Author Topic: k75 backfires, stalls, and bucks like a mf  (Read 1887 times)

Offline Soggz

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #25 on: July 08, 2019, 03:17:18 AM »
. . .
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
One More Of These,One Less Of Those...

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #26 on: July 08, 2019, 08:04:33 AM »
We turned it off, back on and back to a dolphin like sound . . .
That "dolphin sound" is most likely the fuel pump which will be heard running for a second or two after a starting attempt. It's helpful to post a YouTube video for sound identification.

TMG cautioned against throwing parts at this moto but that seems to be your strategy. Rather than doing multiple replacements at one time, I suggest starting the engine after each replacement. That might reveal the exact cause and will be helpful to others who might have a similar problem but not the funds for shotgun-style parts replacement.  Please continue to update us when the ignition control unit and fuel pressure regulator have been replaced.
  • 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 johnny

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #27 on: July 08, 2019, 09:49:35 AM »
greetings...

im thinking you take it to pete at...

bavarian motocycles west of colorado
2312 s delaware st denver co 80223
720 570 2044

he will have it figgered out just rolling it in... however we all know its the fore pin...

j o
  • :johnny i parks my 96 eleven hundert rs motobrick in dodge county cheezconsin  :johnny

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #28 on: July 09, 2019, 04:50:01 PM »
The fuel pressure regulator, icu, ecu, and fuel pump relay have all been replaced with zero changes to the problem at hand. The sound im referring to isnt the fuel pump its the starter motor.
  • Denver CO
  • 1986 K75

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #29 on: July 09, 2019, 04:57:21 PM »
Laitch, I have been starting the moto after each replacement or cleaning. Its run great twice, once after swapping an icu (no start on the replacement) and then plugging in the old one. The second after literally the exact same back and forth on a fuel pump relay. SO we have narrowed down the problem to be an electrical misstep somewhere in- and heres where im lost- we suspect fuel supply system.
Johnny, id love to take it to a shop but frankly im out a decent bit of cash on both the salvage title k75 im using as a parts bike and this '85 lemon that im trying to fix up. Would rather sell the both to the highest bidder and use the cash to pick up a ticket and a little shitter in laos or vietnam.
  • Denver CO
  • 1986 K75

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #30 on: July 09, 2019, 07:01:56 PM »
Crankcase breather hose. When replacing the fuel regulator i noticed it was cracked, i removed it to do a momentary repair with liquid gasket and made the last post before putting it back on. Ordered a new one but the DIY job seems to work okay, giving me the confidence to say with general certainty that that was the issue. Thanks for letting me throw my thoughts against ya here, il keep this thread posted if it continues to run fine or if i run into another problem
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #31 on: July 09, 2019, 10:23:03 PM »
Crankcase breather hose.  il keep this thread posted if it continues to run fine or if i run into another problem
:clap:
  • 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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Re: k75 backfires, stalls, and bucks like a mf
« Reply #32 on: July 10, 2019, 03:18:32 AM »
Crankcase breather hose. When replacing the fuel regulator i noticed it was cracked, i removed it to do a momentary repair with liquid gasket and made the last post before putting it back on. Ordered a new one but the DIY job seems to work okay, giving me the confidence to say with general certainty that that was the issue. Thanks for letting me throw my thoughts against ya here, il keep this thread posted if it continues to run fine or if i run into another problem
yup, that was one of the first rubber bits I had to replace to make it run even. I did do a ‘repair’ with heat shrink wrap, which worked until I could afford to throw a few quid at the bike. The bike was cheap,  it the parts are sooooo expensive for what they are...
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
One More Of These,One Less Of Those...
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Offline DJEwen

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #33 on: July 10, 2019, 07:39:00 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?

I had this exact symptom when I clanged the fuel sender (early pre float type). I changed it back and the problem was cured. You can read about it in my thread from post 240 onwards: http://www.motobrick.com/index.php/topic,10875.225.html  I'd check that the collars for the 4 pin under the tank are tight and clean ensuring a good connection.

