Question
My lawn has 10 rainbird's 1800's pop up sprinklers,recently i noticed 2 of them were not porperly adjusted as far as coverage and distance, so i adjusted them with the little screw on the top and all was fine,the next time that the sprinklers came on the same 2 sprinklers that i had adjusted before were now once again just dribbling water, so i adjusted them again, once again the next time they came on the same 2 were again just dribbling. i replaced the 2 that were dribbling with 2 that were working just fine from the other side of the lawn.the 2 that were dribbling before work fine in another location while the 2 that were working fine else where are now dribbling in the other location.i've flushed the system, checked filters,it's weird how they will adjust just fine while they are on but the next time they turn on it's back to just dribbling, any help or ideas would be great, Thanks..

Answer
Thanks for your question.
This is going to sound strange, but it is the water temperature. When the system is off, the water in the pipe and sprinklers warms to the soil temperature. This time of year the soil is fairly warm.
The adjustment on the sprinklers restricts the flow of water. That restriction is very small, often as thick as a piece of paper. When the system come on, that warm water expands the plastic just a little, but it is enough to squeeze down the adjustment area of the sprinkler. Further compounding the problem is, the warm water can't spray out, so the new cool water can't get to the sprinkler.
My suggestion is to use a smaller nozzle or not adjust the nozzle so low. Rainbird makes over 50 nozzles for the 1800. There is probably one that would be better suited for your application.
Hope this helps.

Matt