Ok guys I thought I’d share this info in case it happened to anyone else. 2004 Zx10r My engine was smoking pretty badly so I got another good used engine and swapped it. I noticed it had a hard time starting and it worried me, but it did start. Checking a few things blah blah and then I couldn’t get the bike to fire… Code 23 cam up for cam sensor. I replaced with the 1 from old engine (that I just took a video of it running) Nothing! Started checking for missed connections etc. Nothing! Started going thru the manual and checking resistance and continuity, going thru the list. Thought I wasn’t getting spark because I sprayed carb cleaner down throttle bodies and not even a hint of it wanting to start. I pulled plugs and had fire….. Changed them to the ones out of my old engine anyways. Tried again…. Nothing BUT as I was hitting the start button I noticed the speedo go from zero to 1 and back to zero….. Well I just changed the engine so there is no chain on it….. Decided to check to make sure the Trans wasn’t like between 1st and natural, it wasn’t but I held the front sprocket form moving BAMM fired right up. It will free wheel in neutral so I let go…. No start. Held on to it BAM fired right up again. Weirdest thing I’ve ever seen. Glad I caught it! Figured I would share incase someone runs into same problem. I searched the internet, searched thru the manual numerous times and NO WHERE did it say…. Chain had to be on. Must be a glitch in ECU with speed sensor and Cam sensor….. And to give me that code and send me looking in the wrong place…. Just glad she’s running.