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Old 04-12-2011, 01:49 PM   #1
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vapor lock or flooding?

After a rebuild and swapping to an MC 2100 Carb, I'm now having an odd issue that has me stumped. The Jeep runs and idles great, however once it gets up to operating temp it won't idle in any drive gears for very long (auto trans). It will idle all day in park but if you spend more than thirty seconds in drive with your foot on the brake it will start to stumble. The stumbling grows progressively worse and after about a minute it will finally stall the motor. If wait 45 seconds or so then try to take off (green light) it will immediately stall. Once it does it takes 10 minutes or so, then it will start right up and run fine again until the next time you stop. I suspect its either vapor lock or its somehow flooding at idle. Once it stalls, I've pulled the air cleaner and the Carb still squirts gas when you touch the throttle though. The routing of my fuel lines never changed during the rebuild either. I'm about to lose my mind so any help would be GREATLY appreciated.

1990 YJ
4.2L
HEI Ign. Upgrade
Mc2100 Carb, 1.08 venturis, #46 jets (that's what's stamped on them)
Nuttered
12* BTDC timing, triple checked with vac. Advance line plugged
Carb tuned with vacuum gauge, ~16 inches peak vacuum
No vacuum leaks (had an intake manifold leak so spent a week checking every vacuum line on the thing)

Thanks again folks, any suggestions or crazy ideas would help, no matter how dumb.

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Old 04-12-2011, 03:44 PM   #2
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Does this carb have idle mixture screws?

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Old 04-12-2011, 03:53 PM   #3
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don't know if its related but I tell ya, it sounds like the same symptoms of fouled spark plugs. They'll idle all day long until you touch the throttle and blaaahhhh... engines don't like that.

If it is related to the carb, it would be that the carb is dumping way to much gas in.
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Old 04-12-2011, 05:53 PM   #4
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Yes, it has idle mixture screws. I tuned it for peak vacuum with a vacuum gauge. Its about 2 and 3/4 turns out each.

I've changed the plugs, rotor, cap and wires. Didn't help any. I thought the same thing though LOL.

It runs fine all day long on the highway, its just idling in drive that it does this. Its got me stumped lol
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Old 04-12-2011, 07:34 PM   #5
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ive never tried it but ive heard if you have vapor lock issues you can put a wooden clothes pin on your fuel line to remedy it, maybe at least long enough to see if that works, and then of course fix it right. im sure there are other things for that now and when i was told that trick (25 years ago) i was into the hot rod stuff so folks had to to things of that nature all the time.

and as always, im no expert so feel free to call me an idiot, if i tell you wrong
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Old 04-13-2011, 12:46 AM   #6
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I should probably also mention that the EGR system, pulse air system, et cetera have all been done away with. The charcoal canister is gone completely. The EGR system has a block-off plate that the PO installed behind the EGR valve against the intake manifold. There is a small exhaust leak in one of the pulse air tubes, but that system too is disconnected, so aside from the noise I doubt that would cause this. Just wanted to mention all of this, perhaps its relevant to something I'm overlooking.
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Old 04-13-2011, 12:59 AM   #7
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I'd say get it to stall out in your driveway and pull a plug and see if it's wet with gas. If it's wet, it's probably loading up at idle and the screws may need to be turned in a quarter turn.
If the plug is dry, your starving for fuel. A quick and dirty way to check for vapor lock is to pull the fuel line off the carb after it stalls and while it's still hot. If liquid comes out, vapor lock probably isn't the problem.
I had an old diplomat with a 318 that had vapor lock problems. Clothes pins helped, but a length of garden hose cut to cover the fuel line from the pump to the carb is what cured it
Oh, now that I think of it, I had a bronco that started dying out at idle. It turned out to have pinholes in the fuel line from the tank to the pump and it wouldn't hold prime at idle.
Oh crap, now I'm just rambling, anyway hope this helps and good luck.
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Old 04-13-2011, 09:38 PM   #8
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is your float level set properly? i would turn your mixture screws in a little also
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Old 04-14-2011, 10:46 AM   #9
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Checked the float level and it was fine, let it stall again at idle with the top off the carb and it never stopped receiving gas, so that rules out vapor lock. One of the things I was considering is perhaps a vacuum leak in the brake booster. My brakes are, of course, soft and I get the "whoosh" sound (as my fiance calls it) from under the dash when pushing the brakes, but pumping the brakes never stalled the motor. Any thoughts, or suggestions on how to further test that theory?
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Old 04-14-2011, 10:58 AM   #10
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All of my Carb experience says its the choke you might want to revisit that just to be sure, also one poster above mentioned leaks in fuel line and losing prime at idle that is possible.
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Old 04-14-2011, 11:24 AM   #11
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that was my second place to go was the choke. i would check it also now that you are sure fuel is not the problem
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Old 04-14-2011, 09:41 PM   #12
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When I installed the carb I just played with the electric choke until it closed completely and firmly when cold and was fully and firmly open when warm. Just double checked it, and its fully open when warm. I dont think the choke is it, but out of curiosity how could the choke cause it to stumble only when in drive or reverse and stopped? On the carter when I had choke issues it just stumbled and wouldnt run right when you got on the throttle (manual choke cured that nicely).

