Stumble at 35-3600 rpm?

[kkm]

A friend took delivery of a ‘22 1250RT recently (his 1st BMW) & I rode with him for the first time last week. He says he has a “stumble” or transitory surge around 35-3600rpm going up or down in rpm. I have a’16 RTW, so no experience with the shiftcam. Has anyone else seen this behavior from your shiftcam? I didn’t get the opportunity to ride it to experience what he’s describing. Is the shift of the cam at this rpm or is it variable with load as well?

[wbw6cos]

The Cam Shift occurs at the 4,000 mark, plus or minus.   On occasion,  I have experienced a skip just about the same range that you describe.    Sometimes when I head home in the afternoon,  I start the bike an ride away - that's when that occurs.  Like a power skip type of hesitation.   But, when I fire the motor and finish puting on the gloves, the engine RPMS settle a little (like it is warmed up,) I take off and the skip does not happen.  Outside temp doesn't matter.   It happens too infrequently to worry.   

Normally when this happens, I am under easy acceleration; it did happen on a harder take-off, but it is rare.

It happens only when I use the shift assist, and I rarely use the clutch for shifting, so I would not know if it happens during clutch use; be hard to tell.  If you know what I mean.

[kkm]

Interesting. He said the dealer brushed it off as being related to the adaptive cruise control. (which he doesn’t have yet as the module or whatever is needed is waiting on a chip,as is his radio)

 

[Cap]

There is a noticeable change in sound as the cams swap over at about that RPM.  I would not call it a stumble.  If you are accelerating slowly, you can hear it and feel it.  If you are accelerating briskly, you probably won't notice it.  Also, there is a flat spot in the power just after the cams swap over.  So, it may be that your friend is simply noticing the temporary reduction in thrust. 

[wbw6cos]

Sorry, I have a '19 model.  I forgot about the adaptive cruise, which I do not have.  But, if that is not engaged, there should not be any issue felt.   

I concur with Cap, but have your friend test it out full power, easy power using shift assist and then with using clutch.  Maybe put a cam on the tach?   See if he can still tell the difference.

[Cap]

6 hours ago, kkm said:

Interesting. He said the dealer brushed it off ...

 

As an aside, this is the type of interaction with BMW dealers that I have experienced several times.  It is common enough to have become a BMW moto meme: "They all do that, sir."  Or better yet: "you are not riding it properly, sir."

 

When I first encountered that, it put me off BMW's for about 10 years.  I came back after I had the time, skill, and money to do all my own work.  I hate hearing that a new BMW owner is getting that treatment.

 

Cap

[Bernie]

I was told my gear change problems where due to improper footwear. I told them I was wearing BMW boots sold to me by their dealership.

I also asked them to take them in trade for the proper boots. That was 4 years ago and 70K miles, I am still waiting for the proper gear shifting boots for a WetHead RT.

No problem, I am a patient idiot. LOL 

[Rinkydink]

On 2/24/2022 at 11:48 AM, Cap said:

There is a noticeable change in sound as the cams swap over at about that RPM.  I would not call it a stumble.  If you are accelerating slowly, you can hear it and feel it.  If you are accelerating briskly, you probably won't notice it.  Also, there is a flat spot in the power just after the cams swap over.  So, it may be that your friend is simply noticing the temporary reduction in thrust. 

I noticed that on today’s ride. I get a little spit or popcorn fart out of my exhaust at about 4k rpm. I’m sure the Akrapovic slip on makes it more pronounced but I’ve experienced no stumble, flat spot, or hesitation whatsoever. 

[Bill Murray]

On 4/10/2022 at 10:21 PM, Rinkydink said:

I noticed that on today’s ride. I get a little spit or popcorn fart out of my exhaust at about 4k rpm. I’m sure the Akrapovic slip on makes it more pronounced but I’ve experienced no stumble, flat spot, or hesitation whatsoever. 

Interesting. My 2021 R1250GSA gives me the spit or popcorn fart at 4700-4800 rpm when under brisk acceleration. It often sounds like a single knock in the engine. I assumed this was the shiftcam changing to the other profile. I can sometimes feel it, but I would not say it is a flat spot or stumble.