So I have a force 18 pull start engine. I've been running it for about six hours. About one week ago i tried to start my engine but I noticed that the pullstart is very hard to pull when the glow plug was in. After I took the glow plug off the pullstart would pull, but when I put the glow plug back on it was hard. I cleaned my engine with WD-40 in case there were any dried up oil or fuel but I still can't start my engine because the pull start is very hard to pull. I checked my pull start and saw that it's not broken, so i guess it's a problem of the engine. I know that new engines are sometimes tight like that, but mine is not new. Any idea what the problem could be?

Manually rotate the engine so the piston moves away from top dead center. The liner is tapered and the piston will stick towards the top. Heating the engine up a good bit will help prevent that tight spot too (and be easier to start).

It could be that your engine is flooded or suffering from hydraulic lock, this is when the cylinder is full of fuel making it very difficult or impossible to turn over, remove your glow plug and then turn it over rapidly and see if any fuel is squirting out of the cylinder head and if there is then you need to turn your engine over rapidly to flush out the fuel trapped in the cylinder.

In future, don't stop your engine using your tx controller, if you want to finish running your engine, pinch the fuel line with your fingers and let the engine die through fuel starvation.

Sounds flooded, pull the plug and give it 5 good pulls to expel the excess fuel. Test your glow plug to make sure its bright it should fire up.

Did you lubricate it with WD-40 after?