I think a carb set up to run rich (and three jet sizes larger for a 2100 is running her rich) and a carb thats intended to feed a 360 FE Big block ford engine for a full size truck, will definetly cause it to run to rich for a 289 powered Bronco.
Not only that but my biggest concern (besides the oversize) in this case would be what other "modifications" have been done to this carb by the rebuilder. I see these "rebuilt" carbs all the time with mismatched jetting, wrong jetting, wrong bench settings, mismatched idle mixture screws, wrong accelerator pump settings, I have even seen one with the wrong butterflies installed.
Even without anything else wrong (and I am willing to bet it has at least been bench set incorrectly) the three jets sizes larger is already disrupting and throwing off the proper flow from this carb EVEN if it was on the correct vehicle.
The 289 is an EXTREMELY sensitive motor when it comes to overcarburetion due to its extremely small valves and tiny ports, and its fast revving stroke.
We have this debate every month or so, but it really comes down to physics, and those physics do not change. An engine is basically an air pump and can only pump a given amount of air through it based entirely on bore and stroke, times its maximum RPM and then factored (decreased) by its volumetric efficiency. The stock 2V 289 is truly not a very efficient VE engine, only about 65-70% efficient if that. And this engine is probably spun out at 5500 rpm.
He's running rich, the carb is the reason. Its bigger than what his engine needs. No brainer.
Remember we are not talking racing or high performance here. Put away the hot rod magazine,talking about larger is better, it isn't.
Replace the carb with the correct application and you will be way more happier. If he showed up at my shop I would not waste his money trying to make this mismatch work, I would replace the carb.
Bill White
White Automotive
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HAVE A NICE DAY!!!!