Probably the first thing you should check is your orifice. If it is blown out, it will widen the cutting stream considerably. The easiest way to tell whether or not you have a bad orifice is to look at the stream of water coming out of the nozzle at full pressure. To do this check, raise the cutting head above the level of the water about six inches. Turn your pump on and bring it up to operating pressure then manually turn on the cutting head and observe the stream. If the stream looks fairly small all the way down to only about 0.25"-0.5" at the bottom then your stream is considered good. However if your stream fans out very wide immediately at the tip of the nozzle then you know you have a problem with your orifice and/or your nozzle.

Another way to check your orifice and eliminate the possibility of a bad nozzle is to remove the nozzle from the cutting head and then proceed to use the first method described above. Although this time you should see a very thin stream, almost like a needle.

The next item to check would be the mixing chamber. First you should visually check that it is intact by removing it, secondly you should check that it is in spec while you have it out. You should gauge the hole where the water enters the mixing chamber. If it is out of round, blocked or enlarged, it would affect the quality of your water stream.