I have a chinese ATV with an engine copied from an ~2006 Honda Foreman 400 (a very simple 1 cylinder 400cc carburetor based engine). It's an exact copy with regards to parts. However it does not start easily.
I don't even know what information to provide here to make debugging this easier.
You can see what I am talking about here: https://www.youtube.com/watch?v=m5XOFfaw0OY The video shows just 10 seconds of it, the actual time it took was more like 40 minutes. It doesn't even make any promising sounds, I can just hear the starter turning and air being pumped out of the exhaust via the engine turning. The least amount of time that damn thing took to start up was 5 minutes and that was in the hottest weather in summer. In winter its like 1 hour or no luck at all.
Usually it starts when I give up, let the ATV sit there for a minute or so and then try again one last time. It starts up a few cranks after the first time I hear any promising sounds of combustion in the engine or sometimes immediately when there is combustion.
I have tried new spark plugs, a new controller, new air filter. Choking the air intake helps (it has no choke of it own). When the ATV starts, the engine will die if I do not keep the revs up. When the engine is warm, it will run for hours on end until it runs out of fuel. The ATV started up fine when it was brand new in 2007, but the amount of time it takes to start the damn thing has slowly grown over the years.
At some point the battery of the ATV was not enough (they kept dying on me), so I now use a dead battery as a terminal of sorts and start the atv with jumper cables and a large car battery. After the ATV has started, I remove the cables and let it run from its own generator.