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Thread: couple of K75 questions, idle, backfiring, etc.

  1. #1
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    couple of K75 questions, idle, backfiring, etc.

    I finally got to ride my bike a bit thanks to a free schedule and decent weather. A couple of things...

    1. When I bought it, it was hesitant to start, but seemed fine once it did start. Last night it started immediately without "choke" in the 85 degree weather, although it didn't sound happy at idle. While cold, it sounded like it was about to die, and was around 850-900 RPM. Once it warmed up it idled at a very smooth 1000 RPM. Do I need to adjust something, or is it normal for the bike to idle a little off when cold? When very cold, if I blip the throttle there's some hesitation, but not under load or when warm.

    2. I added some seafoam to the tank (3 ounces in a nearly empty tank, ran for a bit, shut down for 20 minutes, then rode 4 blocks to the gas station and filled up) hoping that it'll help a bit with some carbon in the system and maybe clean the injectors. Perhaps the seafoam was contributing, but I noticed some backfiring during deceleration on the ride to the station (which also reminds me, should I make an effort to avoid engine braking, or is it okay? I drive a diesel jetta so I'm in the habit of engine braking and slowing down with the transmission engaged), but it got better. This means a lean mixture, so I'm assuming partially clogged injectors could explain this as well, right? I also bought some techron, and I plan to put that in the next tank, I just wanted some seafoam for what it does for the top end. The bike has a bit of carbon build up that I'm hoping to burn off, I think it's from not being ridden properly / sitting / being started and idled but not ridden. It's also entirely possible that the remnants of the gas in the bike were from last year, and I don't know if they were stabilized or not.

    3. Shifting from first to second, or second to first can be a bit rough. Going from first to second was better if I preloaded the shifter, but it was a little clanky/grindy if I didn't. Seems like shifting into 2nd around 4000 RPM is optimal, above that it was rougher and below that it was rougher. That's a comfortable shift point for normal driving, but in slow traffic or when accelerating briskly it's not.

    Anyway, that's about it. I think this stuff is all probably fairly normal for this bike, given that it's had a break in riding. The ergonomics of this bike are great except the turn signals, which will take some getting used to. Good riding position, and I don't know what people mean about the bike being top-heavy even with a full tank, I'm much more in control without having to throw my weight around on this bike at low speeds than I was on my yamaha or on my friend's sportster and I'm able to come to an easy stop without weaving or loss of balance. I need to hit a parking lot for some u-turns and panic stops... er... hard stops, but I feel very confident in traffic already. It's probably the best bike I've ever ridden.

  2. #2
    Benchwrenching PGlaves's Avatar
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    First - the choke is not a choke - regardless of the label on the lever. It is just a fast idle lever. So on cold start needing it for a minute or two is normal.

    Some popping back through the exhaust is normal. Loud fire cracker bangs however happen often, but are not normal. They are caused by one of three causes:

    1. Rider technique
    2. Throttle position switch
    3. Exhaust leak.

    The throttle position switch is the black box on the back side of the back throttle body. It is a 3 position switch. Throttle closed, off, full throttle. It signals the ECU to add fuel at full throttle. It also signals the ECU that the throttle is fully closed. If fully closed at an RPM over 2,000 (coasting, deceleration into a turn, etc) the ECU shuts off the fuel injectors. This prevents pumping raw gas through into the exhaust.

    If the switch doesn't close at closed throttle, or (here is the rider technique part) you almost close the throttle but don't roll all the way off on deceleration then it pumps unburned fuel into the exhaust and when you roll back on it sounds like a loud gunshot. You can normally hear the switch click just as the throttle is fully closed. The switch is adjustable - two screws on the back side - loosened, allows the switch to be rotated to adjust.

    If you do hear the click, you should check for an exhaust leak at the heads, and where the muffler slips on the header pipe.

    The seafoam may have broken some carbon loose. I like Chevron brand injector cleaner with Techron better purely as an injector cleaner though. Take it for a good 100 mile ride - then while hot set the idle to 1050 to 1100. Then see what you get on cold startup.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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    The backfiring was pretty subtle, and it was on decelerating only. I'd be coming to a stop light and let off the throttle and I'd hear a somewhat muffled pop pop pop. Definitely not gunshot loud, and not when re-applying throttle. I didn't notice it after filling up, so it may have been the high seafoam concentration for the trip to the gas station, or the old gas, or a cold engine. Who knows?

    Is it better with these bikes to let the throttle snap shut because of this throttle position switch? I try and ease it off, especially in lower gears just because the engine braking effect can be pretty sudden otherwise. When I took my MSF class, the instructor told us to never engine brake a motorcycle becuase the rings can't handle it, but I've heard otherwise from other people...

