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post #1 of (permalink) Old 06-29-2009, 09:43 AM Thread Starter
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Yet another MC2100 situation

88 YJ 4.2 A/T HEI ignition module, ford dist cap, base and rotor, MC2100 from early 80's Ford van(currently the one with the problem)
mixture leans out/engine stumbles during initial acceleration from any speed. (verified by an air/fuel ratio gauge).
situation and what I've done:
replaced my old junkyard MC2100 with a "new" junkyard MC2100. Got home and rebuilt it. old carb had #52 jets. New one has #48. It has a new 6.5 power valve. Installed carb. I DID adjust the accelerator pump linkage in each possible configuration, and test drove in each possible configuration. it IS squirting fuel when cycled. I tried to richen the mixture. Timing is at 10 BTDC (changed nothing besides the carb itself...well I adjusted the mixture, of course, using a vac gauge). All this and no change in performance. Old carb was replaced due to a vac leak at the throttle shaft, wasn't worth fixing, but ran better than it does now. Again, the only change i made was the carb. I'm thinking blown accelerator pump...but it's new as of two days ago. Any ideas?

'88 YJ
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post #2 of (permalink) Old 06-29-2009, 12:54 PM
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I had a very similar problem after running apparently for awhile with a leaky power valve. I'm guessing the carbon buildup had fubar'd the egr valve which got sticky, and wouldn't close every time after acceleration. Led to lean "bog" on acceleration, and crappy idle after a run. Took off the EGR valve cleaned it up good and my problem is gone (actually took a couple tries before it closed reliably). You can test the EGR in your driveway - just put a vacuum on the EGR nipple at idle - as it opens, the engine runs like crap. It's supposed to be open only at speed.

Is the diameter of your new carb the same as your old one (both 1.08?) If you grabbed a 1.21, along with your smaller jets, you'd have a very lean mix I'd guess.

If you found a vacuum leak around the throttle shaft on your old carb, you know how to find them. Make sure there is not some uplugged port sucking in unmetered air on your new carb. I found the other day that I've been driving for a year sucking in unfiltered air from a portal in the air horn.

Mine is a 1978 4.2, MC2100 1.08 with 47 jets and a 5.5 power valve (late opening). Runs like a scalded cat. Now I've just got to get it past emissions.

Last edited by zshooter1; 06-29-2009 at 01:05 PM.
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post #3 of (permalink) Old 06-29-2009, 01:28 PM Thread Starter
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EGR valve disconnected, port leak there. new power valve. definately a 1.08. when you say port at the air horn I am guessing you are talking about the diaphragm that has a passage way from the air horn to the base of the carb that uses manifold vac to assist the choke at idle. It's plugged. Otherwise, the only other port simply creates a passage way for unfiltered air into the throat of the carb, not having anything to do with the metering of air/fuel...besides its cleanliness. Either way, its capped off as well. Now, there is a port that uses manifold vac to pull exhaust air past the choke. It is not in use so it is plugged as well. Maybe that is what you are talking about? And like I said, nothing has changed except for the replacement of a carb. So I am 92.5% sure that it is the "new" carb (or a component of it) that is the problem. Thank you for your input though. I post this issue not as an easy way out to fix the problem, I do know how to trouble shoot. I post because I really don't want to go out and buy another rebuild kit just because I need an acelerator pump, if that is even the problem. So any input is taken into deep consideration. Thanks again

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post #4 of (permalink) Old 06-29-2009, 02:11 PM
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No vacuum leaks, no EGR problems, too lean probably means ignition is good. A 6.5 power valve is a "late opener" - doesn't open until engine vacuum falls below 6.5 hg. That's basically when you floor it, for the first few seconds you have WOT (as in passing cars on the highway). One other way to enrich your mixture is to get an "earlier opening" power valve - I think a 10.5 is the "earliest" Holly sells. Holley advertises the rule of hg/inch at idle divided by two, rounded down to the nearest .5 size. So for my jeep, idle vacuum is 23 divided by 2 is 11.5 - the power valve Holley would recommend is the 10.5 which would open up alot. I'm more in the economy mode - I'd rather save ga$ by cruising with just the jets - so I keep in the 5.5 power valve which opens up only on the highway for passing basically. By the way - the Checker Auto here in Gilbert has a huge selection of power valves, jets etc for that carb. (you can buy them online also).

