Im going to go with crazyjr on this one also - it sounds like a backpressure problem - as long as when all this happened it happend at the same time - if after replacing the coupler - it was another day or another temperature outside or humidity change - that could lead to it as well... as far as changing glow plugs and then starting - i would check to see how much fuel is in the motor when you change the plug - take out the plug and turn the revo over and shake it - see if any fuel spills out - could be too much fuel and that could cause problems as well... But get the right size coupler and try it again - post back when you get the problem figured out -