BMW K bikes (Bricks)


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Cablebeacher

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Hi all

Background: while attempting my first carby balance I found the bike was revving a little high (1100rpm) The main idle speed adjusting screw was not in contact so I looked elsewhere.

I hypothetically "may have" played with the three Butterfly Balance screws. Then I "may have" read the Haynes manual page 142 16.10 which states "Do NOT muck about with these screws..."

Any thoughts on getting back to original state of play?

Regards

Mal 
BROOME WA
2,420 kms away from BMW service centres

    

Suzi Q

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Somewhere, buried deep in every K rider's psyche, is the primordial, demonic need to fiddle with those screws. It seems bikes can be running without issue, yet those screws just ache to be twiddled with...

Anyway, there's plenty of information (some hypothetical) about getting things right but, before you attack those screws make sure its not the TPS that's preventing the throttles fully closing. That once happened to me, and I pretty much stripped everything to bits to solve the high tickover before realising the very simple cause. This happened hypothetically of course.


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Sometimes I'm not really Suzi Quatro.
    

Laitch

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Cablebeacher wrote:I hypothetically "may have" played with the three Butterfly Balance screws. Then I "may have" read the Haynes manual page 142 16.10 which states "Do NOT muck about with these screws..."
Here is a method developed by this site's founder—including an illustration—to deal with butterfly screws that "may have been played with". It seems to have been acclaimed by several of its reader so now's your chance to weigh in. Laughing

    

Suzi Q

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There is another method which, I don't mind saying, might appeal to the more Neanderthal K-owner. I count myself as one of those and I have used this, ahem, 'method' many times, usually when reassembling the bank of 3/4 throttles after disassembly. (Y'know, the 'disassembly' that you should never attempt)

Hold the throttles up to the sunlight (not electric light - Neanderthals didn't have electric light*) and view the individual crescents of daylight seen peeping around the edge of the butterflies. If you have properly twiddled with the 'Do Not Touch' screws, then the crescents will all be of magnificently different sizes. If, however, you merely and cowardly only turned one screw forwards (and quickly backwards) by a tentative half a turn or so (just to see what happened) then the crescents might be nearly equal. The ability of daylight to 'grow' after it passes through a tiny orifice makes this method very sensitive (Yes, Neanderthals, though dim, had feelings too)

Doesn't matter. Just lay into the 'Do Not Turn' screws until you have matching crescents. When you have done this, play with the throttle stop screw and confirm that the crescents grow and shrink in happy unison. Leave the throttle stop screw so's the tiniest crescents can be seen.

Stick the throttles back on the bike and balance the brass bypass screws as normal, start with a base 3/4 turn out on each and use the throttle stop screw to keep bringing tickover back to 950. If you stay close to 3/4 turns on the bypass screws, then you'll end up with the right size crescents - all you're doing is letting air into the engine.

I found that trying to set the 'Do Not Touch' screws on a running engine using gauges just left me going round in circles. The above method appealed to my inner Neanderthal. None of this is rocket science, just common sense.

*They didn't have K-bikes either, but that's not important right now.


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Sometimes I'm not really Suzi Quatro.
    

Laitch

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Suzi Q wrote:Hold the throttles up to the sunlight (not electric light - Neanderthals didn't have electric light*) and view the individual crescents of daylight seen peeping around the edge of the butterflies.
All you have to do is remove some of that stuff above them—no problem. Even a Neanderthal could do that if spanners were available, which they must have been because they were needed to build the Brick that wasn't there yet. Don't let loose parts roll into the fire.
cheers

    

Cablebeacher

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Hi to all of you who replied to my post...

I had actually gone back to the original posts by Frog and Lentini. This time instead of just reading the words I actually digested, pondered and understood them. Yes it was right in front of me the whole time!

I think this forum may be one of the few good things that have come out of the internet. The rest of it is questionable at the very least...

Keep up the good work and may the neanderthals return one day!

    

K75cster

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The throttle idle screw isn't a throttle idle screw, its a throttle stop screw, the task is to stop the plates from jamming onto the throttle bore. Seeing it's to just lightly touch without banging into the throttle bore is it's job. you must set it to do that off the bike. Once that is done you can recall the "set at factory via a flow bench do not touch screws" you had touched and remember that if touched the best outcome you aim for is to set them to balance the combustion chambers equal to each other on the bike (as even the Germans can't manufacture a motor that's 100% equivalent in each cylinder on the production line) having first set the valve shim clearances to be best case within tolerances. Close the Big Bras Screws and stick a spacer under the throttle stop screw to achieve an 1000 1100 rpm. This time you'll set the throttle balance screws to match the cylinder output of each cylinder using your manometer. Having tightened it all up in as good as you can manage outcome you'll then open the BBS's and remove the spacer from the throttle stop screw and then set about balancing the idle bypass's to your preferred rpm and evenness. It should be smoother all round, Idle and at speed, although the secondary vibes from the chosen rod length and stroke will see it vibe as so many do, A consequence of the piston at TDC travelling further down the bore at 90% than it does to reach BDC, the travel 36.5mm for instance to 90% on the way down and then again after 90% on the way back up is the variance that brings into play the secondary vibe we all know for internal combustion engines. From 90% to BDC it travels 33.5mmor something like that. This strange quirk in piston speed variance gives us the buzz at 3500 to 4000, change the rod length or the stroke and the vibe shifts also. Dialling in the variance between cylinder output gives you the sensation of an improved response engine, all be it ever so subtle. That is how I understand our motors and their throttles.


