Question
I just installed a Rainbird Impulse  (not incased) on a riser, to over shoot near by shrubs..Problem...The sprinkler won't move..I'm beginning to suspect that the water pressure is too high..I've tried more than one and they all do the same thing Hunter encased works fine

Answer
Hi Phil

Before you toss out the impact sprinklers, be sure the arc settings are properly set.  There are trip arms below the diffuser assembly.  Make sure they are set up correctly.

Insufficient water pressure can prevent the head from rotating. Requirements vary from model to model.  Debris in the nozzle or other parts of the sprinkler can also inhibit proper operation.

Also as you've observed, excessive water pressure can also be an issue. Just as each sprinkler has a lower pressure limit for proper operation,there is also an upper limit. For most sprinklers the limit is 80 psi at the head. The optimum operating pressure is around 50 psi. Pressure that is too high will, at the worst, cause the sprinkler to stall, and at the least, cause the premature wear of the sprinkler parts. If the sprinkler rotates or reverses very fast, this a forewarning of excessive pressure.  

If it is excessive pressure, install a regulator and reduce the operating pressure of the zone.  You can also add 1 or 2 more heads to the zone, where it makes sense or increase the size of the nozzle to get more flow.

Good Luck

Tony