Had your seller just power washed the bike? I also had an almost identical issue the last few days after going to pick the moto up from a vehicle detailer, it wouldnt run, then would but not hold idle, would backfire. She dried out over the weekend, then started on the monday, rode her home for her to not restart.  I traced it back to a dirty and slightly damp #4 injector plug as can be seen from this video, The bike has seen 3500 miles of roads in all conditions in the last year and is due a connector "service":



I also found, at least on my first fuel injection wiring harness, that the run from the under tank connector down to the FICM had been water logged due to poor wrap, this had caused a wire break which will give you the same symptoms again. In my case every wire was corroded black, I had to replace the whole harness.

It's always electrical for me I've found. Could it be an earth? As a matter of course, it is beneficial to go through every electrical connector and clean and grease the contacts. I went as far as removing heat shrink around earthed connections to ensure they weren't broken. The electrics are old on these girls, regular tlc or heavy front end refreshing will save you less pain in future.

The good luck in your fault finding.

EDIT: You've cured it - well done!
  • Scotland
  • 1985 K100 'Revive', 1987 K100 LT

Offline Photog

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #34 on: July 10, 2019, 09:20:27 PM »
I had a lot of ignition problems earlier this year getting the bike awakened from the winter hibernation.
It idled slightly rough, but wouldn't make power starting off, and had a consistent miss at speed.
What did the trick was cleaning the low voltage contacts on the coils, both in the connectors and the pins in the coil.
I had unplugged and plugged these multiple times, but attacking them with some contact cleaner is what did the trick.
I got there by pulling the spark plugs and finding that the middle one was slightly black compared to the other two. I cleaned the middle coil first, and the misfires went away, and followed-up by cleaning all three.
YMMV, of course.
  • NNE
  • K75-S

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #35 on: July 16, 2019, 06:01:15 PM »
Alright guys so heres the saga of the last week- I take the bike out first day- rides great go to a movie that night and when i come out i notice a little gas under my bike. Weird but i brush it off. Next day i ride it down to a meeting first thing and by the time i get to the cafe the bike is pissing gas everywhere. I narrow it down to a single injector thats spraying from the rail side. I wheel it over to a friends garage pull the injectors to find one of my O rings had broken up (I had my injectors done professionally/ by a friend) before so a little annoyed but ya know what its alright. Run down to bmw dealer to pick up my 45$ o rings run back and find that my pintle cap on that injector is cracked. New one coming in from good old mr injector and im throwing my rail back on a few days later (today.) Now heres where im stumped, replaced the o rings on all three and did a total overhaul of the problem injector, and now im pissing gas out of the header side. I did everything correctly when putting the guys back on, any ideas?

TLDR: pissing gas out of my header side on injector 1, replaced the o rings already.
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #36 on: July 16, 2019, 07:06:13 PM »
I did everything correctly when putting the guys back on, any ideas?
Describe how you installed them. Did you lubricate the o-rings?
  • 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 evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #37 on: July 16, 2019, 11:26:26 PM »
I installed the spacer then using a loose socket installed the pintle cap, then rolled o ring on. Then took a blowdryer to the cap and used a little motor oil on the end of a q-tip to smother the o rings. Then i twisted and applied pressure to the injector until it seated fully into the header (it was seated on the same level then the other two) then used a block of wood and light hammer taps to seat the rail. So I think i did everything right? Only thought is that the heat from the dryer may of damaged the o ring. Is that even possible?
  • Denver CO
  • 1986 K75

Offline Soggz

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #38 on: July 17, 2019, 02:44:52 AM »
Is everything clean?  Oils be a little bit of dirt sitting on the face of the seat where the O ring sits. Or sometimes, when putting it all back together, the O ring can sort of move up a little bit. Happened to me, and had a tiny fire, as I had a plug cap off when checking the spark on no.1.
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
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Offline The Mighty Gryphon

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #39 on: July 17, 2019, 09:24:36 AM »
I get my o-rings from Carquest.  P/N BWD 274572.  Not only are they cheaper(about $10 for 10), but they include a plastic cone that lets you install them without nicks that cause leaks. 
  • In my garage in Marilla, NY
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Offline natalena

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #40 on: July 17, 2019, 11:22:57 AM »
Then took a blowdryer to the cap and used a little motor oil on the end of a q-tip to smother the o rings.
Q-Tips always leave lint hairs. Probably not a biggie, but they shed more than cocker spaniel in summer.
  • East of Joshua Tree
  • 1987 K75s #0919
On Holy Quest seeking Techron equivalent for splines.