__________________________________________________
Keith - 1987 K75c with r100rt replica fairing and half of a 1984 K100rt 1992 K1100LT a blue one

The Clever are adept at extricating themselves from situations that the wise would have avoided from the outset - QUOTE from david Hillel in Out of the Earth.
    

8Back to top Go down   Hypothetical issue with Throttle Butterfly Assembly... Empty Still have problems... Mon Dec 25, 2023 9:35 am

Cablebeacher

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Hi all my repliers - Merry Christmas

Still not a happy bike. The balance etc is all good i.e. happy at idle, smooth up to 2000rpm but when I take it for a ride it is slow and sluggish when I give it a handful. It revs smoothly and runs at 100kpm smoothly with no funny noises or hiccups. Just lacks the accelerator punch.

Any simply thoughts? When I reset the TPS I get the first click but advice is I should get a second click if I manually twist the throttle cable? I don't. Could this give me my symptoms?

Or do I have to redo the basics: fuel pump/filter/injectors/spark plugs etc.

Regards

Mal
Broome Western Australia
(2200kms south and 1980 kms north to the nearest 4 cylinder motor bike shop. Yes. Really)

    

jbt

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Check fuel pressure.
Measure it.


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That fills our fairest day.
    

Point-Seven-five

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Are you absolutely certain there are no air leaks??? 

What is the condition of the crankcase breather hose(AKA "Z tube")?  Are there any cracks in the throttle body boots or gaps between the throttle body bushes and the top of the cylinder head? 

Air leaks can be compensated for to allow the engine to idle and run at low rpm, but when revved, will lean out the mix causing weak performance because they allow unmeasured air to bypass the air flow meter so inadequate fuel is injected. 

Also, is there any varnish in the fuel injectors?  It wouldn't hurt to throw some fuel system cleaner in the tank and run it through the system over the course of a week. 

How old are the spark plugs and wires?  When accelerating, the amount of air introduced into the cylinder increases due to the wider throttle opening, and results in a higher cylinder pressure at the point where the spark is fired.  The higher the pressure the harder it is to spark and the more likely it is for the spark energy to leak or fail to make a hot spark.

Last, how old/dirty are your air and fuel filters?  Fuel and/or air starvation will hurt higher rpm performance.


__________________________________________________
Present: 1991 K100RS "Moby Brick Too"
 
Past:
1994 K75RT "Ilsa, She Wolf of the SS"
1988 K100RS SE "Special Ed"
1994 K75S "Cheetos"
1992 K100RS "Moby Brick" R.I.P.
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

Laitch

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Cablebeacher wrote:Or do I have to redo the basics: fuel pump/filter/injectors/spark plugs etc.
All that should be reviewed when faced with a conundrum. If fuel pressure meets spec, valve clearances meet spec, coil resistance is at spec, if spark plugs are the correct type and set to spec, if no air is leaking into the intake stream from split intake hose connections, and if all electrical connections are clean and tight then two relatively simple conditions come to mind. Laughing

The hose clamp on the #4 throttle body is out of position and restricting throttle movement.
Hypothetical issue with Throttle Butterfly Assembly... Scree295

Secondly, because the airflow meter takes over at full throttle, acceleration will be affected: if its plug connection is dirty and/or loose, if its air temperature sensor is dirty, and/or if its air vane is sluggish or stuck. Its internal wiper assembly could be faulty but that seems to be a rare fault and I personally would not get involved at that level.

    

K75cster

K75cster
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THey do lift up they're skirts, they are not like Jap bikes for lighting up the pavement. Do check the Filters and ensure you have good airflow, They do have a close ratio road box on them, so you should get a full head of steam and be looking for 6th gear on them. That's natural.


__________________________________________________
Keith - 1987 K75c with r100rt replica fairing and half of a 1984 K100rt 1992 K1100LT a blue one

The Clever are adept at extricating themselves from situations that the wise would have avoided from the outset - QUOTE from david Hillel in Out of the Earth.
    

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