Not cutting right - or at all

Blade may be getting dull, or you're pushing it too hard.

Go to
http://www.ereplacementparts.com/black-and-decker-7390-type-circular-saw-parts-c-4167_4252_12157.html

Its on ebay http://www.ebay.com/itm/NEW-BLACK-DECKER-ZIP-SAW-11-PIECE-CUTTING-SET-74-691-/260888941452
only 15.00

Yes. Suggestions:
- make sure you're using the least amount of extension cord possible to keep the motor cool
- use a sharp blade, prefferably carbide tipped
- don't force the saw through the cuts if the motor is bogging down slow down the cut speed.

first find out what wire guage it is and then remove the handle half, making note of where the wires go, you might want to take a picture for ease of reassembly and then remove the old one and replace it making sure not to pinch the wiring.

This could possibly be down to the material you are cutting is a hard substance or possibly that the blade needs renewing, check to see if the tips are broken, missing , or worn?? That is 2 possible reasons unless the smoke is coming from out the motor?

Unplug it and loosen the bolt in the middle of the blade. It's a standard thread, so turn it lefty-loosey. But you need to hold the blade from turning. That's easy by putting a screwdriver in front of a blade tooth, under the sole plate. Let the sole plate prevent the screwdriver from rising up with the blade as you turn the bolt. If your current blade has small teeth (like a plywood blade) then you can raise the sole plate as for a maximum depth of cut, then push the saw against the front edge of a workbench (or block of wood somehow held still) as if you were going to cut it. Before you remove the bolt, take note of the exact orientation of the washer (or washers)so you can replace them the same way.

Draw the circle pattern on the wood where you need it. Drill a 1/2 or 3/8 inch hole inside the circle, near the circle line. Insert the jigsaw blade into the hole and start cutting, following the line.

If you have the scrolling feature on your saw, don't use it, lock the scroll.

Sounds like a very dull blade.

According to the service manual this saw has a 120VAC single phase 2.5 Hp motor with a 5/8" x 1.5"long arbor. It has a maximum vertical cutting depth of 2 and 15/16". It has a maximum cutting depth at 45 degrees of 2.125" (2 and 1/8") without the lower guards. The maximum cutting depth at 45 degrees with the lower guards in place is 1.625" (1 and 5/8"
The service manuals (approximately 24 megabytes each) are available on line at Dewalt.com After creating an account. Then you should go here:

http://www.dewaltservicenet.com/Products/DocumentView.aspx?productid=18624&typeId=1495&documentId=574

for the manuals.