I'd think your 48 jets should be fine - if you are at Yuma you're almost at sea level. Here in Gilbert (1200 feet), I actually ran 44 jets for awhile, but the engine is alot happier (and runs cooler) with the 47's. 44's were good for the mountains.

However, I found this post on another site: I am at sea level here in Virginia, the jet we ended up running were 55's. I tried a set of 47 than 49 and it just didn't run well. It would idle but no power and poor performance was easily noticed. Take it for what it's worth.

I highly doubt it's your accel pump since it's new and you can see it work. That basically just gets you out of the hole shot anyway. Sounds like you are lean well into midrange rpms. If you've got no air leaks, my best guess would be too small jets and/or a too late opening power valve.

All this advice may be worth exactly what you paid for it!!

Here is a pretty good source for MC2100 information.

Good luck.

Last edited by zshooter1; 06-29-2009 at 02:58 PM.
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post #5 of (permalink) Old 06-29-2009, 02:22 PM
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zshooter do you know of any place to snatch up a mc2100 in the valley? Im in fountain hills unable to find one.

1990 YJ 258 l6 3.5 BDS lift

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post #6 of (permalink) Old 06-29-2009, 02:44 PM
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Originally Posted by PHX? View Post
zshooter do you know of any place to snatch up a mc2100 in the valley? Im in fountain hills unable to find one.
You can find them in any pick n pull salvage yard. I prolly saw 20 of them when I was looking for mine - any older Ford pickup or van, or midsize sedan if I recall.

Try Ecology Salvage (Buckeye road and maybe 24th ave. - the one near the big onion shaped water tower). Lot's of the yards have sales going now- too darned hot for most sane people.

Good luck
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post #7 of (permalink) Old 06-29-2009, 03:26 PM
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Verified by an air fuel gauge? All the AF gauges I've seen aren't worth looking at. The only way is to measure the CO with an infrared machine. Even then it usually won't help with a stumble.

If you want, look up "propane trick" on here - a sticky - it will tell you in seconds if the stumble is from a lack of fuel. Shoot a bit of propane to it while accelerating, if the stumble goes away it was lean.

But - I doubt it.
You said the pump's working, giving a nice shot - make sure it starts shooting just as the throttle cracks open, not part way open. That's simply a linkage adjustment.

Two carbs, both with the exact same symptoms? What are the chances 10 more will be any different?

My feeling - ignition.
1. The TFI coil MUST HAVE A FULL 12 VOLTS TO IT AT THE + TERMINAL. Check it while running, not with the engine off, key on. The old ignition resistance wire or resistor MUST BE BYPASSED!

2. Check the advance - you said 10 degrees at idle - that must be with the distributor vacuum disconnected! The distributor vacuum MUST BE PORTED VACUUM - near 0 at idle, smoothly progressing as the R's are raised.

3. Use a timing light - vacuum disconnected - it should SLOWLY advance to about the low 20's at about 2000+ RPM. That's the mechanical advance.

4. Connect vacuum, check advance again - it should SLOWLY advance to about 35 degrees at 2800.

If in either case it "jumps" or "slams" from low to high advance - fix that.

5. Pull off a plug wire at the plug - look at the spark as it jumps to ground. It should be a scary sounding, snappy, blue/white spark at least 3/4 of an inch long. If not, find out why.

