Very important part of getting successful runs is the plug. However, it's not uncommon for owners to say that the plug is still glowing. Just because it glows doesn't mean that it's a healthy plug. And then you have those that will insist on using fuel that's 30 years or older. USE NEW fuel!!!
My tip however is that when the engine is apart, hook a syringe with a piece of tubing to the venturi on the tank. You shouldn't be able to withdraw the syringe AT ALL!!!. If you can, the reed isn't working properly. With the engine assembled, hook the syringe to the filler pipe blocking off the overflow. Screw the needle all the way down tight enough to seat not tight enough to destroy it. Use the syringe to pressurize the tank. If you hear air leaking note where it's leaking. If you hear air internally, the o-ring is screwed. If the tank perimeter is leaking, place dental floss in the groove and reassemble. Oil can hold the floss in place. Air can be leaking from the screws on the backplate as well. I have used cotton from cotton swabs twisted up and wrapped around the screws like Teflon tape. This works great and is reusable.
Insure the back of the case is flat by sanding it lightly on a piece of glass with 400 paper or finer and a bit of oil in circular motion. The extrusion usually has a lip around the perimeter of the threaded hole due to the screws pulling the material out. Chase the holes with a 2-56 bottom tap which cleans them out and insures the screws aren't bottoming out on crap.