Intermittent starting problem

[Ben sIII]

So it was cranking, the starter motor was spinning at normal speed but the engine not starting? Or the starter was not spinning?

in that instance the starter motor wont spin, you'll hear the relay click, but unless the starter motor is in the right position it wont get enough current to spin, hence it is intermittent.

The push start means fuelling, spark, air etc is all ok.

 

[TownsendsFJR1300]

Sounds like a failing starter,

Yamaha sells that entire plate, all assembled for a reasonable price.

Mine failed at about 36,000 miles. You can see the one brush really worn out:

 

[jomor147]

I am having an intermittent crank but no start issue where I was told by a mechanic to replace (or rebuild) the starter motor with the explanation that it sucks too much current (measured at 75A) and/or creating electrical noise above the ecu's noise cancellation filters threshold, confusing the ecu to read the cps singal (hence the error 12). The starter motor had the brushes replaced by another mechanic 10.000km before this problem appeared. I have a new Yuasa battery, 0.3V voltage drop at starter's relay terminals during cranking, 0.2V voltage drop between battery negative and chassis during cranking, cps signal seems ok on oscilloscope, 10.7V efi supply (d09) voltage and coil positive during cranking and not consistent spark during failed start while cranking. No issues after starting. Not knowing what else to do before replacing the ecu (will also try a new cps despite its good signal before replacing the ecu), i ordered a new magneti marelli starter motor and wait to see if the mechanic was correct or wrong.