrajet.
Quote:
Originally Posted by CSP View Post
Yes, the 2100 won't have secondaries. A secondary is on a carb with multiple venturies that one (or two) open before the other(s). A non-double pumper 4bbl carb for example. Q-jet is one of many of this type.


Sounds very much like a vacuum leak or bad power valve to me.
Thank you guys.

after a little more research i think this is a mc2150 because it has the fuel return line.

can a blocked return cause issues like this since there's no FPR?

Thank you for walking me through this.
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Unread Today, 10:27 AM   #6
Mike Romain
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Have you tried running with the gas cap cracked open. This makes sure the gas tank vent hasn't been messed with or has a dirty air filter on the bottom of it. (Charcoal canister)
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread Today, 10:33 AM   #7
2k2wranglerx
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Quote:
Originally Posted by Mike Romain View Post
Have you tried running with the gas cap cracked open. This makes sure the gas tank vent hasn't been messed with or has a dirty air filter on the bottom of it. (Charcoal canister)
no, i'll give that a shot
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Unread Today, 11:01 AM   #8
2k2wranglerx
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interesting... this carb apparently is the 1.23 venturis...

i'm wondering if it's too much carb for the stock motor... some threads say their stock 258 runs like crap with the bigger venturis, some say it's fine.
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Unread Today, 11:33 AM   #9
Mike Romain
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Well, its a V8 carb right. I haven't heard of them working well on a stock 6. Might be the issue. Too much in and no place to go.
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread Today, 02:44 PM   #10
JeepHammer
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Falling flat when you stomp on it is almost exclusively Accelrator Pump.

From a standing start,
Or idling rolling start with a manual transmission,

If it chokes right away for a second, then takes off,
It's a late pump shot.
The Accelrator pump linkage in most cases (but can be the pump/diaphram) simply isn't adjusted to start the fuel into the carb fast enough for the amount of throttle opening.

It can also be an internal valve letting fuel escape back into the float bowl instead of being forced into the Venturi bores of the carb.

-----

If it tries to take off, then falls on its face,
The pump shot isn't lasting long enough.

This is usually one or both of two things,
The linkage is already partly depresses, so the pump is partly through its stroke when you start, and can't deliver a full duration fuel shot,
Or the internal valve in the Accelrator pump is letting some of the fuel back into the float bowl.

Short pump shot (Duration) can also be caused by weak spring in the Accelrator pump.
The spring/diaphram is what refills the pump when you let up on the pedal,
So weak spring or old/stiff diaphram will short the pump duration by not fully refilling the cavity there is supposed to be fuel in.
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