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Throttle snatches at low revs


sinzuber

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Evening

 

Previous bike was R80RT. Now have 1100RT. Noticed that the throttle responce at low revs and slow riding that the throttle is very on/off. Either on or off. Quite jerky and unsettling. The carbed R80 was much smoother and I could trickle along in traffic without a problem. It's more jerky now and have to slip the clutch more to avoid it.

 

Is this a genuine observation about the diff btw carb and injection? Or is something not quite set up right on my machine. The R1200RT played with for a weekend recently was much worse still.

 

Ideas?

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Injected bikes DO have something of a "captain to control room: more speed, please" feel sometimes, but what you're describing sounds more like the bottom-end engine braking.

 

For instance: my 1150RTP and 1150GS have a definite "bottom" on the throttle, where you've pretty much hit the brakes - compression braking, very noticeable. Our Ducati 620, which has a similar injection system, is precisely the same.

 

There comes a point where the butterfly position is such that the injection controller pretty much kills all but the idle flow, and it usually seems abrupt.

 

My apologies if that sounds rambling, but I'm writing code and doing this at the same time :D Short story: pretty normal.

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I know what you mean sort of on = too much off a stumble. I think you learn to ride around it. It is harder to ease the throttle on in the R1100 series.

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Hi and welcome to the board.

 

The FI cuts all fuel to the engine with a closed throttle above a certain RPM, so giving small amounts of throttle is literally on/off.

A well set-up throttle synch helps but the best way to deal with the problem is to use 2nd gear down to very low speeds, you have to keep some throttle on then so you get less jerking.

 

Andy thumbsup.gif

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Some of this can be cured with careful LBBPSs adjustment, the idle air bleeds. Do a caful valve lash adjust (use the two feeeler gauge method) then set the LBBPSs to provide idle RPM at the lowest RPM for your model. This will enrichen the mixture slightly at off throttle RPM and provide a bit of smoothing of your off throttle performance.

 

If the condition is severe, a zero = zero adjustment may help but, I recommend this only to proficient mechanics who know their BMW well.

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Some of this can be cured with careful LBBPSs adjustment, the idle air bleeds. Do a caful valve lash adjust (use the two feeeler gauge method) then set the LBBPSs to provide idle RPM at the lowest RPM for your model. This will enrichen the mixture slightly at off throttle RPM and provide a bit of smoothing of your off throttle performance.

 

If the condition is severe, a zero = zero adjustment may help but, I recommend this only to proficient mechanics who know their BMW well.

 

thumbsup.gif What he said.

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ShovelStrokeEd

Some of this is, indeed, a result of a much more responsive engine than you are used to with the R80. The flywheel and, in fact, the entire rotating assembly is lighter. Throttle response with the fuel injection is also a bit more abrupt than the R80's with its CV type carburators.

 

Def is close with his idea that setting the throttle air bypass screws lower might slighty richen the mixture but, that is by no means the whole story or the proper way to overcome this.

 

The way the Motronic on your bike works is that below 2500 RPM, if the throttle position sensor voltage falls below 0.400 VDC, the fuel will be cut off and not resume till revs drop below 13 or 1400 RPM. Quite often, the factory setting for the TPS is quite a bit lower than optimum. One of the foundation settings in the zero=zero is setting the throttle position sensor voltage properly. Without going through the entire procedure and discounting any possible other problems such as worn throttle shaft bushings or grossly bad idle stop adjustments, one can simply re-adjust the throttle position sensor base voltage upwards with the throttles at their stops. A good setting, and one I have used on my previous 4 oilheads, is 0.385 volts. This serves not only to slighty richen the idle mixture but also to greatly narrow the range of throttle position in which the fuel will cut out.

 

Another helpful thing is don't ride at any RPM below 2000 or so. Either downshift to maintain higher revs or, if your already in first, simply pull in the clutch and coast till the opportunity comes to use more revs. With the bike running well and the setting I described above, I can ride my 1100S at a pace slower than a walk with the clutch fully engaged. I don't often make this a practice but it is one of my little tests to see if maybe it is time to do a little tuneup.

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Some of this is, indeed, a result of a much more responsive engine than you are used to with the R80. The flywheel and, in fact, the entire rotating assembly is lighter. Throttle response with the fuel injection is also a bit more abrupt than the R80's with its CV type carburators.

 

Def is close with his idea that setting the throttle air bypass screws lower might slighty richen the mixture but, that is by no means the whole story or the proper way to overcome this.

