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Unread 08-02-2002, 01:15 PM   #1
mpbspb
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258 Upgrades

I have a 86 Cj7 with a recently rebuilt 258 and was wondering what some good mods were for the engine. I dont do much aggressive riding but would still like a little more power than stock. The carb is original and it sucks(stalls at stops). Fuel injection is too expensive. What is the best replacement carb and what are some other mods that would help performance?

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Unread 08-02-2002, 01:30 PM   #2
johny1i
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There's a lot of pretty good information floating around related to this question. Start with a search for "nutter bypass" and "timing" in both the YJ (we had 258s until '91) and the CJ forums, then also check the "Engines" forum. Do you have emission testing in your area? If no, there are a pretty good number of options, some of them free (timing and vacuum advance source after you get the computer out of there).

Regarding your carb question, I put 135,000 miles on my Carter before converting to a Weber a few months ago and I am very pleased.

Start with this, and then come back if you have any questions--
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Unread 08-02-2002, 06:17 PM   #3
Happypappy
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Johnny gave you excellent advice! The engine is a great low end and mid-range engine, if you take it hunting you will appreciate how quietly it runs.
Now, about mods to the engine. The cylinder head is the biggest restriction to horsepower on the engine. Basically any cylinder head that has the intake and exhaust on the same side of the head is not going to be conducive to building horsepower! Second the two center exhaust ports are Siamesed, or share the same opening, again not conducive to building horsepower due to exhaust gas reversion. Machine work can correct the center ports by adding a divider.
Have added exhaust headers on a couple of different CJ's and by themselves I can honestly tell you they don't do much. (more for the ego and ears) If you add headers and an Offenhauser intake and larger carb such as a 390 Holley, injection, or a 2100 you may feel a seat of the pants difference from midrange up. As far as bolt ons probably the best improvements I have made are the timing adjustments, TFI ignition, and Holley Pro-Jection. The Holley gave me no more horsepower but it sure runs great!
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Unread 08-03-2002, 09:19 AM   #4
mpbspb
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Thanks for the info. I think i will start with the weber carb. I was wondering which one to use the 300cfm or the 400cfm. I dont really care about gas milage(Its a Jeep not a Honda!) What is important is smooth operation at idle and at speed. That has been my biggest problem.
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Unread 08-04-2002, 08:17 PM   #5
johny1i
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I went with the 300 cfm Weber--got it from www.carbs.net for about $320 (they had a special), but their regular price was better than anyone else, also. I did some calculations and figured that when I rebuild the 258 and bore it .030 over, the carb should still suffice (barely)--with a cam and/or other upgrades, I'm not real sure. As far as driveability, it is much better than the carter--one mixture screw and one idle adjustment, one choke adjustment. I tuned it, but it wound up right back where the factory had set it--just bolt it on and go. Also, in Louisiana I'm sure you don't need the factory air cleaner assembly for cold-weather condition, so I'd say forget the adapter they try to sell you if you go with carbs.net--the air cleaner supplied with the Weber is plenty good, and you can add a K&N if you want.
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Unread 08-05-2002, 08:49 AM   #6
mpbspb
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Thanks Johny1i. Did you notice any gains in performance with the weber or was it just smoother.
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Unread 08-05-2002, 09:23 AM   #7
johny1i
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Like I said, it's been such a variety of changes in the past few months (nutter bypass, high-flow cat, much higher timing w/93 gas, Weber, change in vacuum advance source) that I hate to point at any one thing. However, the Weber does pull strong, and when you stomp it enough to open the secondaries, it takes off and makes a really cool low-frequency roar (my friends like that one). Gas mileage went from about 13-14 with the Carter (I can pinpoint this one) to at least 17-18 with the Weber, and up to 20 at one point when we spent the day puttering through trails.

Sometimes I feel like the Weber will only allow the jeep to top at 80 mph, but I just got back from an 8-hr roundtrip over the weekend and I could fairly easily maintain 80 on the interstate (unless the hills got too bad), so I think aerodynamics are just too much for the vehicle at that point (the carter struggled at that point, also). Plus, 80 is enough in a jeep .

The only thing that I would really like to do next--and I would probably wait until the engine needs a head rebuild--is the 4.0 head modification to get it flowing a little better. Maybe a cam, but nothing that would screw up the idle (I commute everyday and I don't need it shaking while sitting in traffic). My only experience with cams has been on "lopey" V8s, so it may not even be that bad on an I6.
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Unread 08-05-2002, 12:42 PM   #8
Hooch
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I see a lot of you recommending the Weber and no doubt this is a fine carb. What I don't understand although one of you mentioned it is why pay so much money while the Motorcraft 2100 is just as good an alternative and a lot cheaper.

I live in the U.K. and have no junkyards around like you lot in the States and would certainly not pay this kind of money if I could get the same performance from something I can pick up from a junkyard (even if you exchange the junker as a core for a rebuild one at places like NAPA it is still a hell of a lot cheaper than the Weber).

See below:

Motorcraft 2100 (sometimes called Autolite) 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.

Power Valve: 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.

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).

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.

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.
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Last edited by Hooch; 08-05-2002 at 12:46 PM..
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Unread 08-06-2002, 06:54 PM   #9
cj5v8
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hooch,

What do you mean no junkyards. You guys produce the show Junkyard Wars (my favorite).

Brad
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Unread 08-07-2002, 05:16 AM   #10
Hooch
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Oh yeah, plenty of junk yards, but not American ones... There might be a few, but you'd need a sniffer dog to find one and I can safely say that finding one of these carbs is out of the question...
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