Intermittent Misfire

Sorry J. about the bike woes (I wonder about fuel pump failing). Regardless, congrats on 177,000 miles on that ST! :yes:

J.Murphy said:

By the time I get home the bike is running sweetly so I guess all the readings, etc will show normal.

Click to expand...

A scope trace of the current draw waveform of the fuel pump will show speed of that pump motor. Most fuel pump motors have 8 commutator bars on the armature, and these 8 "humps" will show on the scope trace. by counting out 8 "humps" and noting the time base between those 8 "humps" (in millisesconds) that would be the time it actually takes for the motor to make 1 revolution. divide that number into 60,000 and you have motor rpm. A bad and failing fuel pump motor will run substantially slower than say 6000 rpm ALL the time, possibly even when the engine is running well. Add some heat (stop and go , 100 degree weather, etc) and that pump motor speed will drop like 1500 - 2000 maybe even less, starving the engine for fuel. The way the engines' fuel rail is set up with 2 sides means that it is possible to have bank or the other completely misfiring as the rail starts to drop in pressure available to injectors.

Seems like once an ST develops a misfire, it stays!