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #41 on: July 17, 2019, 03:20:35 PM »
Huh maybe I just nicked or broke one when installing it, I gave em a good look over but if that's easily done then I guess it's an option. I just got the bike back to the shop today, will update. Thanks yall


Side note is it possible that the washers that connect the fuel rail to the header could be bad and therefor not giving enough tension on plug one to keep it in place? Easy fix and just wondering because mine look original.
  • Denver CO
  • 1986 K75

Offline Soggz

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #42 on: July 17, 2019, 06:29:06 PM »
Huh maybe I just nicked or broke one when installing it, I gave em a good look over but if that's easily done then I guess it's an option. I just got the bike back to the shop today, will update. Thanks yall


Side note is it possible that the washers that connect the fuel rail to the header could be bad and therefor not giving enough tension on plug one to keep it in place? Easy fix and just wondering because mine look original.
Old bike now, replace ALL rubber parts.
  • 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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Power loss quick questions
« Reply #43 on: September 02, 2019, 04:57:17 PM »
Well folks, here i am again with the bucking problem. I set out on my 86 k75 last week to head up to montana for some good old brick-camping and hanging, annnnnd the bike sputtered to a stop on a mountain pass. Heres what happened and what i think, but im still new to my brick so id like to run it past yall. I was going around 70-80 (speedo dont work) and felt a little bump, like a buck but tiny so much so i thought it was something i went over. Then about a 2 minutes later, again, and again with increasing intensity and speed and as they increased i slowly dropped rpms, to the point where i was wide open and had nothing. Pulled over and the bike died, couldnt get it to start back up. So i post up, smoke a cigarette and try her again, and boom shes all good, though my first thought was vacuum leak so i look around the bike and find a single drop of something. stayed by it for a while longer to see if i was leaking and where from but cant find it and dont see any more mystery liquid I limp, keeping the rpms under 3000, the 10 miles down to butte misfiring the whole way and here i am. Now i reckon its either a fuel problem or a vacuum leak but those are both aspects that i refurbished when i got the bike.
New pump
new filter
new fuel lines
refurbished injectors
new rubber bits all around
etc etc

And body got a lead? I read someone had power loss when his exhaust valve went out, but i did a compression test before i left and clocked in on all 3 with 145. Also when i left i had a good amount of oil above the dot, now i look through the view glass and its at the dot. Could be unrelated but i thought it was strange. Taking the bike down to slc tomorrow where i can use a garage for the week.
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: Power loss quick questions
« Reply #44 on: September 02, 2019, 05:07:20 PM »
  • 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 evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #45 on: September 02, 2019, 05:14:17 PM »
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
  • Denver CO
  • 1986 K75

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #46 on: September 02, 2019, 05:15:53 PM »
whoops didnt mean to post that yet

Also replaced the crankcase breather hose and fuel injector o rings
Did a few more things to it but nothing having to do with the engine itself
  • Denver CO
  • 1986 K75

Offline evanwellsk75

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #47 on: September 02, 2019, 05:16:48 PM »
cleaned my air filter too, and checked the rubber there. it looked fine.
  • Denver CO
  • 1986 K75

Offline Laitch

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Re: k75 backfires, stalls, and bucks like a mf
« Reply #48 on: September 02, 2019, 05:20:56 PM »
Vacuum port caps on the throttle bodies? Did you check the manifold bushings for leakage? Have you done the Hall sensor test?
  • 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 evanwellsk75

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  • Posts: 37
Re: k75 backfires, stalls, and bucks like a mf
« Reply #49 on: September 02, 2019, 05:23:47 PM »
Havent worked on the bike at all, this happened last night. Like i said getting it down to salt lake tomorrow, though those do all sound like good places to start. How do you do a hall sensor test?
  • Denver CO
  • 1986 K75

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