The gun fires but the pellet will not leave the barrel. The barrel is clear and black smoke comes out when fired.

Check the end of the barrel for any imperfections. Small nick can send pellets off course.
put the gun in a vice and fire a shot. Then adjust scope for dead center. Make sure mounts are on correctly. If they are you may need a shim on the mount-to-scope assembly.

All air guns need gun oil down the hole the air comes from in order to keep plungers from wearing out . Treat them every few times out with a small drop or two. Also check to see if you had a bad pellet get stuck.

The piston has seal

The best way I found to fix this is to break the gun down. Once you've done that you can slide the barrel out and use a push rod to remove jammed pellets. To stop this issue, I've read to not use short skirted or pointed pellet.

The only way I found to fix this was to break the gun down and remove the barrel. Once you do that you can use a push rod to remove the pellets. By sure to push them out the way they went in.

Sounds like you've got grease in the bottom that's catching on fire. Take the grill apart and clean out the bottom of the barrel. You'll probably need a shop vac to clean out the ash, too.

This gun is a break barrel or break action gun. Hold the stock in your left hand and pull down on the barrel with your right. Once the barrel is down you feed your pellet right into the chamber. Pull back up to set the barrel ready to fire.

Upon cocking your gun is there the same tension with the cocking spring as there was befor? Also check the condition of your barrel seal for tears or cracks in it.

It looks like you pull back the slide, drop in your pellet, hit the slide release and you are ready to fire, 400 FPS, single stroke pneumatic, Made in germany, recoiless, formerly the Marksman 2004, $50.

I would check your pellets.

Light pellets typically perform horrible in break barrel rifles due to their tendancy to fly around

Make sure you have a stiff, tight fitting pellet. Smaller soft pellets also will contribute to accuracy issues as they deform much and can let air escape from around the skirt when fired (called "blo by")