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Double deceleration? Not sure how to describe...


Trobinson

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Bike: '96 R1100RT, 165,xxx miles.

 

Here's the problem. In fifth gear when I let off the throttle I get what feels like two decelerations. In other words I get the normal feeling of letting off the throttle and then a second or so later (may be rpm dependent but I'm not sure) I get a second feeling of deceleration. It's as if I closed the throttle slightly then fully although this is not the case. I've experienced this since buying the bike 2 years ago and have put 69000 miles on it since then. Anyone experienced something similar? It is only in fifth gear this happens.

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Afternoons Tom

 

Without riding the bike it is difficult to tell exactly what you are feeling but my guess is you are feeling the decel fuel shut off then fuel return for idle requirements.

 

Watch to see if you can determine IF it always happens at about the same RPM.

 

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Good evening DR

 

I was about to post the same problem I have with my 96 RT with 75,XXX kilometers. It only occurs with sudden throttle closure from about 4000 RPM or above. I try to avoid shutting down abruptly, but sometimes there is no choice. I have another 96 RT with 180,XXX kilometers but it does not display the same issue.

 

Is this "normal" for the 1100's?

 

Ron

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Morning Ron

 

As I posted above, I would have to ride the bike to tell you anything definitive.

 

I don't know about normal but some have a fairly defined fuel cutoff & cut in.

 

You might try to hit the kill switch as you close the throttle & leave it off all the way to de-clutching just before a stop. If that effects or removes the problem then you are probably feeling the fueling cut off & on.

 

The throttle valves are fully mechanical & solidly hooked to the twist grip so we know those are not moving (opening/closing) on dropped throttle, & the idle air passages are mechanical (not electronic steppers) so no way those are moving, & the camshafts are chain driven with solid mounted sprockets so the cam timing can't be changing-- So that pretty well leaves fueling as the only thing that can change on decel.

 

Are you still running the OEM CCP with no added fuel controller (like Fuel Nanny or Techlusion)?

 

You can also try putting the choke on then riding the bike to see if the problem goes away. (the choke on the BMW 1100 is ONLY a fast idle lever & doesn't add any extra fuel)--If that makes the problem go away then you have a direction to maybe look in.

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roger 04 rt

Here is some technical information on Overrun Fuel Cutoff that I posted a while back: Overrun Tech Specs. It basically says if the engine is hot and the engine is decelerating, the throttle is closed to less than 2 degrees and the RPM is above 1800 RPM, the Motronic does not inject fuel to the engine.

 

So there are two rates of deceleration: modest if the throttle is open 2 degrees or more, stronger if it is fully closed. Once the engine is warmed up you should always get the stronger rate when you fully release the throttle.

 

Two degrees is a very small angle. So a couple things come to mind.

 

1) The TPS, which the Motronic uses to determine throttle angle was set to a higher voltage at idle by someone tinkering.

 

2) Your throttle cables are tight such that it requires full closed-throttle engine vacuum to close the throttle below 2 degrees and it takes a moment or two for the engine to pull the throttle shut.

 

Both the above can be remedied by correctly adjusting TPS and cables.

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I notice this between 3000 - 4000 rpm. I have adjusted the throttle cables several times throughout my ownership and had to re-adjust the stops (along with getting a matching TB for the left side, somewhere along the way I believe the right TB was replaced with one for a GS and I was able to find a GS match for the left) which I used Roger's 0=250mv procedure. When I have more time (getting ready to move) and appropriate garage space I'll check this again. Right now it's a minor irritation but one I'd like to fix eventually.

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Afternoon Tom

 

You might try what I suggested to Ron above--

 

You might try to hit the kill switch as you close the throttle & leave it off all the way to de-clutching just before a stop. If that effects or removes the problem then you are probably feeling the fueling cut off & then back on.

 

This should tell you if you are chasing a fueling issue or a throttle plate movement.

 

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As mentioned in a previous post, there is a decel fuel cut that is noticeable only when the fuel is turned back on somewhere above normal idle speed (to prevent stalling). The decel fuel cut is to achieve an overall emissions reduction as required for bikes in the EU. Engines are inherently 'dirty' on decel hence the fuel cut. It's off and on again for a short window to prevent the catalytic converter from cooling down.

FWIW

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Good Evening DR

 

Interesting response and information.

 

As far as I know everything on the RT is stock, I will check the throttle cables for tension and try hitting the kill switch on throttle closure to see what effect that has on the issue.

 

It was interesting reading the other responses, I too thought that I had lost my touch on smooth shifts, but now that I understand more of the background I will try a little more finesse and a little less sloppiness.

 

Ron

 

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Bike: '96 R1100RT, 165,xxx miles.

 

Here's the problem. In fifth gear when I let off the throttle I get what feels like two decelerations.

 

It is only in fifth gear this happens.

 

I think you guys are missing a big clue here.

 

I'll add that my 96 also has this same odd problem. I've chalked it up to the transmission, possibly a shift fork or more likely rounded dogs.

 

To the OP and others, try this next time you're out. Shift into 5th, then decel, you'll get that skipping sensation. Now stay in 5th and decel again, does it happen now? Mine doesn't, only once per shift into 5th. It doesn't happen in any other gear than 5th and only 1 time, if I downshift and then shift back to 5th, it'll happen again, but only once.

 

I have a M97 sitting on the bench ready to go in when I get the chance, I'll also disassemble the M94 and find out the real cause someday.

 

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Afternoon bmwnomad

 

I finally got out on the highway today with the RT to test your theory, you are right, it only happens as you describe.

I understand drive line backlash on closing the throttle, but am trying to picture how it would induce the secondary deceleration and only once in fifth and not in the other gears.

I thought at one time I was experiencing clutch slippage initially and when the speed and RPM's decreased it would hook up, but then why would it only occur under the conditions we both experienced.

 

Ron

 

 

 

 

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bmwnomad,

 

What you are describing is also what I get in second during acceleration. I'll try your suggestion and get back to you.

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Ah, the joys of non-undercut gear dogs.

 

The second (and sometimes 3rd) gear slip under acceleration is common when the dogs wear. If I hammer my 96 RT hard in 3rd gear I can make it happen.

 

The only solution is the M97 transmission with undercut dogs, either the whole transmission or just the intermediate gears/shaft and output gears/shaft. I guess you could have your gear dogs undercut, but that would cost more than a nice used M97 transmission.

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Good evening bmwnomad

 

I took my other RT 1100 (185,000 Kilometers) out for a long ride yesterday, I tried everything I could think of to duplicate the secondary deceleration I get on my low kilometer RT, nothing.

 

The last owner had the transmission overhauled a few years back at about the 125K mark and if I recall, he indicated that besides some new parts, the dogs were "modified", I should also mention that it shifts much smoother then then the low K one.

This would tend to support your case, but I still find DR's argument has merit as well.

 

I think I'll just keep racking up the smiles.

 

Ron

 

 

 

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I've been paying more attention to when this happens and I've noted at partially closed throttle decel it occurs. It definitely seems to be tranny related, but since it is good otherwise I'll not worry about it.

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