How to fix nitro RC engine priming problem?

Nitro can be tricky to figure out. Although most of the time it becomes a "doh" moment when the answer comes to light. First thing I would check is the fuel tank, it sounds like you have looked at all the other common symptoms. If the fuel tank has a leak the back-pressure from the motor will not push the fuel through. Also check that your head bolts or screws are completely tight. Again that will be a loss of compression and therefore a loss of pressure to the tank. If that doesn't seem to work then go to the carb, although they are very simple in design and don't wear out as often as people think, though could be a probable cause if the motor is old and old fuel was stored in the tank. If all else fails pull the fuel line that attaches to the muffler and blow through it, if fuel moves through the lines it must be a loss of pressure before that point, if you cannot blow through the line and move the fuel, then you may have other problems, just make sure the carb is open slightly when you manually pressurize the tank (blow through the fuel tube attached to the muffler). Hope that gives you the information you were looking for. Probably not "dirt" in the motor, although if the motor is old enough may not be enough compression to run anyhow. Please let me know if you need more information.

Nitro Rc Engine