There's reasons for doing it.
The first problem is when you get into these factory part numbered carbs, date codes, like DZ holley carbs, L78 holley carbs, CJ holley carbs etc... these things can run upwards of $2500 and still need refurbished. You want things like the correct part number meter blocks and associated parts to start with.
The second problem is that these parts are 50+ years old and no telling how many hands have been in it. So I check things like passages, metered orifices etc...and often times I find that things have been drilled above factory specs from hot rodders. Not so easy to go backwards once that happens.
Idle feed restrictors for instance, when too big, causes the mixture screw to be way over sensitive to the point they can't be out more than a 1/4 turn or there is an overly rich idle condition. In these cases the IFR needs to be reduced. In a case where they have been drilled too large the best solution to save a hard to find meter block is to remove that brass insert, tap the upper hole and install a brass set screw and drill accordingly like I mentioned above. This saves an otherwise perfectly fine and rare meter block.
This is the biggest reason for the change. There are plenty of 40+ year holley carb experts in the field that feel the IFR works better in the upper hole. I can't say for sure and don't really care to dispute that, but I can say this mod works fine on the carbs I've done and gets a rare carb back into service. I don't do it because I think it's better.
|