Stumble on cold starts

[Mike]

Originally Posted By: hefy_jefy

One thing I do wonder about is the decompressor mechanism that's fitted to the front exhaust cam on each side, this seems to prevent the exhaust valve from closing at very low cranking speeds, relieving the load on the starter, now if that was just on the point of operating...?

 

 

 

Evening Geoff

 

That is a thought-- I have seen the centrifugal valve lift compression release fail on the KLR 650's & if it fails in the non-working position it will barely crank over & if it fails in the valve lift position the thing will crank like crazy but have a difficult time firing with enough power to stay running.

 

I do have a difficult time believing that both side decompressors can fail on the same bike at the same time so that would probably leave across the board low cranking RPM's as about the only way both sides could fail on this many bikes.

 

I guess it would be easy enough to test using a compression tester-- If it won't make decent compression on the first cranking try but will on the 2nd cranking try then you might have found the smoking gun.

 

With all the cold start complaints it is very difficult to believe that BMW hasn't issued a service bulletin about this if the problem was even remotely related to the auto decompressors.

_________________________

I doubt that this is the cause! The symptom is similar to having low battery voltage problem, and the engine seem to crank slower than normal. I know that it isn't the battery, in the few times that it had happened to me because one of my routine is to plug in a maintaining charger as soon as the bike is parked in the garage. OK, so it isn't definitive, but the fact that the symptom went completely away and the bike started quite normally subsequently seem to point in the direction that the battery is OK.

 

I don't know if any of this adds to the discussion, but when I have had the problem, there's no apparent ignition on the first attempt; it cranked and just didn't light up, so it's not really a "stumble." The second attempt always resulted in near instantaneous ignition and smooth running.

 

Not a lot of data points, but the latter has been what I've experienced in the half-dozen or so post-kill switch starts I've attempted: the engine barely cranks at all before firing up and settling into a steady idle.

[roger 04 rt]

If someone attaches a GS-911 to a bike with this problem and records a CSV file during the starting sequence, you will likely shed a lot of light on what is happening. It could eliminate a lot of guesswork.

[hefy_jefy]

Yeah I don't think its that either - just throwing an idea out there, the answer probably lies in some software quirk... funny thing is that having said my bike hardly ever shows the problem -this morning it did. I have a GS-911 so I could try it but I bet when it connected the fault won't happen!

Geoff

Edited September 15, 2016 by hefy_jefy

[roger 04 rt]

You're right, depending on the bike it might take several tries to capture the fault condition.

[KeithK]

Just another post to confirm shutting down with the kill switch & letting the system "boot up" before turning on kill switch and starting completely solved the cold start stumble.

YMMV

 

KEITH

[LIRider]

I tried the kill switch thing and didn't see one bit of difference (still experienced the stumble on occasion). What has had an effect was adding a little StarTron fuel treatment to my last tank of gas. Not one stumble since and I almost always stop the engine by putting down the side stand.

[KeithK]

One thing that is critical is you must leave the kill switch off until it goes through the "boot up" and the neutral light lights up,then immediately push start from the off position. First time start every time. Won't work if you don't follow this sequence.

 

Keith

[gordiet]

I have used Star Tron for ever and my bike has always had a stumble when I would start it. Tried this shut down method and I can't tell you why.....but it works.

That's good enough for me.

GT

[hefy_jefy]

Just checking in on this, every time I connect the GS-911 the bike starts perfectly!

Perhaps with some cooler California nights I may be able to "catch" the problem, watch this space...

Geoff