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post #1 of (permalink) Old 09-26-2006, 11:00 PM Thread Starter
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258 idling rough with MC2100

I'm getting a stumble when slowly moving through the beginning of the rpm range. It runs rough at idle. The exhaust smells like its burning rich. I've adjusted the mixture screws countless amounts of times per everything I've read in different posts. When I rev the engine and let it come back down, it almost dies. The idle speed screw is set at about 750 rpm. I havent road tested it yet. There is no vaccuum from a ported source(the one under the choke assembly) at idle and up to about 1200 rpm. the timing is set at 8 with a timing light. When the engine is slowly revved, the timing mark seems to bounce back and forth a little at first, only about 2 degrees, and then travels slowly upward as rpms increase. In my mind, all signs point to vaccuum leak. Most of this just started with the install of the MC2100. My old weber ran rich but had seemingly uncontrollable vaccuum at idle from a ported source. The MC2100 has none. I used the Holley 2bbl to Rochester 2bbl adapter from Mr. Gasket and the gaskets that came with the kit, part number 1937. The gasket mounting the carb to the adapter plate is from the rebuild kit and is about 1/4-3/8 thick. The adapter plate doesnt cover the bottom of the carb without a gasket but the mounting holes line up so I'm thinkin thats where a leak is coming from. is this the wrong adapter plate? Is there a different gasket I should use? Am I correct on assuming that there is a vaccuum leak? Thanks
Tim
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post #2 of (permalink) Old 09-26-2006, 11:25 PM
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Re: 258 idling rough with MC2100

"""""The adapter plate doesnt cover the bottom of the carb without a gasket but the mounting holes line up """""

Betcha that's BINGO!

Try something to verify it - spray some carb cleaner or starting fluid at that point. Note how it reacts - it should make no difference.
Yes, sounds like you have the wrong adapter.
The gasket alone won't seal it.
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post #3 of (permalink) Old 09-27-2006, 10:10 AM Thread Starter
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Re: 258 idling rough with MC2100

Ok. Anyone have a good part number for the correct adapter? From what I've read, it requires the holley to rochester manifold adapter, which is what I thought I had. And its not like the gasket is sealing it alone, the adapter plate does cover most of the bottom of the carb, just not part of the back. I'd be willing to bet that there isnt enough pressure from where the adapter ends and where the carb ends to create a proper seal with the gasket. I will spray the area with carb cleaner but I will still probably need a new plate.
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post #4 of (permalink) Old 09-27-2006, 10:45 AM
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Re: 258 idling rough with MC2100

Ford 2100 Parts Manual, PDF Format

I'd also suggest reading This JU thread...

Summit sells the TD Performance Products adapter no. TRD-2086 for $12.75. It comes with 2 gaskets. If you're not running one, I'd sure suggest it.
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post #5 of (permalink) Old 09-27-2006, 12:49 PM Thread Starter
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Re: 258 idling rough with MC2100

Thanks LEVE. That thread didnt work but it could just be the computer I'm on. I'll try when I get home. What do you mean if I'm not running one? Running one what? I'm using the adapter from Mr. Gasket and one of the gaskets that go with it on the manifold, and the gasket I'm using for the manifold to carb mount is one from the rebuild kit, that really thick one. I am going to see if the parts store will take back the adapter I got from them and then order the one from summit. Thanks again
tim
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post #6 of (permalink) Old 09-27-2006, 07:50 PM
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Re: 258 idling rough with MC2100

Ford often times used a spacer under the carb. Look for that factory gasket application. Also, I had this problem with mine at first. A closer look revealed some slight differences between two gaskets in the kit, thus the vacuum leak. I've since had the carb off a couple of times and used Ford/Jeep replacement gaskets to go back together.

I used the Mr. Gasket adapter you have and had great results.
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post #7 of (permalink) Old 09-27-2006, 09:43 PM Thread Starter
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Re: 258 idling rough with MC2100

There was a hose that went from a manifold vac source on the carb to a little plunger, so I took that off and plugged it since I run a mechanical choke. I get no suction from ported vac up til about 1200 when it starts to slightly pull. when i rev the engine, the suction jumps up but doesnt hold. it drops down a little bit. is this normal? I am using a vaccuum gauge, by the way, to test. What is that telling me? And, to adjust mixture using a vac gauge, i'm supposed to use manifold or ported?
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post #8 of (permalink) Old 09-29-2006, 02:31 PM Thread Starter
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Re: 258 idling rough with MC2100

Found the vac leak. It is at the throttle shaft. Remember, when I got the carb from the junkyard, the shaft was stuck in there. I went to the only two yards within about 50 miles today and neither had a good 2100 with 1.08 venturis. They had good 2100's with 1.21 and 1.23, as well as 2150s but not what I was looking for. How do I go about fixing this problem, aside from using the carb as a core? I was told by RRich that the bushings can be drilled out and new ones added but I didnt see any bushings in there to begin with. I assume I would just be able to replace the shaft since the aluminum body of the carb isnt rusted. Yah, nah? Thanks
Tim
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post #9 of (permalink) Old 09-29-2006, 02:40 PM
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Re: 258 idling rough with MC2100

See if there's a carb bebuild shop in the area. Maybe call a good independant shop or a good parts house to find them.

They'll have the drill, reamer, and bushings. It takes them only about 5-10 minutes to re-bush it. $20? The tooling would cost more.

No bushings? It's probably never been rebuilt by a good rebuilder before - many times new they don't use bushings - only those they screw up get bushings, otherwise they just drill the casting.
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post #10 of (permalink) Old 09-29-2006, 02:50 PM Thread Starter
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Re: 258 idling rough with MC2100

Ok thanks. I called a machine shop earlier and they were asking about 40 to rebush it. That was just an estimate off of what he has done with holley's. But definately not something I would want to do at home with the cordless and a drill bit, huh? [img]/ubbthreads/images/graemlins/blush.gif[/img]
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