Join Date: Nov 2000
Location: Manitoba, Canada
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Re: YJ - MC2100 Questions.
If you pull one off a Ford, make sure it has a pcv connection on the carb. Some don't have it on the carb, and you need it to keep your crankcase ventilated and the environment healthy.
Here's the info I've gathered on the swap. It's worthwhile, it does all the things a jeep carb should, idles well, makes power, runs perfectly on inclines.
Motorcraft / (sometimes called Autolite ) 2100 Carb Install on 258 Jeep
Parts List: MC2100 Carburetor, adapter, air cleaner assembly, throttle arm replacement (fabricated).
This 2bbl carburetor will replace the troublesome Carter BBD and produce a superior idle. It also appears to boost horsepower. It has a good reputation for being reliable and good off road. I am very happy with the conversion and thank all of those jeep enthusiasts who helped me with information on this swap.
This conversion will not meet with any legal requirements that prohibit the removal of original equipment or the tampering with emissions control equipment. Check with your local authorities if your vehicle will be subject to inspections in this regard.
The cost of this conversion will be approximately $150 or less using a junkyard carb you rebuilt yourself, rebuild kit, new air cleaner, adapter plate, and manual choke kit. You may opt for buying a rebuilt carburetor, and I would suggest that you order one made for an AMC 304.
1. Motorcraft 2100 Carburetor – Sources: AMC 304 or Ford 302. Unfortunately Motorcraft does not stamp 2100 anywhere I can see on these carbs. The correct size will have “1.08” in a small circle cast on the drivers side of the carb body, somewhat obscured by the accelerator pump linkage. As difficult as this is to measure in the junkyard, the 1.08 is the size of the venturis (the barrels). I have been advised that #108 is 450 cfm, #114 520 cfm, and #121 575. Use the 1.08. This carburetor body is cast from two pieces, a top cover and a combination baseplate / venturi / float bowl. Apparently you can set / observe the float height by idling the motor with the top cover removed. The top cover has the word “Motorcraft” cast into it on the passenger side above the float bowl.
Be sure to replace the power valve on this carb. It’s function is to enrich the mixture as manifold vacuum drops (signaling load increase). It is closed at idle, and opens to add more fuel as the manifold vacuum drops. If it is not operating correctly, your mixture may be too rich. It is located on the bottom side (front) and is contained under a cap with 4 screws. There is also an accelerator pump on the front, also under a cover with 4 screws, but it is identifiable by the accell pump linkage. I have obtained good results using an oem 2 stage power valve. This valve is longer than the single stage and is original equipment. It is 39mm long from top to bottom and mine was labeled #16. I have been advised that this means that it will not enrich the mixture until manifold vacuum drops to 16 inches HG. I had to go to a carburetor rebuilder to get this part, it was $5.00. Take in your old one as a sample. A single stage may work fine as well.
2. Carburetor Adapter – It is the 2 barrel Rochestor to 2 barrel Holley adapter. Also known as the large 2 bbl to small 2 bbl manifold adapter. Summit sells TD Performance Products adapter no. TRD-2086 for $12.75. It comes with 2 gaskets. I replaced the slotted bolts that came in the kit with allen head bolts to make tightening the adapter to the manifold easier. I also used a hardening gasket maker instead of the gasket between the adapter and the manifold (Permatex Form a Gasket 1A). I also used a Ford gasket between the carburetor and the adapter. It is made of black plastic and looks like two gaskets sandwiched together with plastic bushings inside the stud holes. Is this gasket necessary? There is nothing special about it other than that it will seal very well and also provide a lot of heat insulation from the manifold. The hole in the adapter is slightly larger than the jeep manifold. I have thoughts to switch to an aluminum intake manifold from an 84 and blend the manifold to the adapter with a die grinder.
3. Air Cleaner Assembly – This carburetor has a much large air cleaner base than the Carter BBD. It is the standard 5 1/8 “ air cleaner base used on most 4 barrel carburetors like Holley’s. Unfortunately the close proximity of the power brake booster and master cylinder limit you to a 9 to 10” (9” preferred) open element air cleaner. If you swap booster to a dual diaphragm GM booster (a great upgrade) you will not be able to use a filter above the carb at all. A K & N 9” filter (2” height) with top and baseplate is $49.39 from Summit (pn. KNN-601110). I ended up fabricating an adapter to a K & N conical filter out of fibreglass.
