Had a 4100 that i rebuilt. installed a 48 pri and 54 sec jet (stock '66 289), replaced and cleaned all the usual stuff. Starts great, even cold, idles great, drives great, and secondaries kick in very smooth. The one problem I can't seem to fix is the WOT from a dead stop. It stumbles as if a cylinder is misfiring, and then eventually catches up with itself. I'm thinkin' the primary circuit may be clogged internally, causing a severe lean condition under WOT. What da yu think? I triple checked the float levels, and they were right on the money, 17/32" if I recall correctly. I got so tired of chasing this one that I put on a Edelbrock.

Personally, I don't feel a huge difference between the two carbs other then WOT. Of course, that's where it counts the most, right?

No hurry for a response, just curious. I've been an observer to this forum, and you seem to have some pretty good insight.

 

Trust me, there are a lot of guys on here a whole lot smarter than I. But, I'll give this one a stab. The carb jets are changeable in this model. The Motorcraft kits give you jets to use. If you do some drag racing, keep them and experiment next time at the track. Don't try using Holley jets in your carb, because the threads aren't the same. So, experiment with the jets to see if that clears up the stumbling.

The accelerator pump rod hole could be moved to the last slot in the diaphragm arm to improve your low-end power. On some cars, it's too much juice, so be ready to go back a notch. Another upgrade part useful for performance is the primary accelerator pump check valve upgrade. Old carbs used a return spring and check ball arrangement. By 1965, this was replaced with the elastomer valve.

 

Thanks much!