The only two plastics WD40 has an adverse effect on are polystyrenes and polycarbonates. (spelling) It doesn't eat rubber any faster than any other petroleum product. I can not think of a single plastic part in any carburetor, but there are some external plastic bits of course. Gasoline eats the same plastic WD40 does. Only faster.
For those of us who have raced and crewed alcohol cars, we know that the weekend ritual involves opening AN fittings, and hosing copious amounts of WD40 in to the fuel lines, and into the carbs or injectors.
For those using pump gas on their small gas engines, in a perfect world, tanks would be drained, WD40 sprayed into them, and into the fuel lines, and into the float bowls. The "WD" part of WD40 means water displacement, and the alcohol added to pump gas condenses and caused a build up of water vapor which corrodes brass washers, and jets, steel springs, and aluminum.
In this case, I have to wonder why the guy pulled off an OEM carburetor, and then replaced it with another OEM carburetor. We can fill pages up with what it might be, fuel, or spark. I'll guess fuel, since he is on the second carburetor, and I'll guess air leak, causing him to lean out during pilot to main transition, only because you guys already guessed that the crud that was laying on the bottom of his fuel tank was loosened up by his fresh fuel, and migrated into his orifices.
Sorry, too much coffee, and I only want to be in internet expert- like the guy I just read who said "WD40 is mostly water." I guess he doesn't know that "WD" stands for "Water Displacement." A bit of fun- WD40 was first used as an anti-corrosive compound on the external skins of missiles. It is indeed a solvent, but then so is H2O.
Edit: And that can of CRC is fine for electrical contact cleaning. It is a petroleum distillate using an alcohol solvent. We used it on aircraft electrical systems. Since it is so expensive, (for production work) isopropyl alcohol is a better choice with an acid brush for cleaning, flux removal etc followed by application of lithium or dielectric grease.