The company says:
"Best results for stain blocking are obtained with no thinning. However, KILZ 2 Latex primer and KILZ Premium primer may be thinned up to 10% per gallon with water."

Also, this subject is usually covered by the instructions on the can.

Well, I tried thinning it today about 1 cup water to a gallon of primer and the sprayer still plugged up after about 5 minutes of use. Very frustrating because in that 5 minutes I was able to cover an area equal to what it took two hours to do later with a roller and brush.

Spray painting is for professionals where time is money. Stick with the roller and get it done.

I really don't want to dedicate every weekend for the next six months to painting the exterior of the house. I was hoping to use the sprayer to get some quick coverage with the primer and then use roller/brush for doing the actual painting.

Do you have the right sized tip installed? Sounds like you may be using too small an orifice for a heavy bodied latex primer. I am not familiar with this particular sprayer, but don't have much expectations from anything with the words "Wagner Power Painter" in its name. Go to a local paint store like Sherwin Williams or Benjamin Moore that deals with professionals, and rent an airless for about 50 to 60 dollars for a day. As for the stain blocking, don't get your hopes too high for these latex primers to block much of anything.

Yup, renting the correct pro model will allow you to paint much faster and better.

I tried it with a Wagner PP---the thing it did best was put my hand to sleep from all the vibration. Borrowed a pro airless sprayer from a friend and did a 2 story 3 bed 2 car garage split entry with a hip roof in less than a day.

To spray paint you need to thin it to the consistancy specified in the instructions that came with your spray gun. This is commor pratice, whatever the can says. Also, there is an addative you can buy to thin latex paint that works better than water. I don't want to walk out to my garage to get the name. but you can just ask at the paint store and they will, or should, know. Just make sure you get the one for latex and not the one for oil base.

Use Floetrol instead of water for thinning, but you would be better served by renting a pro unit. Go a a full service paint store and talk to them. You may be able to get a pro unit on weekendds for a good rate.
All you have to handle then are the gun and the hose, and the tip size can be adjusted to elininate thinning in most cases.
Thinning primer is just a bad choice, almost as bad as thinning the paint intself (sometimes unavoidable but not often). I see a lot of paint being thinned to avoid having to get the correct tip ($$ and a trip to the store if forgotten).
Primer especially should be back brushed (or rolled) to make sure you have a solid coat and good adhesion.

I had the exact same experience with identical products. Except, however, I thinned with 8oz water per gallon of Kilz AND used Wagner thinner for latex.

No luck, but for the first 15 minutes worked great. I thoroughly cleaned all parts and retried. Works for a few minutes at best. The ONLY thing I did not do was re-lubricate interior parts. I recall a very strong order of oil which I believe made a huge difference. Will repeat, but so far not sure if Optimus PP by Wagner was the right choice.

I will update if I can resolve this issue. Otherwise, the Wagner Power Painter Plus deserves to be off the market!