It seems like you have a pretty good relationship and I know you are easy to talk to kind of guy, so consider that your strength. Also, the client is happy with the bike. Perhaps a phone conversation where you explain that it's important for you and your partner and your business to receive the payment, and ask if the client can pay right away (maybe by credit card or paypal - maybe even send a fund request via paypal). See how the client responds. Ask "when should I expect to receive the payment by?" Perhaps you could point out, we've gone around on this a couple times now and I really like to get it all squared away and complete, can we do that?.
Hope that helps. I would bet the client likes you, the bike and would also like to get things wrapped up and square and maybe isn't so skilled and the follow through.
Good luck,