Author Topic: k75 backfires, stalls, and bucks like a mf  (Read 1885 times)

Offline evanwellsk75

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k75 backfires, stalls, and bucks like a mf
« on: June 06, 2019, 02:00:12 AM »
Hey so i just bought a 86 k75, test drove fine and the title was clean. Rode it about 25 miles home, and around 10 miles in the bike started to lose power (felt like a piston isn't firing) then started to backfire, and the farther i rode it the worse it got. Would stall out at stop lights, and was an absolute pain to get off the line, when it didnt stall the rpms would sit super low then kick into it and jolt me like a bronco. It seems like everyone is saying this is a fuel line problem but the lines look brand new, injectors dirty maybe? But i can figure out why it test rode so well, or why it got worse as i rode. Anyone experience anything like this?
  • Denver CO
  • 1986 K75

Offline Chaos

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  • Mars needs women!
Re: k75 backfires, stalls, and bucks like a mf
« Reply #1 on: June 06, 2019, 08:06:31 AM »
possibly fuel filter.especially if it hadn't be run for a while before you bought it.
  • sw ohio
1987 K75S    VIN 0231
Original owner, Original litter
200,000 miles (plus or minus) and 5 paint jobs
2012 Ural 2WD sidecar (BMW's bastard step child)

Offline The Mighty Gryphon

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #2 on: June 06, 2019, 10:24:32 AM »
What do the spark plugs look like? 

A bad water temperature sensor or dirty connections can make the engine run very rich.  This will make it start and run well when cold, but get worse as the engine warms up.

How does it run when you start it in the morning after sitting all night.?   Unless the plugs are badly fouled, if it's the temperature sensor I would expect it to start and run nice, getting progressively worse after a couple minutes.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE
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Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #3 on: June 06, 2019, 12:51:02 PM »
Lol this would be my second flying brick. With that said can i swap fuel system parts from my 94 75?
  • Denver CO
  • 1986 K75

Offline The Mighty Gryphon

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #4 on: June 06, 2019, 01:06:22 PM »
IMHO, blindly swapping parts is the path to having more non-running bikes.  Especially when you have poor mechanical skills.

Much better, not to mention productive, is to begin a program of refurbishing parts that impact the proper running of the engine.

Having purchased a Brick with running issues, I would strongly advise a teardown and close inspection of the external engine components, especially the rubber bits, the stuff lurking in the fuel tank, testing and cleaning of all engine related electrical thingies, followed by a complete tune up.  This will reward you with a properly running machine in the shortest amount of time, and make you intimately familiar with your engine.

I can't avoid repeating that these bikes are like women, and will reward you abundantly for the time you spend fussing over them.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE
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Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #5 on: June 06, 2019, 01:20:50 PM »
Fantastic thank you, im busy right now but im gonna pull the spark plugs when i get home later. If i figure out exactly whats off il post the steps i took, thanks.
  • Denver CO
  • 1986 K75

Offline volador

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #6 on: June 06, 2019, 02:04:24 PM »
Has the moto sat for some time? Old fuel?  What does the inside of the fuel tank look like?

Rubber fuel pump damper will disintegrate over time sitting in fuel making a bloody mess.
A missing pump strainer will allow the tank debris to be sucked into the fuel pump. No buono!

  • NYC
  • 1991 K100RS 1993 K75S ABS
5 BOROUGHS SISYPHEAN SOCIETY "When Life Throws You A Curve... Lean Into It"
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Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #7 on: June 06, 2019, 02:42:46 PM »
update: all plugs are black, plug 2 is absolutely coated with oil and plug 3 had a little oil on it also this dude was running autolite plugs..... worrying sign for any work he might have done.
  • Denver CO
  • 1986 K75

Offline volador

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #8 on: June 06, 2019, 02:48:54 PM »
spark plugs Bosch X5DC or NGK D7EA with spark plug 5KΩ wires
  • NYC
  • 1991 K100RS 1993 K75S ABS
5 BOROUGHS SISYPHEAN SOCIETY "When Life Throws You A Curve... Lean Into It"