6. Plugs - make SURE the plugs are the ones the mfgr called out for the ENGINE, not the ignition, not what the parts kid said. USE STOCK ORIGINAL PLUGS designed to work with the heads. Avoid all the aftermarket trash, many of them will cause stumble or internal damage.
Gap at .035 - .040, not at .080 like some fools will tell you.

Stumble is rarely caused by jets - running troubles yes, but not tip-in sag or stumble.

Wrong advance, weak ignition etc are very good places to start looking.
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post #8 of (permalink) Old 06-29-2009, 03:56 PM
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I got a ton of stuff saved on the computer for the MC 2100, way too much to post up but are few items that might help that haven't been mentioned yet. Oh yea, most of my problems usually lead back to a vac leak. As for Jet size, use Rich's Propane trick.

It sounds like you are almost there with your MC2100 carb my friend. OK, here is how you tune an MC2100. Please note that you will need to do all of these steps in order without leaving any of the steps out. Trust me on this, I've completely tuned MC2100s before, only to end with a carb that was running like caca, all because I forgot to do something simple, like check the pump pressure. OK, here are the steps:
1) Check the fuel pump pressure. MC2100s like 4 to 5 PSI, no more and no less. Do this step first and don't skip it! Your carb cannot be properly tuned if the pressure is below 4 PSI or above 5 PSI.
2) Float level. You need to adjust the float dry. It's much simpler to remove the carb and turn it upside-down above your head to do this. Measure 1/8" from the end of the float and place a mark there. This is the spot where you will measure the height from. Next, turn carb upside-don as previously stated and measure the distance from the mark on the float to the fuel bowl flange. It should be 9/16" maximum. 1/2" is generally OK, but it is better if you can set the float to EXACTLY 9/16". If it is more than 9/16" the carb will want to run like poo.
3) Idle mixture: Warm engine to normal operating temp and set the curb idle RPM to around 650 RPMs for a standard tranny equipped Jeep. Using a vacuum gauge that is hooked to a manifold vacuum source, turn each idle adjustment screw gently 1/4 turn inwards until the engine starts to stumble and run rough. Back off each screw in 1/8 turn increments until the engine start idling smoothly again. It is important that you turn both idle screws equal amounts. If you have turned both screws equal amounts and then you turn one screw slightly and the engine starts to stumble, back off the screw to it's pre-stumble position and leave both screws alone. This is the best lean position. Try this and post the results.
Your MC2100 lovin' friend;

Mc 2100 & 2150

AMERICAN MOTORS (8 cyl) ............... ........... 1968-79
EDSEL (8 cyl) .................................................. 1959-60
FORD / MERCURY (6,8 cyl ) ............... ............ 1958-86
FORD TRUCK (6,8 cyl) ...................... ............. 1963-86
JEEP (8 cyl) .................................................. ... 1971-92
LINCOLN (6,8 cyl) ............................... ............ 1977-83

************************************************** *
The Ford/Autolite 2100 was first introduced in 1957 and was installed on applications till 1974, In 1975 it was revised to become the 2150 and were phased out in the mid 80's.
The number on the side in the circle is the venturi size, Ford made 8 differing venturi sizes of the 2100/2150 depending upon the applications used. Since this was the ONLY FORD two barrel carburetor used from 1957-the early 70's Ford installed it on MANY differing applications. As you have noted even AMC bought this carb from ford for its applications. The venturi sizes ranged from .98, 1.01, 1.02, 1.08, 1.14, 1.21, 1.23, and the hard to find 1.33
The 2100 is very easy to tell apart from the 2150 as the 2150 is labeled Motorcraft on the carb top and has a choke vacuum break extending from the rear of the carburetor. You can also tell the later 2100's (70's versions) by the "raised" float bowl section and the choke unloader located just aft of the choke flap. (sometimes not used in the earlier 70's models).
OF course all 2100/2150's will have the part numbers stamped on the drivers side front foot which will tell the applications and year of manufacturer.
The earlier 2100's are probably more desirable as they’re are simpler carbs and were not tuned with economy in mind as are the later ones. Consequently the later ones tend to run a bit leaner and are often more temperamental, plus often have extra vacuum accessories.
The 2100/2150 carburetor has been reputed to be the BEST CARBURETOR series ever built. The 2100's are basically two barrel versions of the famed 4100 4 barrel carburetor and many parts are compatible.
Any other questions about 2100/2150's feel free to write me
Bill White, White Automotive


