Hi Robert,
Did you clean the barrel yet??? If not get yourself some "Goo Gone",, they sell it in Walmart, Eckert Drugs, CVX Pharmacies, and most good hardware stores. Take a length of weed whacker line and cut one end with a point to it. Take the other end and, using a lighter or match, burn it to form a small ball. Let it cool. Now use the pointed end to pierce a cleaning patch soaked in the Goo Gone and run the patch to "ball end" of the line. Insert the pointed end into the barrel at the breach where you load the pellet. Push the line until it comes of of the muzzle then pull the patch through. Keep doing this until, alternating with dry patches, until the barrel is clean and the patches come out clean. That should take care of your dieseling problem.
Not quite sure what other problems you're having with the rifle. Is it just that fact that it's not shooting accurately??? If so one of the best things you can do to greatly improve your accuracy is to buy one of the GRT triggers from Bob (aka: charliedatuna) ,, here's the link..
Home CharliedatunaHis trigger will literally transform your gun.
You also didn't say what pellets you're using..... Do yourself a favor and try a number of different pellets. You can get a good "sampler pack" from straightshooters.com ,,,, using a different pellet can make all the difference in the world.
I own a .22 GAMO 1250 which is basically the exact same gun,, just an older version. Great gun, accurate, powerful, a real game getter. Bobs trigger helped me tame my 1250, I know it can help you also.....
Hope this helps ya some. I"m sure some of the other GTA members will chime in with more advice......
Jeff