Great fun, good challenge, but poorly working design. Marble explorer doesn't gain enough speed to reach the firing point. Being a very mechanical person cannot help overcome bad design. So you spend hours (about 30) on a flawless assembly of parts and end up with a cool gadget. then it doesn't work. This is very disappointing in the end. I would like to add that I was able to change the design and it works fine. However, I see that many people give up at some point. I took a two week break and decided to make it work. Here, just a few simple design changes with no difference in cost make this item really impressive. Call me for a Rokr solution.
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