Problem With Delta Replacement Tub Spout

Using a delta replacement spout, no problems with install, but when I pull the diverter up to enable the shower, I am getting enough blowback that water is leaking out of the back of the spout and through the caulk around the copper pipe where it comes through. There seems to be ample flow out of the shower head, but water some water is also coming out of the spout.
The only components are the spout, the brass sleeve that is threaded on the back end that the spout screws onto, and a bolt that holds that on via a small set screw (allen head). There is an O-ring on the forward end of the sleeve, which came with the sleeve.
Why is there soooo much water blowing back??

Welcome to the site. This is a link to Delta Canada, they should be able to help you if not just google delta costomer support and you should get your local site.
http://www.deltafaucet.ca/customersupport/assistance.html

I'd replace the spout with the thread-on spouts and loose the o-ring setup.

I hope it's not too late for this post, but here it goes anyways.
It sounds like the type of spout you have needs to be soldered on to get the proper seal. The brass sleeve has an o-ring and threads on the outside of it. Check the instructions carefully. The o-ring would need to be removed, the pipe cut to about 2 1/2" and the adapter soldered on. Allow the pipe to cool, put the o-ring back on and thread the spout on.
There are other universal spouts that have only an o-ring seal with a set screw to hold the spout on. When using this type it is important to deburr the end of the copper pipe and smooth it with sandcloth to avoid damaging the o-ring that is on the inside of the plastic insert of the spout.
The third type is as kok328 mentioned above and also requires the ability to solder. The copper would be cut approx. 3 1/8" from the back wall and a male adapter soldered onto it. Teflon tape and a little pipe lubricant is helpful in getting the spout to screw on straight and getting a good seal.
With the first two types of spouts mentioned it is important not to leave the copper pipe too long, that could be part of the problem with the diverter still allowing water out of the spout when the water is diverted for shower.