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #9 on: June 06, 2019, 03:17:23 PM »
  What does the inside of the fuel tank look like?
The tank looks good, im gonna try with new plugs and rule out any electronic malfunction before messing with the fuel system. Any idea why the plugs would be covered with oil? other then the piston not firing? didnt know they made bmw 2strokes
  • Denver CO
  • 1986 K75

Offline volador

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #10 on: June 06, 2019, 03:41:23 PM »
Moto was sitting on the side-stand too long
  • NYC
  • 1991 K100RS 1993 K75S ABS
5 BOROUGHS SISYPHEAN SOCIETY "When Life Throws You A Curve... Lean Into It"
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Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #11 on: June 06, 2019, 06:25:34 PM »
This stain looks like evaporated fuel, these problems could be symptomatic of a gas leak right?
  • Denver CO
  • 1986 K75

Offline The Mighty Gryphon

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #12 on: June 06, 2019, 06:38:06 PM »
Something is making your engine run very rich.  Try a set of new, clean plugs to see what happens.  I suspect you'll have the same issues as with the Autolite plugs.

I am going to reiterate my earlier advice to thoroughly go through all the engine operating stuff on the bike before you attempt to do any serious riding.  It appears the bike has some issues that need to be addressed.  It will better in the long run to address them now, especially since the previous owner appears to have had some novel maintenance procedures.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #13 on: June 06, 2019, 07:41:07 PM »
Right tried new plugs (correct bosch ones too) and same problem as before, you seem to know why? You think its a faulty sensor or dirty points then? Im not planning on serious riding for at least two months, but dont have access to a shop for 20 days, so I'm just trying to get the engine running so i have something to ride before a tear down/ restoration.
  • Denver CO
  • 1986 K75

Offline The Mighty Gryphon

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #14 on: June 06, 2019, 08:47:38 PM »
Right now you need to go through the engine control system point by point.  First off, oily plugs possibly indicate stuck rings.  A compression test would be a good start. 

The next thing would be the injectors possibly sticking open and over fueling.  Then you need to check the engine temperature sender.  Are you sure the fuel pressure regulator is working properly?

Do you know if the previous owner made any "adjustments" to things like the MAF and the Hall Effect Sensors.  Being a new owner, you really have no idea of the maintenance history of the bike.  You need to establish a starting point of the engine controls. 
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE

Offline stokester

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #15 on: June 06, 2019, 09:04:54 PM »
To add to all the previous good advice on checking all the items that may be deteriorated from sitting - is it oil or fuel on your plugs?
  • Yorktown, Virginia
  • '94 K75S Dakar Yellow - '93 K75S Seiden Blau - '91 R100RT Dark Blue- '78 R100S Smoke Red

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #16 on: June 07, 2019, 12:50:54 AM »
Okay cheers. No i think the guy knows he sold me bunk bike and is ghosting so no luck on engine history. Its oil for sure, maybe an oil and gas mixture but its black and thick, also left deposits just on a test run with the new plugs- also getting spark but spark is orange.
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #17 on: June 07, 2019, 06:38:13 AM »
Im not planning on serious riding for at least two months, but dont have access to a shop for 20 days, so I'm just trying to get the engine running so i have something to ride before a tear down/ restoration.
There are no motorcycle gods that allow useful riding of a stalling moto without repairing it.

You're all over the map with this thing and diagnosing without skill. Follow Gryph's advice. Proceed system by system. We just had a member whose moto was running intermittently rough and he seemed to relieve those symptoms by tightening the fuel injection control unit's plug to its connection. So review that connection.

Pull the spark plug wires that connect to the coils—one at a time. Check their ends and the coil terminals for corrosion.  Check the spark plugs' top connections. Does each have a rounded terminal nut, or a bare screw? Does the coil wire fit tightly to that connection or loosely?

Let us know what you see.

This stain looks like evaporated fuel, these problems could be symptomatic of a gas leak right?
Another member had a stain in that area, too. It turned out to be dog piss, or at least he said it tasted like dog piss.