    And I realize the "choke" isn't a choke, but I guess my assumption was that if it's 85-90 degrees out even a cold start shouldn't need it. I know it won't hurt anything to turn on to smooth out the idle, I was just wondering if this is typically necessary on a warm day.

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    Quote Originally Posted by PGlaves View Post
    You can normally hear the switch click just as the throttle is fully closed. The switch is adjustable - two screws on the back side - loosened, allows the switch to be rotated to adjust.
    Does the bike have to be running to do this test, or do I just squat down by the engine and listen as I work the throttle?

  5. #5
    A bozo on the bus deilenberger's Avatar
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    It's actually difficult to hear the switch with the engine running. Engine off.

    Backfiring is common coming down hills and using engine braking since you won't be fully closing the throttle - instead using the throttle to moderate your engine braking. Less common on flat ground. While snapping the throttle shut (if the TPS is working correctly) will avoid it - it isn't a way I like to ride (throttle dictated by backfiring rather than riding conditions.) And the backfire isn't really a backfire (which is firing back through the carbs - or in this case the throttle-bodies) - it's an afterfire, meaning an explosion in the exhaust system. It's actually due to a rich mixture - unburned fuel - getting into the exhaust and igniting. The K75 seems to do this with some regularity, and theory is - it's partly due to BMW using the same air-flow sensor (the barn-door one) they used on the K100 engine.

    Whatever - if it doesn't do it all the time - consider it normal and learn to ignore it. Your friends will want you in the back of the ride usually..
    Don Eilenberger http://www.eilenberger.net
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    Registered User jeneralist's Avatar
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    What is this "seafoam" of which you speak?
    - Jeneralist
    1992 K75 RT "Kaype"

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    Quote Originally Posted by jeneralist View Post
    What is this "seafoam" of which you speak?
    It's a general additive. I've had good luck in the past using it in bikes. It cleans the injectors and is supposed to be good for clearing out carbon and general top end conditioning. I guess you can also add it to the crankcase oil. I think it's originally sold as a marine engine product, but a lot of bike people swear by it.

    Techron is probably comparable. I hear it's better for the injectors but doesn't do as much to break up carbon deposits, etc.

  8. #8
    BMW uber alles! Zagando's Avatar
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    Arrow Best Stuff on Earth

    SeaFoam is, quite simply, the best stuff on earth.

    It is widely available in auto parts stores, Wal-Mart and anywhere motorcycle/car/truck accessories are sold.

    Try some in your gas tank and also your engine crankcase 40-50 miles BEFORE your next oil change and you'll know why.
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

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    Kbiker BCKRider's Avatar
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    Paul Glaves is almost always right - especially about K bikes. As one who rode a K75 for 8 years, my best guess (and I had the same back-firing problem) is that the trailing throttle is the main culprit. If so, you might want to try gearing down sooner so you get serious engine braking when you need to slow but not stop.

    The advice about simply ignoring it is also solid.

    Also ignore (if you can) the smoke that will frequently come out of your tailpipe if you park the bike on the side stand without leaving it vertical for at least 10 seconds after turning off the key.

    Both the back-firing and and cloud of oil smoke are embarassing, I know from experience, but neither seem to cause any harm to a bike that has an incredible reputation for longevity.
    Doug
    1992 K100RS

  10. #10
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    TPC - Backfire

    Hi guys, I really hate to have you go over this again but as I need your help please forgive me. I too have the backfire issue on my 94 K75, .

    I opened the throttle without any click, and when I fully close the throttle it does not click, either, where do I go from here?
    Adjust the TPC , ie: 2 scews and rotate ( adjust ).

    I checked a friends 94 K75 and he has the click on open throttle and a click on closed throttle.

    Thanks
    2007 R1200R ( trade in for the 2011RT )
    2011 R1200RT
    2010 CanAm RTS Spyder( The Wife's)
    2009 F650GS Single (Also the Wife's)

  11. #11
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by tony2733 View Post
    Hi guys, I really hate to have you go over this again but as I need your help please forgive me. I too have the backfire issue on my 94 K75, .

    I opened the throttle without any click, and when I fully close the throttle it does not click, either, where do I go from here?
    Adjust the TPC , ie: 2 scews and rotate ( adjust ).

    I checked a friends 94 K75 and he has the click on open throttle and a click on closed throttle.

