Hello,
I have got a simple question.
Can I power Arduino UNO with motorcycle battery?
Arduino has build-in voltage regulator, so it should not be a problem, right?
Check out the Quickshifter thread.
TL;DR, engines are hell if you're going to be powering this from a running motorcycle.
Typically, motorcycle batteries are just small lead acid car batteries and when the engine is running, they are often in the normal charge range for Lead Acid batteries - 13.7 - 14.3 In addition, vehicle electrical systems are notoriously noisy with spikes and other things on it. I would use a small (inexpensive) switching regulator instead that can handle up to 40v. I would also put a fuse in-line (I have seen too many things directly wired to the battery and when they short or otherwise fail, you can get LOTS of "magic smoke" (which can be really exciting if you are in traffic and the smoke is coming from between your legs !!!!! :o ). A switching regulator would also lower the current consumption although you don't indicate if that is an issue and what else you are planning to power with this (or if the battery is actually even on a motorcycle?)
You could add an external 7812 regulator to be safe.
Also if noise becomes an issue, you could put a 470uf cap between gnd and 5V pin.
Funny thing, because I'm building my own quickshifter as well 
So, 7812 regulator will help? Thats inexpensive so I can afford one
Then I should connect battery + to Vin in voltage regulator, battery - to ground in regulator, Vout from regulator to Vin in Arduino and arduino GDN to - in battery, right?
motor cycle batteries are normally lead acid and are available in 6v and 12v, so check the voltage..
lead acid batteries can produce a lot of current , so always have an inline fuse...........
I've got 12V in mine bike.
Yeah, right, I forgot. Then i should add 1A fuse between battery + and Vin in regulator?