No i think the guy knows he sold me bunk bike and is ghosting so no luck on engine history. Its oil for sure, maybe an oil and gas mixture but its black and thick, also left deposits just on a test run with the new plugs- also getting spark but spark is orange.
He didn't sell you a "bunk bike." You bought a neglected K75 that is typically acting like a neglected K75. You're likely to get it running if you can follow directions.
  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline Soggz

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  • 2 wheels are better than 4,but 4 wheels,2 bikes...
Re: k75 backfires, stalls, and bucks like a mf
« Reply #18 on: June 07, 2019, 07:38:47 PM »
Read my mega thread, if you can read for that long, it will be helpful to you, eventually...(the political pics and references are nothing to do with me, bored mods, I think...). My bikes running great now, by the way.
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
One More Of These,One Less Of Those...
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Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #19 on: July 02, 2019, 06:48:41 PM »
Hey so i was finally able to get it into the shop and this is what i've done:
Replaced the fuel pump with a used but working one
Cleaned the tank
Brand new fuel filter
Replaced the fuel hoses inside and outside the tank
Replaced the oil/oil filter
Had the injectors cleaned
Cleaned the air filter
Blew out all electrical points with compressed air
Replaced both the icu and ecu with used but working ones
Replaced the spark plugs (twice)
Replaced the throttle butterfly switch

What i've found:
Fuel delivery is working fine though when we plugged in the new icu the bike started great but the idle continued to rise and fall without the throttle, though after turning it off and turning it back on we were back to no idle and ignition failure after 15 seconds . We swapped the icu back to the original and the bike ran fantastically, it sounded like it should and revved up as far as we wanted it. We turned it off, back on and back to a dolphin like sound and no idle with ignition failure (again) after about 15 seconds or as long as we held the starter.

Im going to swap the injection relay tomorrow but wanted to consult you guys first, again if anyone has ever experienced this sort of thing?


Read my mega thread, if you can read for that long, it will be helpful to you, eventually...(the political pics and references are nothing to do with me, bored mods, I think...). My bikes running great now, by the way.

Soggz i would love to is there a link or something of the sort i could follow, im at a bit of a loss on this guy and could use all the help i could get though i cant seem to find this thread your talking about. Im pretty new to this site.
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #20 on: July 02, 2019, 08:39:39 PM »
Based on your latest descriptions, consider dismantling and cleaning your moto's ignition switch. A faulty or filthy ignition switch will cause electrical current to fluctuate or fail intermittently.
  • Along the Ridley in Vermont.
  • 1995 K75 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline natalena

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #21 on: July 02, 2019, 10:25:21 PM »
As above, clean ignition switch, compression and leak-down, and do more than blow air on the 4-prong connectors. Deoxit and Dielectric grease are your buddies.
Worse case scenario is that it's something so fundamentally simple, you'll kick yourself for missing it.
Good luck, continue to caress and wax her, and she'll eventually come around to purr for you.
* I also had crappy Autolites when I got my K75, must've been a period thing.
  • East of Joshua Tree
  • 1987 K75s #0919
On Holy Quest seeking Techron equivalent for splines.

Offline evanwellsk75

  • Motobrick Curious
  • Posts: 37
Re: k75 backfires, stalls, and bucks like a mf
« Reply #22 on: July 02, 2019, 11:56:56 PM »
Hey thanks guys, never would have thought of an ignition switch for this sort of problem but makes sense.
Yeah it does feel like its right on the verge of wanting to start and run, I certainly hope its something easily missed. That'd be one gooooooood celebratory beer.
  • Denver CO
  • 1986 K75

Offline evanwellsk75

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  • Posts: 37
Re: k75 backfires, stalls, and bucks like a mf
« Reply #23 on: July 06, 2019, 01:20:25 PM »
Update: Not the ignition switch, ordering a new fuel pump relay and replacing the fuel pressure regulator.
  • Denver CO
  • 1986 K75

Offline Soggz

  • ^ SuperNatural Motobricker
  • Posts: 698
  • 2 wheels are better than 4,but 4 wheels,2 bikes...
Re: k75 backfires, stalls, and bucks like a mf
« Reply #24 on: July 08, 2019, 03:13:34 AM »
I think the mods renamed it to something like soggz maga thread or something, for a laugh. Ask Laitch or the mighty griffin. It’s on here somewhere. I can’t find it either.
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
One More Of These,One Less Of Those...

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