 

The way the Motronic on your bike works is that ABOVE 2,000 RPM , if the throttle position sensor voltage falls below 0.400 VDC, the fuel will be cut off and not resume till revs drop below 2,000 RPM or the throttle is re-opened. Quite often, the factory setting for the TPS is quite a bit lower than optimum. One of the foundation settings in the zero=zero is setting the throttle position sensor voltage properly. Without going through the entire procedure and discounting any possible other problems such as worn throttle shaft bushings or grossly bad idle stop adjustments, one can simply re-adjust the throttle position sensor base voltage upwards with the throttles at their stops. A good setting, and one I have used on my previous 4 oilheads, is 0.385 volts. This serves not only to slighty richen the idle mixture but also to greatly narrow the range of throttle position in which the fuel will cut out.This has very little affect on the fuel cutoff, remember, it cuts off at 0.400 and you cannot change that

 

Another helpful thing is don't ride at any RPM below 2000 or so. Either downshift to maintain higher revs or, if your already in first, simply pull in the clutch and coast till the opportunity comes to use more revs. With the bike running well and the setting I described above, I can ride my 1100S at a pace slower than a walk with the clutch fully engaged. I don't often make this a practice but it is one of my little tests to see if maybe it is time to do a little tuneup.

 

Fixed it for ya Ed, my corrections are in BOLD

 

Mick

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ShovelStrokeEd

Bold they are but, the last one is wrong. Yes, it cuts out at 400 mV regardless. The thing is that if you start at 232 mV, where my 1100S was, you will reach that 400 mV at a far larger throttle angle. In other words, fuel cutoff will occur, with my setting, only if you have completly closed the throttle, not at throttle positions quite a bit more open than that. Since the OP's problem is occuring at low RPM, this change can make a significant difference. Take a peek some time at the change in open area that occurs at small throttle openings and you'll get some idea of the facts. I'll even give you a start. Bore=50 mm. Throttle blade angle at the fully closed position is 7 degrees (may be 6 or 8 but that is a good starting point).

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How about the throtttle cabels being worn and hanging up? I didn't realize how stiff mine were till I read about it here and checked them out.

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The thing is that if you start at 232 mV, where my 1100S was, you will reach that 400 mV at a far larger throttle angle. In other words, fuel cutoff will occur, with my setting, only if you have completly closed the throttle, not at throttle positions quite a bit more open than that.

 

Ed,

I thought ALL the R1100S bikes had Motronic MA 2.4? The only thing you need to do to set the TPS on that bike is to go through the reset Motronics, key on, throttle open, close, open , close key off to set the high and low voltages on the TPS.

 

Mick

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ShovelStrokeEd

True enough, I actually reset mine before I learned of the 2.4 version being the norm on the 1100S models. It made a small difference in the already superb driveability of the 1100S and a huge difference in both my 94 RS and my 2000 RT. If truth be told, at a very short time after I made the TPS voltage change, I also changed the pipe to a Ztechnic and added a Laser chip, all that more than 40K miles ago so I'm working from a badly beat up memory here.

 

Doesn't change the fact that on older versions of the Motronic this adjustment can make a considerable difference on the low rpm throttle response snd in the transition from partly open throttle to zero throttle.

 

BTW, thanks for the correction on the 2000 RPM being the fuel cutoff point. I actually vacilated between 2000 and 2500 rpm while composing the orignal post, and, as usual in my case, a 50-50 proposition will prove wrong 82.8% of the time. At least that is my track record with wiring 3 phase motors. grin.gif

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I'd like to check the TPS voltage on my 2000RT, does the on/off routine apply to this bike; or do I need a voltmeter and which wires on the TPS to check voltage?

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I'd like to check the TPS voltage on my 2000RT, does the on/off routine apply to this bike; or do I need a voltmeter and which wires on the TPS to check voltage?

 

Gordon,

Measure between pins #1 & #4 (the numbers are on the TPS itself).

 

Mick

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My '02 1150 was the same way with throttle snatching at off idle openings. Techlusion R259 cured it(among other things,see my post).

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Well, Gents, I'm impressed with the number of responses I got here. I'll look into the TPS business - hope the shiny new Haynes manual on my shelf deals with this. Understand the theory well enough, but the practice awaits.

 

Might revisit this if:

A) Can't fugure it out

B) Get results.

 

Thanks all.

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ShovelStrokeEd

Gordon,

Pin 1 and 4 gives the voltage. Yes, you should pull fuse 5 and wind the throttle to full open twice. The older version such as you have doesn't learn the closed throttle voltage but does need to know the 100% open voltage.

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Gordon,

 

If you have a go, be sure to report back on the forum. Will try it myself next weekend, Anything you learn would be well received. Vice Versa of course!

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