I2C Bus is Not Responsive When Engine Is Running. How to Diagnose and Fix?
Start by hooking up the scope to your MCU's 5V power. First look in DC coupling, then look in AC coupling for the real fun view. For the mostestest (tm) fun, also look at the incoming "12V" at least once.

Motorvehicles are extremely noisy things, the power rail can contain noise many times more than a simple linear regulator and a few caps will solve. In fact the spikes in a motorcycle or car can go so high (or low) that they may break a simple 7805 if it's not protected by a MOV, a filter and reverse protection diode. (The diode will also act as a sort of filter with a 100uF or higher cap).

I2C is very sensitive because of its pull up resistors and a reasonably high capacity in the breadboard as well that couples in noise too. 

Also: Vibrating contacts on a running motorcycle. Solderless + vibrations = not good.