    Thanks
    I would use an ohm meter or continuity tester to verify whether the switches are or are not making contact. I don't recall which wire is which and in on-road mode do't have the manual. Maybe Don E. has the pins or wire colors.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  12. #12
    What, me worry? GILLY's Avatar
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    I had heard that the backfire is caused by something which is actually described in the owners manual (the system, not the backfiring, btw I have never have heard the term "afterfiring". BACKfiring through the intake yes, but generally a backfire is from the exhaust).
    Anywho, I'm sure Paul can describe this better than I, but when decelerating, above a certain speed, the fuel injectors are doing nada, shut off. Below a certain speed (as determined from the wheel speed sensor, not engine rpm), the injectors will again inject fuel, enough to prevent the engine from stalling, and I guess this will sometimes trigger the popping or in some bikes even a solid backfire or two. Unsure if this would affect exhaust valve health or not, I know with cars, constant backfiring is normally not consider good for the engine.
    It all made sense on my old K75 as I would decelerate and I would have to get the speed down to below somewhere between 20 and 30 mph before it would pop, and sometimes it was pretty nasty.
    Gilly
    87 K75S, bought new, now sold
    07 K1200GT Bought new, now traded in
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    Quote Originally Posted by deilenberger View Post
    It's actually difficult to hear the switch with the engine running. Engine off.

    Backfiring is common coming down hills and using engine braking since you won't be fully closing the throttle - instead using the throttle to moderate your engine braking. Less common on flat ground. While snapping the throttle shut (if the TPS is working correctly) will avoid it - it isn't a way I like to ride (throttle dictated by backfiring rather than riding conditions.) And the backfire isn't really a backfire (which is firing back through the carbs - or in this case the throttle-bodies) - it's an afterfire, meaning an explosion in the exhaust system. It's actually due to a rich mixture - unburned fuel - getting into the exhaust and igniting. The K75 seems to do this with some regularity, and theory is - it's partly due to BMW using the same air-flow sensor (the barn-door one) they used on the K100 engine.

    Whatever - if it doesn't do it all the time - consider it normal and learn to ignore it. Your friends will want you in the back of the ride usually..

    I have been looking at a K75 for a second bike so this thread interests me. I am somewhat confused regarding afterfiring. I will agree that the term is more appropriate for the condition of fuel burning in the exhaust. Backfiring occurs thru the intake system.

    I am confused as to the cause. I have heard that both too lean and too rich can cause the condition. Both explanations seem plausible.

    For example, my airhead would afterfire after I removed the air injection system and installed a less restrictive exhaust. The air injection system was there to burn any remaining fuel before it escaped into the atmosphere. The less restrictive exhaust should have leaned the mixture. Modern engines are lean to begin with so now it is even leaner.

    An explanation given to me regarding a too lean mixture as a cause of afterfiring is that if the mixture is too lean (just as too rich) there is incomplete combustion in the chamber, thus allowing some unburned fuel to enter the exhaust. If conditions are just right, this fuel will burn in the exhaust, thus causing the popping.

    So, which is it--too lean, too rich, or could be both, depending on specific conditions?

  14. #14
    Just another hack... :) 122832's Avatar
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    Talking

    So I'll take it I'm the only one who enjoys the bike 'afterfire' under a highway overpass and listening to the echo (and perhaps a startled pedestrian and/or following cage)? {Not that I know how to do that 'on command'... }
    "Quis custodiet ipsos custodes?"
    "... Love Much, Laugh Often..." - Amanda Kay Corso (January 18, 1980 - April 16, 2008)
    Josh / Austin, Texas / '93 K75S

  15. #15
    Stage Crew beemerPhil's Avatar
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    Quote: "I am confused as to the cause. I have heard that both too lean and too rich can cause the condition. Both explanations seem plausible."





    Both are true, under varying circumstances. A lean mix burns more slowly, and can result in both unburned fuel and fire exiting the exhaust valve. A rich mix will also feed unburned fuel into the exhaust, where it may mix with fresh air and re-ignite.

    The injector circuit on the K-series is controlled by engine rpm, not road speed. If the throttle has been closed on deceleration far enough to trip the tps switch, injector function ceases completely until engine speed drops below 1770 rpm.

    This is why this adjustment is so crucial; if the throttle switch doesn't re-open until the throttle is applied more than a hair, there will be quite a sudden surge as the engine comes back online-

    I also use trailing-throttle engine-braking; on a shaft-drive motorcycle, you'd be crazy not to, since throttle control so profoundly affects the attitude of the bike. A shaft drive 'stands up' on open throttle, as the pinion gear tries to climb up the crown gear, and the opposite on closing the throttle. (This is the opposite of chain/belt drives) In a corner, sudden throttle changes can make it very tough to choose and hold a line; in extreme circumstances, closing the throttle suddenly can compromise your cornering clearance- airhead twins will ground a jug. (The purpose of the paralever drive is to mitigate this effect.)

    The backfire/afterfire is pretty easy to produce on command, if you're so inclined, and it can be useful at times; if you're riding with someone who wants to be right on your shoulder, cannon them a few times, and they'll back off...



    I wonder what the msf instructor thought was different about motorcycle rings that they couldn't take engine braking? I've never heard that one before.
    Phil Keppelman #20331
    MOA Rally Stage Manager
    The shortest distance between two points.............
    ain't how I got here......

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