nope, i don't have any problems with mine at all. (chinese 50 c.c. slant head 2 stroke) been riding it on and off for about a year now and i have not had one problem.
other than going over it every once in awhile to check for anything loose, or to modify something to make it look or perform better, i don't have to tinker with my engine.
i took my time when i built it, took the time to route all the wires and tie them down, i also cut off the cheap plug in connectors and soldered all of the electrical connections.
I did the standard engine break in, running it at 16:1 for awhile (about 3 tanks full), varying the rpms as i rode, limited ride time to about 15-20 min. each time out, and let the engine cool down for a few hours before riding it again.
i took the time to tune the carb. and that was the one thing that took the most time and patience.
I put on an expansion chamber, and a high flow air filter after the break in period was over. threw the stock spark plug in the grabage from the beginning, and put in an ngk B6HS. i also threw away the stock spark plug wire and boot, and made a new one from some left over 8 m.m. resistor spark plug wire that i had left over from when i made them for my car.
take the time to do it right as you build it and do the break in as recommended by the manufacturer. if you get a good engine, it will perform and run good all the time.
altho sometimes with these chinese 2 strokes, every once in awhile someone gets a lemon and they can't make it run well from the beginning.