Calling all Motorcraft carb gurus!

Bought a Motorcraft 2100 1.21 carb from a used parts place and rebuilt it. Put it on my 2.3 and during the feature tonight it would stumble every now and then about half way down the straight aways. If I feathered the throttle it would recover and pull well. On the last lap it started falling off really bad like it was running low on fuel. I am not familiar with these carbs so I don't know what problems they could encounter.

I haven't pulled the plugs or anything yet but will do that tomorrow and maybe have some more information.

Main problem is tops coming lose and sucking air.

Check your fuel supply from end to end:Air leak between pump and tank,kinked line,dirty filter,fuel tank vented,3/8 or bigger lines,low float level.....................Pressure is important:4-5lbs at the carb.Volume is more important than pressure.Check for good flow.

Check your fuel pick-up line inside the tank. I've seen this symptom a few times caused by a crack in the hose used for that pick-up. As the fuel sloshes through the corner, or the level is lowered as the race goes on, it starts sucking air.

Also, if that carb had the brass float, get on ebay and buy a phenolic one ... they don't leak and sink.

Something I tried. There is a small hole in the accel pump area, at the bottom of the circle. It is a return fuel oriface. I plugged it with some JB and then when the pump works, it forces all the fuel to the squirters. I also opened the main jets up a bit. It depends on what you have done to your head as to how much you open them. usually in stock form they are about 57. I opened mine up by getting some regulare holley jets and measuring them. then drilled the Motorcraft jets according to what I wanted. The holley jets will not work on the MC carb unless you do some work on the jet holes which are bigger than the holley jets. It is a trial and error thing until you get the performance you want. Be careful not to go too big. The MC works pretty well with what it came with. However, a little bit of drill wont hurt the flo. Otherwise, all the above advice is right on. The MC carb is well suited for mini stock engines, even ones that have some modifications done to the head, ie, cam and some mild port work. Be careful. too much is not better. I have had good luck with MC carbs. I currently have a 123 on a modified 2.3 engine and it seems to work fine. Good luck.

i've ran autolite/motorcraft carbs off and on for 40 years on v8s and most recently 4cyl and they can be made to work very well. like listed above the things to watch are float level, fuel pressure, and float type. use the plastic float as i've had the brass float bend from vibration. lower fuel pressure like stated above. more like for a webber then a holley. i switched to a low pressure electric instead of a mech pump to stop flooding on rough tracks. also check your pickup hose like stated above. i prefer the older autolite to the newer motorcraft carbs. i don't run the 1.21 or 1.23 carbs as i run the 1.33 carbs. size wise it fits nicely between the 500 and 350 holleys. not sure what lre means about the top coming loose because you can take the top off and run the carb to watch the float level or other diag. i think my 1.33 runs 61 jets(ford, not holley size) and i use holley powervalves. if your carb has the open vent on top of the bowl then run a small piece of cotton t-shirt between the gasket and cover. it will stop chunks of dirt from getting into the bowl and plugging the jets or other openings. also look over the bottom of the carb and fill all unneeded cavities as they do seal a little different from a holley.best thing about the ford carb is hold one in one hand and a holley in the other.

olin

Good advice guys, something that stuck out was a mention of rough track conditions. It was rough last night. I pulled the plugs today and they were dark brown. Not wet but the entire plug was colored dark brown.

The head has 100 thousands milled off of it and a 480 lift cam with mild port work. I had a Holley 350 on it but couldn't get the bugs out of it. It would work for a while then start acting up again so I wanted something less temperamental.
The MC has 55 jets in it now and whatever power valve came with the kit, I think a 7.5. Does the Holley power valve work in the MC? I read they were different threads.

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Originally Posted by return to dirt

i've ran autolite/motorcraft carbs off and on for 40 years on v8s and most recently 4cyl and they can be made to work very well. like listed above the things to watch are float level, fuel pressure, and float type. use the plastic float as i've had the brass float bend from vibration. lower fuel pressure like stated above. more like for a webber then a holley. i switched to a low pressure electric instead of a mech pump to stop flooding on rough tracks. also check your pickup hose like stated above. i prefer the older autolite to the newer motorcraft carbs. i don't run the 1.21 or 1.23 carbs as i run the 1.33 carbs. size wise it fits nicely between the 500 and 350 holleys. not sure what lre means about the top coming loose because you can take the top off and run the carb to watch the float level or other diag. i think my 1.33 runs 61 jets(ford, not holley size) and i use holley powervalves. if your carb has the open vent on top of the bowl then run a small piece of cotton t-shirt between the gasket and cover. it will stop chunks of dirt from getting into the bowl and plugging the jets or other openings. also look over the bottom of the carb and fill all unneeded cavities as they do seal a little different from a holley.best thing about the ford carb is hold one in one hand and a holley in the other.olin

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If the five bolts on the top come lose on the track you will have the problems he speaks of. I'm speaking of 5200 series carburetors.

The siphon tubes under the air horn can also be a problem.Have seen them break causing similar problems.The air horn screw WILL come loose eventually,check it regularly.

hello guys, new to the forum. i run a 2.3 in the scca and it has a MC 1.08. the problem is a rich condition at idle and up the rpm range. i just put in some #50 jets, 4.5 power valve and still runs rich. cant let it run in the garage for more than 10-15 minutes and you have to get out. BTW its a 88 mustang with a carb conversion done. im running the lower part of the EFI manifold, ported head, milled about .140" and a step up header. Help

Send it to Pony Carburetors. They are not expensive and you will be pleased with their work.

Tc65