Water flowing from shower head

Hi. I recently installed a new tub/shower valve. I have read a few previous posts on this site, but they didn't quite fit my situation. The valve is a Tuscany Brass single handle navona tub & shower faucet with a lever handle. What's going on is whenever the diverter is down and the tub spout is running wide open, water will come out of the shower head in a pretty steady trickle. It IS plumbed with 1/2" copper, not PEX. Do not have exact measurements now, as I am not at the house, but the shower head was actually raised a few inches from the old one. Pretty sure there is plenty of height. What's got me puzzled is if the spout is removed from the nipple, the water will not back up far enough to come out at the top. Does anyone know of any design flaws with these cheaper models. Perhaps the diverter stem actually blocks too much flow? I did have to use a double 90 in the spout line to get around some framing. I realize this will increase the friction, but like I said there are no problems when the spout is not screwed on. Any thoughts???

https://terrylove.com/forums/showthread.php?480-Tuscany-Brass-bath-shower-faucet-parts

Are you sure the valve was installed with the proper side up?
Normally the larger port goes to the tub and should be marked up and down on the valve body.

Yep. The valve is marked "up" and is in that position. Before I checked though, I still couldn't explain the spout thing. The valve is one with the double port on the down side and the single port going back throught the valve body to the shower riser. Even if it was upside down, that would have meant the primary outlet on the valve was pointing up and the water was gravity flowing back through the valve to the spout. So pulling the spout off the nipple should affect that.

The two elbows may be producing enough resistance to cause it to back up.

That's what I was thinking at first. But why is the problem going away with the spout removed and just the straight nipple flowing all the way on. Doesn't this basically indicate the problem is in the spout itself?

It is all about the supply pressure, and the back pressure of the spout. Since the spout and its hookup are "normal", next check the static water supply pressure in the house. If it is too high...meaning >75 PSI, you could have the issue.

You should inspect the spout in detail; you may have to try a another one just to be sure. Two 90s (while not preferable) should not make much difference in a short tub spout installation.

The spout DOES create some back pressure, and when you add that to the two elbows, it creates enough resistance to cause the backup to the shower. Just because it does not happen with the spout off does not mean ANYTHING, (it is the same as if I can drive 125 mph on an empty section of highway with no police around, but that has nothing to do with how I have to drive normally). You might be able to find a different spout, such as the Delta "pull down diverter" model, which would have less resistance. IF the valve were inverted with the "single port" facing downward, the flow from the valve would be MUCH MORE that gravity could drain back down that smaller opening and the water WOULD come out of the shower.