I have one zone with 4 Hunter I-20s. When the zone comes on only 3 heads pop up and the water flow is very poor. They spray maybe 10 feet. I thought if there was a leak somewhere in the PVC there would be puddling or bubbling of water. I don't have anything that resembles a leak. All my other zones are fine. Thanks for your help.
Did you ever try to turn it on with the bleeder screw like Gator suggested? If it comes on full blast after doing so then change the diaphragm.
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If you still can't figure it, I'd be looking for a break in the pipe. Let it run longer. If the break is near a sidewalk or driveway it can travel under the cement for many feet before surfacing. I saw one once that went 70 feet and came up under the neighbors driveway.
Let it run a half hour if need be.
Hot diggity dog!! I turned out the bleeder screw and took it out so the water flowed at full force. Bam! The head popped up with a real strong flow and the flow in the other 3 heads were even greater! In haven't seen that much water coming from those heads in several years. Flowing just like new. Thanks guys!! Wow what a difference. I guess I should do that to my other zones just for maintainence purposes.