If the engine has a good plug, good fuel, and good compression, it WILL fire and run. It's all in the adjustments. However, R/C car engines get a lot of abuse as a rule, so you have to check them carefully. The most common problem is an overheat and the second most-common problem is dirt damage. Both will affect compression. If the compression is off, then starting will become problematic as well as idling once the engine starts. Running can become problematic once the engine warms up.

With an overheat, the piston will expand to nearly seizing and then shrink down to be a bit too small when it cools. You get a looser-than-normal piston/liner fit. Dirt damage just erodes the piston down to a looser fit. The only way to tell, other than performance, is to disassemble the engine and see if the piston will push up through the top of the cylinder liner. If it does, the fit is shot.

You could also check the glow plug. A bad plug can be a contributor, but if it glows bright cherry red, it's getting hot enough to fire.