got a 1980 motobecane, which for the most part runs very well. However, every so ofte, it starts to run really slow, and starts to make a pop noise, like a backfire. It did this yesterday, for seemingly no reason. i then left it to idle in my driveway a bit, and the problem went away. Then i was doing some donuts in a gravel parking lot today, and it started to run like crap again. So, like yesterday, i let it idle for a bit. When i went to go ride it, it back fired, and ran like crap for about 15 seconds, and then all of a sudden started running great. whats up with my bike????