Learning as I go here, so any info helps folks. Please keep the suggestions coming, I'm at a complete loss on this one. I will check for fuel line leaks tomorrow in the light if i get the chance.
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Old 04-14-2011, 10:15 PM   #13
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Most 2100's in stock set up had a thick thermal spacer under them. If you are not running a stock gasket set up under it...meaning 2100 stock gasket set up, Make sure your gasket fits the carb properly. Make sure you dont have a vacuum leak. Then tune it again.
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Old 04-14-2011, 10:21 PM   #14
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I've actually been through that already LOL. Has the thick gasket, and I ended up putting fuel-proof RTV between each layer. No more vacuum leaks anywhere ( though I'm considering the possibility of a faulty brake booster, but not sure how to verify that) and it was retuned after.
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Old 04-14-2011, 11:37 PM   #15
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I found another, older, thread where a grand cherokee was having a similar problem and it ended up being the computer. I've nuttered the jeep, but the computer is still behind the glove box. Regardless, at this point the computer doesnt have any control at all over the motor, does it?
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Old 04-15-2011, 08:11 AM   #16
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i don't think so. i think all it does now is send fire for the dist.

google how to check your brake booster. mine is bad but i don't have the trouble you have even though i have the same hissing noise when braking
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Old 04-15-2011, 10:31 PM   #17
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I've been at work all day so haven't been able to try a few of the more recent suggestions, but a thought occurred to me I wouldn't mind some opinions on.

When the previous owner installed the block off plate he just installed a plate between the egr and intake manifold. The egr pipe is still there, and has a hole in it. Is there any way the egr could cause it to stall when idling in drive for extended periods?
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Old 04-18-2011, 11:38 PM   #18
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Alright, so I've been playing with it and I'm STILL having the same problem. On top of that, when I pulled the intake off to fix that leak, I now have an exhaust manifold leak. I found one of the studs loose and tightened it and all the bolts down but its still there, just not as loud. I'm thinking I may have to pull both the exhaust and intake off and replace the gasket again, but it was an unimaginable b*tch to get the exhaust manifold back on last time so I'm not looking forward to that one.

This still has me stumped. I have the next two days off so I'm going to try to spend a lot of time troubleshooting the two issues. Any suggestions you guys can provide of other things I can try would be greatly appreciated. As stated, I'm pulling my hair out here and there is the possibility that its something small I'm overlooking, so feel free to give me ANY ideas.
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Old 04-19-2011, 08:26 AM   #19
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Did you check the float and are sure the float is set to the correct hieght I think it should be a 1/4 inch from the top in the full up position.
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Old 04-19-2011, 09:26 AM   #20
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I checked it, but I was under the impression its supposed to be 1/2" from the top... that could perhaps be why. I was basing mine on the info for a 66 mustang with the 302 and MC2100.
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Old 04-19-2011, 11:30 PM   #21
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Anyone know what the correct setting for the float in an MC2100 is? Is it 1/4"?
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Old 04-20-2011, 08:09 AM   #22
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I pulled out a old carb kit for the Carter-Weber 2731 and yes it is 1/4 I was prety sure but wanted to double check

p.s. make sure you lay a straight edge accross the bowl when measuring down dont eye ball it. also one of the most important thing is when setting the top cover back on make 100% sure you get the 2 needle pins back in the metering holes a lot of people will have them not back in the holes and tighten down the lid and bend them and you wont even know it and the carb will never work right if those get bent.
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Old 04-23-2011, 05:05 PM   #23
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Alright, bit of an update. I hooked the charcoal canister back up, as the previous owner disconnected it. Next chance I have I'm going to check and see if that helped the stalling any. I had to replace the intake and exhaust manifold gasket as it was leaking. A correction to what I previously posted though, the Carb screws are about 3.5 turns out each and if I set it to 2.5 each it runs real rough and wants to die. If I try to turn it in any from where its at, its the same thing. Could I still have an intake leak somewhere? What would cause it to need to run so rich? I sprayed Carb cleaner all around the Carb and couldn't find a single leak. Any thoughts at all would be greatly appreciated.
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Old 04-24-2011, 02:13 PM   #24
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Alright so its still flooding at idle, despite hooking up the charcoal canister (I knew that one was a long shot, but I'm willing to try anything). Verified its not vapor locking again by pulling the top of the Carb off. Checked the float level and its a little low if anything. Turned the idle mixture screws in about half a turn and it was running pretty rough, and would still stumble at idle, but it never stalled. Back them out again and it runs super smooth but will start to stumble after idling for a few then stall. Given that, I checked the timing again and set it at EXACTLY 8* BTDC ( was 9*), still no help. Any other suggestions? Is there something I'm missing here?
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Old 02-07-2012, 09:17 PM   #25
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Sounds like a clogged cat. Just a thought.
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Old 02-07-2012, 11:32 PM   #26
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Actually turned out to be a bad ICM in a bad location. Ive since switched to using the ford ICM thats only $20 at autozone and has a lifetime warranty and relocated it to the fender well so it gets more airflow. No problems since.
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Old 06-22-2012, 01:30 AM   #27
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do u have a part number for the ford ICM?
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Old 06-22-2012, 08:10 AM   #28
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Float level for that carb is 7/16" +/-.

If I had to guess, I'd say the flooding was due to 2 possible causes, (if you've ruled out fuel boil):

1) bad needle/seat.

2) excess fuel pressure.

I've got that same carb in a warm climate & the only related problem is heat soak after I shut it off for 10 mins. I swapped in the thick, rubber grommet Ford gasket under the carb & it cured the problem.
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Old 06-22-2012, 03:28 PM   #29
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the icm, hmmm. it would have been way down on my list of htings to check. glad you got it fixed, that was a tough one

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