"The Heep" 73 Commando, 258 I-6, T-18, Scout Dana 300 w/Tera 4:1 Kit, SOA, PS, Thru-dash Cage, Alcans, OBA, ARB's, HEI, Ft disc, MC 2100,Dents, Mud & Rust.
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post #9 of (permalink) Old 06-29-2009, 08:53 PM
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I don't have much to add, but it does sound like we are running a very similar carb. Mine is off a 1982 Ford E150 van. The first time I rebuilt it I actually took the tag into the parts store and they looked up the kit the old fashioned way... in a book. The Walker kit number I've used ever since is 15890. The carb has always run great minus a vacuum leak on the original install. Just in case you do end up back at the counter, it's an option if your using something else.

I believe the MC2100s are very forgiving and will run well even though you may not be running the same jets, adjustments as the next guy. For instance, I ran size 50 jets for the first few years. Plugs looked good and the exhaust didn't stink. But every time I mentioned it, I got bombarded with "dude you better jet down" comments.

I changed to 47s two years ago for a trip to Colorado and never put the 50's back in. It seems to run as well here now, and the plugs don't indicate a lean condition. It does run better at altitude though.

Is this the only hammer you've got?
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post #10 of (permalink) Old 06-30-2009, 01:19 PM Thread Starter
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WOW! I just spent the last hour writing a reply and it all went away somehow when I went to post it. Basically, I replaced the power valve with a 7.5. I must have forgotten to replace the old de-formed orange one way valve in the accelerator pump housing initially, unless rebuild kits come with 2 The shot is now stronger than before but still shoots just after a VERY slight movement in throttle plates. I don't think it is of a huge concern though, I could be wrong. I also replaced the 6.5 PV with a 7.5. BTW, when replacing a Power Valve, make sure to use the THICK circular gasket. The thin one will create a leak. Found that one out the hard way. It's not the original problem though. So once I got those issues figured out, and the idle mixture adjusted, I drove it and it did not seem to stumble quite as bad, at all really. But there is still a lack of's a Jeep, right? True, and I don't expect to run 9's on the quarter mile by any means but I believe there is more there...

RRich: you are one of the gentlemen I was hoping would reply with your seemingly endless knowledge...but you have me confused. I know quite a bit more than I may put off about engines and how they work, how to set timing, idle mixture, etc.(obviously there is a lot more to it than that). You said: "2. Check the advance - you said 10 degrees at idle - that must be with the distributor vacuum disconnected! The distributor vacuum MUST BE PORTED VACUUM - near 0 at idle, smoothly progressing as the R's are raised."
Ported vac.-got it, 0hg at idle, progressively gets stronger as R's increase. Manifold vac decreases as load increases. Therefore, ported is used for the distributor to help advance timing as R's increase.
that being said, why would having the port disconnected make a difference when setting initial timing advance? Is 10 a bad number to go with?
About mechanical and vac advance...should vac advance occur first, and then mechanical? Is that backwards? Or do they work together at the same time? I ask because the advance jumps in quickly, up to I believe 24*around 15-1600 R's, and then slowly raises from there to umm, not sure. Plugs are regular autolite gapped to .035. I never use "high performance" plugs. Always mfgr recommended.

Not sure if my carb is a 2100 or 2150 now. labeled motorcraft on the top of the carb, but stamped 1.08. I'm hesitant to say that it makes a difference, I don't think so.

Thank you to everyone who replied. Very useful. I need to change my profile, not in AZ anymore...I'm in Missouri.

'88 YJ
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