Motorcraft 2100 carb on 258 -some additional info - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 09-03-2002, 06:01 PM Thread Starter
 
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Motorcraft 2100 carb on 258 -some additional info

Just a little advice for those with MC2100 carb's adapted onto 258's and using a manual choke. The vacuum drawn through the choke mechanism should be disabled by plugging the port in the casting if you are not using the choke as originally installed on the 304. This can be done with a small amount of epoxy at either end of the port. Idle the motor with the plastic choke "pot" removed to see where this port is inside the choke casting. You will know it when you place your finger over it. It's barrel shaped and about 3/8 in diameter. Removing this intentional vacuum leak will reduce your idle rpms somewhat, and that may be desirable for some of you with too high of an idle speed. No sense drawing unfiltered air into the manifold through the choke mechanism. The original purpose of this port was to draw heated air up through the choke mechanism to assist in sensing the engine warmup. There is a threaded fitting cast on the outside of the choke casting, and this should also be plugged (a brass, rubber or vinyl cap will do), just to keep dirt from getting into the choke casting. But plugging it alone will not prevent the intentional vacuum flow that was designed into this mechanism for a purpose that is likely no longer employed.

Be sure to reset your idle mixture after plugging this "leak".

Just thought I would pass this on.
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post #2 of (permalink) Old 09-04-2002, 02:29 PM
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Re: Motorcraft 2100 carb on 258 -some additional info

Tim,

Thanks for all your info on the 2100 swap. I have done this swap following your instructions and it works great for the most part. If you don't mind I would like you thoughts on a related 2100 issue.

I have an 84' CJ7 258 with the 2100. Following the carb swap I used the stock O2 sensor to determine the fuel/air ratio (1.0 being rich side and 0.0 lean). I had read somewhere that an ideal mix would register about .5 (milivolts) on a multimeter directly connected to the O2 sensor. My 2100 came from Texas with #48 jets. I live in Colorado at 7000' so the initial setup was very rich (.88 on the meter) plugs very oily and black. I ordered and installed some #46s and my O2 indicated .65 which is close and suited me fine. Plugs were a bit rich but not too bad. After about one week I retested with the meter and the O2 sensor showed .89, a mixture that was even richer than before the #46s were installed ?????

I am baffled and was hoping you could point me in the direction of what could so drastically change my fuel ratio in a short time. I verified the richness by examining the plugs and sure enough they agree with the sensor.

I am confused and would sure appreciate any input, comments, advice.

Thanks again for all your informative posts.

-Rob
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post #3 of (permalink) Old 09-04-2002, 07:39 PM
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Re: Motorcraft 2100 carb on 258 -some additional info

If you are checking the mixture at idle it will not give you any accuracy at all. The idle mixture screws adjust the idle mixture.not the main jets. you will have to put it on the high step, at least, to check the mixture through the main jets. Better yet test drive it at hiway speed with your meter connected. That will give you a true reading of your jetting.

Jackie
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post #4 of (permalink) Old 09-05-2002, 07:46 AM
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Re: Motorcraft 2100 carb on 258 -some additional info

Yes, the readings that I am refering to are on the highway at about 65 mph, for about 45 minutes. This vehicle is my daily driver and I have a meter sitting on my console and taking continous readings. I set the idle mix using a vacuum gauge.
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post #5 of (permalink) Old 09-05-2002, 08:05 AM Thread Starter
 
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Re: Motorcraft 2100 carb on 258 -some additional info

If you are right that a sudden change in mixture did occur (and I'll say more on this assumption later)then the carb needs a thorough inspection. Power valve or the needle and seat or float come to mind a potential problem area. The power valve is closed by vacuum level. When they fail,they often stick or remain open and constantly enrich the mixture. Replace the power valve if you have any doubts about your ability to test it's function. Consider checking the float level with the engine idling as well.

I would consider the appearance of the plugs as the best indicator of fuel mixture. Oxygen sensors are only accurate at stoich (chemical balance) mixture point. How rich or how lean is very inaccurate.

If the power valve and float level check out, I would continue with rejetting leaner. I have no experience with rejetting for altitude, but a change of 48's to 46's is only a small change. You may need to take it further (if all else is functioning correctly on the carb).

Good luck, and keep us posted.
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post #6 of (permalink) Old 09-05-2002, 12:47 PM
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Re: Motorcraft 2100 carb on 258 -some additional info

One more posibility is that you where running at different vacumes for the different readings. It is posible that your older readings were taken when the engine was pulling more vacume keeping the power valve closed. On your later test the throttle may have been open more lowering vacume and opening the power valve. This would have effectively increased the size of the jets. I would get a vacume gauge and repeat the remeasure with the O2 sensor only this time take multiple readings at different vacumes. You may find that you will want to change power valves as well.

bandhmo
post #7 of (permalink) Old 09-06-2002, 08:00 PM
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Re: Motorcraft 2100 carb on 258 -some additional info

TiminMb hit it on the head. Check the power valve. When it fails it sucks gas right into the intake. I sure missed on that one. Usually you can pick up the whole carb kit for about the same price as the power valve.

Jackie
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