According to the Holley master parts list, the correct secondary diaphragm for my 4180 carb is #35R-235A. When I search for this part number, Holley says it's the same as #135-4 which is used on the standard 4160 carbs. I have a #135-4 and it is too short. Even with the longest spring there is not enough preload to close the secondaries.
Comparing the 4180 carb side-by-side with a 4160 it looks like the diapragm rod needs to be nearly 1/4" longer due to the thicker throttle body on the 4180. I haven't been able to find anyone who sells a 4180 secondary diaphragm outside of a kit, and I don't want to buy a whole kit just for the diaphragm.
Assuming the kits have the correct diaphragm, someone must still be making them. Does anyone have a source or alternate part number that I can check?
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