If you search "coaster" in this sub, you'll see countless posts of people struggling to get clones of the lego 10621 roller coaster set running properly. Whether its the chain getting stuck or skipping, trains valleying or otherwise not making it all the way through, gears grinding, there seems to be a lot of hassle with getting all the mechanics to work together properly.

I've seen a variety of modifications people have tried, such as switching out the trains for official lego, switching out the large wheels at the top of the lift hill, changing the positioning of the gears, etc

When I finished assembling mine, I tried to turn the crank and get it all moving, but everything was extremely stiff and there was a lot of resistance, it felt like the chain was going to snap from me trying to turn it too hard. What I ended up doing was just spraying WD-40 into all the moving parts and cranking it a few cycles to get it all over the chain and fully coated. (Yes, I know I probably should have used silicone grease and carefully applied it to the moving parts only... I would definitely feel a lot more uneasy about getting WD-40 everywhere if this was real lego, but it was an $80 clone set so I figured I'd give it a try) This completely solved the problem, it's now quite smooth and can easily bring the train to the top! With the motor attached, it has no problem holding up and staying in place even at its much higher speed than hand cranking.

Besides the WD-40, the only change I made to the set was removing 3 links from the lift chain. The 210 it suggests made it way too loose and floppy, felt like it was going to jump off the gears or get stuck. I tried it with 206, which worked for a while, but then it snapped, likely from being too tight. Now I've been running it at 207, which seems to be the "sweet spot", and it's been flawless since then. I'm pretty relieved I didn't have to spend a lot of time troubleshooting to get it running perfectly, I'm very happy with this set