M900Sie won't keep running - fuel delivery problem?

spuggy · 30-04-2023, 06:46 PM

Bike down for a couple of years with a drained tank. I didn't run it dry, never occurred to me at the time.

"Running great when parked", indeed. Sigh.

New belts a while back. Fired it up right after - ran lumpy and kept getting worse until it fell back to one cylinder (the front) and/or died. Weather was pretty iffy anyway, walked away. Tried a few more times whenever it stopped raining with much the same results. Except it stopped running on the rear pot entirely, confirmed via the time-honored "lick finger, dab on header" test.

Yesterday, weather was really nice. Champions that came out were pretty grotty looking - had been cleaned/gapped with the belts. Fitted new NGKs, charged the battery completely, tried again.

Turn the ignition on a few times for pump prime & to wake up the lithium battery. Little slow to start on the button (2-3 stabs for a few seconds each), but fires both cylinders, revs to 3-4000 RPM, sounds a little ragged/woolly - but responds to throttle.

After a few seconds running, 10-20 maybe, will run slower and slower, until eventually drops below 1000 RPM - and then eventually stalls entirely. Sounds like it drops onto one cylinder (the front) for a little while before it dies.

Giving it more throttle once it starts to struggle just makes more induction noise - no throttle response at all. Hitting the start button again after it has died may get a few pops and bangs - but seems it'll never fire properly or run again.

Turning the ignition off/on a few times, letting the prime cycle complete, it'll fire right up (sort of), rev to 3-4000 RPM, respond to throttle. And, after a little while, the revs just drop on the same throttle opening until it dies. Again.

At least both cylinders are involved now according to the spit test.. So progress.

Kind of stumped and spitballing here...

It's got a gallon of fresh gas. Tried running with the tank propped up to ensure pickup entirely submerged while on the Abba stand - no change.

This really seems like fuel delivery. But can't be a clogged injector/bad spray pattern, borked pressure regulator or anything systemic - because that issue wouldn't change, it would never start/run at all, and doesn't fit these symptoms, surely?

My current working theory is that the fuel pump prime cycle gets the rail to something close enough to nominal working pressure that it starts/runs initially - but that the pump doesn't run afterwards (for whatever reason) - so as soon as residual rail pressure drops off, game over...

Is there a relay that runs the fuel pump - which operates independently of the "ignition prime" function? Because if there were, I think that'd be my prime (heh) suspect to poke at right now...

Or, any other ideas for these symptoms? Something clogged - like fuel pickup - would seem an obvious suspect...

30-04-2023, 06:46 PM	#1
spuggy Registered User Join Date: Apr 2016 Location: Farnborough Bike: M900sie Posts: 185	M900Sie won't keep running - fuel delivery problem? Bike down for a couple of years with a drained tank. I didn't run it dry, never occurred to me at the time. "Running great when parked", indeed. Sigh. New belts a while back. Fired it up right after - ran lumpy and kept getting worse until it fell back to one cylinder (the front) and/or died. Weather was pretty iffy anyway, walked away. Tried a few more times whenever it stopped raining with much the same results. Except it stopped running on the rear pot entirely, confirmed via the time-honored "lick finger, dab on header" test. Yesterday, weather was really nice. Champions that came out were pretty grotty looking - had been cleaned/gapped with the belts. Fitted new NGKs, charged the battery completely, tried again. Turn the ignition on a few times for pump prime & to wake up the lithium battery. Little slow to start on the button (2-3 stabs for a few seconds each), but fires both cylinders, revs to 3-4000 RPM, sounds a little ragged/woolly - but responds to throttle. After a few seconds running, 10-20 maybe, will run slower and slower, until eventually drops below 1000 RPM - and then eventually stalls entirely. Sounds like it drops onto one cylinder (the front) for a little while before it dies. Giving it more throttle once it starts to struggle just makes more induction noise - no throttle response at all. Hitting the start button again after it has died may get a few pops and bangs - but seems it'll never fire properly or run again. Turning the ignition off/on a few times, letting the prime cycle complete, it'll fire right up (sort of), rev to 3-4000 RPM, respond to throttle. And, after a little while, the revs just drop on the same throttle opening until it dies. Again. At least both cylinders are involved now according to the spit test.. So progress. Kind of stumped and spitballing here... It's got a gallon of fresh gas. Tried running with the tank propped up to ensure pickup entirely submerged while on the Abba stand - no change. This really seems like fuel delivery. But can't be a clogged injector/bad spray pattern, borked pressure regulator or anything systemic - because that issue wouldn't change, it would never start/run at all, and doesn't fit these symptoms, surely? My current working theory is that the fuel pump prime cycle gets the rail to something close enough to nominal working pressure that it starts/runs initially - but that the pump doesn't run afterwards (for whatever reason) - so as soon as residual rail pressure drops off, game over... Is there a relay that runs the fuel pump - which operates independently of the "ignition prime" function? Because if there were, I think that'd be my prime (heh) suspect to poke at right now... Or, any other ideas for these symptoms? Something clogged - like fuel pickup - would seem an obvious suspect...