Whoever rebuilt the carb didn't do their homework and just used…

Whoever rebuilt the carb didn't do their homework and just used the secondary diaphragm that came with the kit. The standard rebuild kit diaphragm uses the 135-4, but the 1848 uses the 135-2.
I purchased one separately off of eBay.

Also worth noting is that the 1848 uses an 8.5 powervalve stock, as opposed to the 6.5 that most others use (and that comes with rebuild kits).

Owner caption January 2015 · Photo 795 of 1562