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I'm running a rebuilt Motorcraft 2100 carb on top of a rebuilt 4.2L, I6 w/ 4.0L head mild cam and slightly overbored cylinders. So how do you choose? I was able to get a bunch of different sizes from the yard. All the way from 44 to 85. Right now I'm running the 49's. But I'm just guessing. I found a calculator which calculates the proper size for Holley carbs. Different inputs from elavation to temp. to relative humidity are required. Does anyone know if Holley jet #'s are similar to the Motorcraft numbers. Meaning the same # being the same size. Someone once told me that Holley produced the Motorcraft carb. Any Ideas.
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Dude this Motorcraft 2100 is an amazing carby.It is so simple yet very efficient.Very popular on Ford cars during the 70's.The jetting that came stock should be near correct to your application.To really find out if it is rich or poor is a matter of pulling the car on a chassis dyno with an emissions probe attached to it.Nowadays the Lambda probes can check it with incredible precision.Ask around.
 

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jetting

Unless you have done some port work and /or are running headers, your stock jetting should work just fine. If you have an extensively modified exhaust system, watch your plugs and if they indicate lean burning, then go up 2 sizes in jet. Keep in mind that being an inline 6, your # 1 and 6 cylinders could well read lean anyway, due to fuel air distribution, in that long skinny intake manifold. I would check all 6 plugs and compare them. If you show lean burning on the other 4 cylinders then you will definately need a jet change. I would go up 2 sizes, if this indicates a need for richer jetting, as going just one size, wont make that much of a difference. And dont worry, going up 2 sizes wont richen you up so much that you start washing the cylinder walls with raw gas. By extensively modified exhaust, I mean running headers. A free flowing exhaust with the stock exhaust manifold, while helpful, wont lean you out that much.
Some signs of lean running are:
lack of power
dieseling on shut off
popping back through the carb when under acceleration
knocking on acceleration, when you know you have the correct fuel octane.

If you experience any of these, try adjusting the choke so it will be partially closed when the engine is fully warmed up ( requires adjusting the choke after warm up), then drive it and see how it performs. If any or all the symptoms go away, then you know you need to go to larger jets.
 
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