4. Throttle arm – The OEM Carter uses a similar linkage, but pulls downward instead of the rear pull on the Motorcraft. You need to simply remove the throttle linkage ball stud from the Motorcraft, bolt on a small metal plate (1” by 3” and 1/8 thick) to the Motorcraft’s throttle arm with 1/8” bolts so it is rearward facing, and drill a new hole in that plate for the OEM throttle linkage. I would suggest bending a slight offset into the plate to move the ball stud slightly away from the carb to prevent binding of the linkage rod. In addition, bolt the plate on first then using the linkage rod, mark the plate for the new ball stud hole, remove the plate and drill the hole for the ball stud. Some people have removed the throttle arm from the old Carter BBD and bolted it on, but that seems like a waste of the old carb just to make a simple throttle arm with 3 holes in it. If you are not happy with this carburetor, you may need that old BBD back ( I doubt that will happen if you start with a healthy Motorcraft.)
5. Throttle return spring – I found that mounting the original downward pulling throttle return spring caused binding as it wrapped around the shaft. I instead fabricated a small bracket to bolt to a front bolt of the carb base to relocate the spring to the front.
6. Vacuum hoses – The motorcraft has two ported outlets. One on the front, just outside (to drivers side) of the drivers side venturi idle control screw. I used this ported outlet for my distributor. There is also a ported outlet on the passenger side of the carb base. I am not sure if there is any difference between these two. Mark all hoses you remove from the Carter BBD and connect them back up to the corresponding ports on the MC 2100. The only connection missing from the MC 2100 I used, compared to the Carter BBD is the vapour line running from above the float bowl to the charcoal canister for the collection of evaporated gas fumes. I would suggest plugging this line that ran back to the canister, as the MC 2100 vents the float bowl into the air cleaner housing.
7. PCV – There is a large PCV inlet port on the rear of the carb at the base. If you are looking in the junkyard for a MC2100 be sure that the version you take home is not an older one missing this port.
8. Idle Adjustment – Like most other carburetors there are two screws on the front base to adjust the mixture. Turning them in leans out the idle mixture. Never seat these screws with a lot of pressure at they are needles which will distort and damage the soft metal of the carburetor. I would start with around 2 turns out on each screw and adjust equally from there using a vacuum gauge attached to manifold vacuum to adjust for peak lean vacuum. In other words, turn them out till you get maximum vacuum, then turn them in till it starts to drop off, and back them out slightly from that point to regain peak vacuum. This carburetor is very forgiving for idle adjustments, so don’t sweat it.
9. Choke – There was an electric choke assisted by exhaust gas on both of the MC2100 I looked at. I tried the Motorcraft electric choke and it did not seem to work well so I used the Carter BBD’s electric choke by removing and reversing the bi-metal coil inside. This worked well till temperatures hit –30 degrees and I switched to a manual choke. I found the electric came off too fast in these cold temperatures and annoyingly came back on after short stops. The manual choke works well. The choke mechanism on my carb has a passage to the base of the carb where is picks up manifold vacuum. This draws heated air or exhaust from the larger fitting on the choke casting (which should also be capped off by you) and warms up the choke’s bi-metal coil. If you install a manual choke, you should plug the inner passage with epoxy or rtv sealant to reduce your idle speed and keep dirty air from entering the manifold.
10. Fuel line inlet – The original fuel line will work fine. You may want to replace the short metal section that screws into the float bowl with a brass fitting.
11. Ford Base plate – There is a cast aluminum base plate with a vacuum operated EGR assembly on it. These are common in the junkyards. As the oem gasket covers all of the bottom of the carburetor except the venturi openings I did not use the Ford base plate. I picked one up from the junkyard just in case. Using it will require lengthening the throttle linkage rod to compensate for its thickness.
12. Miscellaneous – The rebuilt carburetor I used has a small spring on the float assembly shaft which I am told is used to dampen the float movement for rough conditions. The other MC2100 I have for parts does not have this spring on the pivot shaft. Jets are probably available from Ford. They are the same size as used on the 4 barrel motorcrafts. After a winter’s driving I have examined the spark plugs and am happy with the jetting which is #47’s. Both MC2100’s from 304 AMC’s I have seen came equipped with #47 jets. I have found that for my application the accelerator pump linkage works best set to the third hole down from the top (of four holes) on the arm at the throttle shaft. It’s easy to tinker with to verify of this works best for you.