This is probably obvious to most of you WH40 users, but just in case there is someone out there committing the same error, I though I would share my recent "discovery."
I found that the coil spring (filter), when placed correctly in the funnel, greatly improves accuracy during heavy rain. I noticed this while cleaning my WH40 after a bird used the funnel as a toilet. Anyway, while running a slow stream of water through the unit, I saw that the small steady stream of water overwhelmed the tipping cup and held it in the down position - not allowing it to reset for the next tip. When I added the spring filter, I pushed it in and it hooked the bottom of the funnel (by mistake I thought), and viola! I don't know if it is the spiral channeling though the spring or the hook slowing the drip to the cup, but immediately, I could see that the cup was tipping much more consistently.
Somehow I missed it, but the instruction manual does say to install the spring hooked to the bottom of the funnel, but as far as I am aware, it only refers to it as a filter for small debris. For two years I have just been placing the filter in the funnel just kind of sitting there (not hooked in) to catch small debris.