MC2100 Sudden Loss of Low Idle

[84_CJ7]

Here's the Reader's Digest condensed version.

84 CJ7
258 - Nutter Bypass - Stealth HEI - 8mm wires - Large Cap - TFI E-core - MC2100 - EGR blocked/sealed

Rebuilt my MC2100 that had been sitting a while. Mounted and ran fine.
I tried an HEI distributor for a few days and decided to go back to my Nutter Bypass/Stealth HEI setup.

Retimed 1600 @ 9BTC and set idle to 650.
Manifold vac @ 21
Air fuel mix @ 2 1/2 turns
Carb float @ 9/16
Oxygen sensor/turned air fuel meter has normal mixture
Electric choke - working
Plugs gapped @ .45 - cleaned
PCV clean
(Had a problem with needle and seat after a fuel filter swap - it has been remedied by swapping back - allowed too much pressure)

Now the problem

Without warning it started idling down, stumbling and struggled to stay running
I could keep it running by giving it gas
The choke was completely off
If I let the motor stop it would easily crank right up and idle high for about 20 seconds before it started stumbling
It will stay idling as long as I keep it @ 850 rpm
If I let it drop below it begins to die

I can't find any vacuum leak - possibly due to the high idle

Any thoughts? Tricks?

[jdoggmoney]

Correct me if I'm wrong, but with the geyser you suspected too much fuel, correct? Is this still the case or is the engine starving for fuel now?

If you hold your hand over the carb and choke it off slightly will it run better or run longer?

I am going to bet some sort of vacuum leak. Probably in one of the base/adapter gaskets. One thing I did to help make sure things seal up is to move to a Ford application base gasket. 1982 E150 van with 302. This gasket is about 1/8 thick and seals very well.

Keep plugging away... I predict this will be one of those problems that turns out to be something very simple.

[84_CJ7]

Originally Posted by jdoggmoney

Correct me if I'm wrong, but with the geyser you suspected too much fuel, correct? Is this still the case or is the engine starving for fuel now?

The geyser has been fixed. The glass inline fuel filter I put in for viewing seemed to be the culprit. I put the stock filter with the two outputs back on and it has solved the problem.

Originally Posted by jdoggmoney

If you hold your hand over the carb and choke it off slightly will it run better or run longer?

No. It will choke out and die.

Originally Posted by jdoggmoney

I am going to bet some sort of vacuum leak. Probably in one of the base/adapter gaskets. One thing I did to help make sure things seal up is to move to a Ford application base gasket. 1982 E150 van with 302. This gasket is about 1/8 thick and seals very well.

I replaced all of the gaskets at the start of this problem. RTV on the manifold to adapter, and the 1/4 inch base gasket from adapter to carb. My EGR is sealed with 3/8 aluminum plate - that I took off and resealed.

The high idle needed to keep it running makes it real hard to pinpoint anything. I am beginning to think about the intake/exhaust manifold area. It's the only place that hasn't had gaskets replaced recently.

[Caver Dave]

You haven't mentioned *IF* the "rebuild" was last week, last month, or last year, so...

I'll say pull it off (so you don't drop anything down the bores/intake) and then pull the idle tree out as a minimum. While they're fairly robust, it's not unheard of for the tubes to get a clog of some kind. A length of tag wire run thru the holes should insure everything is open... Since you'd already have it off, might as well pull everything out (jets, needle/seat, idle mixture needles, etc.) and give everything a good blast of compressed air before re-assembly.

While I'm sure *I* caused most of the issues I had (NO choke, crappy fuel lines, etc), mine ran like a top most of the time, but when it didn't... required a field strip. Had it down to <15min start to finish (no biggy), but got tired of the inconsistency and went TBI! It NEVER ran this good with the 2100

[84_CJ7]

I have had the carb off several times this weekend - rebuilt it twice - once after a backfire just to be sure.
Like you, I have been able to pull these carbs apart and put them back together pretty quick. I swear this thing is clean - I've run wire, cleaner and or air through every hole on the thing. Even done a few Mexican tune-ups.
Looks like I need to pull it one more time just to satisfy my curiosity.

[cordova]

"Mexican tune-up"...whats that??? Sounds like it WORKS.

[84_CJ7]

It's a trail/backyard fix - sometimes even works.

Choke off the carb and let it flood itself to a stop.

Helps vacuum out any particles that might be stuck in any of the passages.

[cordova]

Oh yeah, I've actually employed that technique before. I wasn't aware of the technical term for this. I did this to my buddies sled once. He thought I was full of S-word until it worked to clear up his idle.

[84_CJ7]

Here's what I've done.

Pulled the carb, mounting adapter and gaskets. Pulled EGR block-off plate.

Removed ALL RTV sealant.

Made new gasket for EGR plate and reinstalled.

Mounted new manifold adapter gasket and carb adapter.

Stripped/blew out and completely rebuilt carb with NAPA kit.

Replugged vacuum hole for choke assist under carb.

Reinstalled carb and new thick gasket.

Replaced fuel filter (stock)

Fresh gas

Wet bowl just under 1/2 inch

Fired it up. . . .

Same problem. Only will idle high (850ish)

Great throttle response - no lag

It idles just a little rich according to the meter but the venturies didn't look wet.

Watched the venturies and my air/fuel meter when it started cutting out - it was on the lean side.

I would like to believe I have eliminated the carb/mounts/filter

Ideas?

Could I have an intake manifold leak or an intake manifold with so much built up carbon it is struggling?