Author Topic: '91 K75 fires but won't fully start  (Read 780 times)

Offline LGT

  • Curious
  • Posts: 3
'91 K75 fires but won't fully start
« on: March 06, 2019, 06:02:55 PM »
Hey everyone,
My dad and I have been working intermittently on a K75 project for around the last year and a half, and are just about done. At this point though, the starter cranks, and there are some occasional pops coming from the exhaust, but the engine won't idle (video attached). The fuel injectors squirt gas, the spark plugs produce a healthy spark, the plugs are connected to the correct coils, and we've been through just about every page of Bertrand Vogels K100 troubleshooting guide. The airflow meter and the water temperature sensor both seem in line with what the guide specifies. The idle switch was giving me some trouble (it wasn't flipping with the throttle closed, so I tried to adjust it, and now it doesn't flip at min or max), but the guide says you should be able to unplug it if there are any issues, and trying that didn't make a difference.

At this point, neither of us are sure what to do short of jumping into the timing and hall sensors, which I'd rather avoid if possible. It seems like it has fuel, air, and spark, so I don't know why it wouldn't be running. Thanks for your help!
  • Durham, NC
  • K75

Offline johnny

  • TrailBrakingThrottleWhacker
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  • Posts: 6669
  • Whacking...n...Chopping Sliding...n...High Siding
Re: '91 K75 fires but won't fully start
« Reply #1 on: March 06, 2019, 06:22:58 PM »
greetings...

thank you for poasting the video...

1... did you see this moto run prior to working on it for a year and a half...

2... battery does not sound 100% at this point...

3... be advised that jumping a motobrick with a car generally causes irreversable jetronic whack...

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

Offline LGT

  • Curious
  • Posts: 3
Re: '91 K75 fires but won't fully start
« Reply #2 on: March 06, 2019, 07:04:02 PM »
Hi Johnny, you're most likely correct, as we've kinda abused this battery, but we've been jumping the bike with a car while testing, so the battery probably isn't a factor. Thanks!
  • Durham, NC
  • K75

Offline alabrew

  • ^ Proficient Motobricker
  • Posts: 120
Re: '91 K75 fires but won't fully start
« Reply #3 on: March 06, 2019, 07:33:17 PM »
Sounds to me like it is at least part of the problem too. Check out:
Be careful, you can fuse the starter relay with low voltage.
I didnít think I heard the fuel pump either before or after, can you hear it running?
  • Alabama
  • 1985 K100, 1991 K100RS
Also:
2005 K1200LT
1979 R65
200,000 miles on BMW motorcycles

Offline Laitch

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Re: '91 K75 fires but won't fully start
« Reply #4 on: March 06, 2019, 10:30:46 PM »
It certainly sounds like the engine is getting insufficient current to start and idle. Vogel's guide will only be effective if all the basics are in good shape. The battery must be in good condition. Charge it and get it load tested. The frame ground connections and the transmission battery ground connection must be clean and tight. The fuel hoses within the tank must be of the correct type, in good condition and tightly connected. The fuel filter must be in good condition. OEM filters have dates on them. What is yours dated? When the moto stalls, the fuel pump should be heard for a couple of seconds before it shuts down.

The fuel injectors squirt gas. How much fuel is sprayed compared with what?
  • Along the Ridley in Vermont.
  • 1995 K75T 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 Chaos

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Re: '91 K75 fires but won't fully start
« Reply #5 on: March 06, 2019, 10:35:37 PM »
hope the gas has not sat for a year and a half
  • sw ohio
1987 K75S    VIN 0231
Original owner, Original litter
200,000 miles (plus or minus) and 5 paint jobs
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Offline The Mighty Gryphon

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Re: '91 K75 fires but won't fully start
« Reply #6 on: March 07, 2019, 09:22:36 AM »
I don't know how long since your bike ran, but you might want to try this trick.



Timing is easy to check.  Remove the "T" shaped timing cover and make sure the cutouts in the plate and the timing chain cover line up.  That gets you the factory setting.

Get another set of spark plugs.  I like the NGK DR7EA.  Recently I had my K75 that ran well, refuse to start.  Plugs came out wet, and sparked, but wouldn't start the engine.  New plugs got it running. 

I have found with my bricks that if they don't start right away the plugs get soaked and won't fire.  With a fresh battery I unplug the tank connector and crank until the engine blows out the excess fuel and fires.  Plug the tank back in and it usually starts.

  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too

Offline LGT

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  • Posts: 3
Re: '91 K75 fires but won't fully start
« Reply #7 on: March 07, 2019, 11:50:16 PM »
@johnny, can you elaborate on 'jetronic whack?' Is the ~13V from the car alternator enough to fry the IO circuitry on the module? Additionally, no I did not see the bike run. I should also mention that the drivetrain, frame, tank, and gauge cluster are all from different bikes. The drivetrain was from an incomplete cafe racer project, and the previous owner said that the bike had run somewhat recently while still in RT form, although the timeline isn't 100% clear.

@Laitch, would a weak battery significantly shunt current from a healthy car battery connected in parallel? We didn't try it just connected to the car battery. For ground connections, what is the maximum resistance I should be seeing between the ground on the jetronic for example, and the negative terminal of the battery? ~1-2 ohms? I'll be sure to check those connections though. As far as the fuel connections go, I assumed the hoses were the submersible type, as the inside of the tank looked unmolested, but I'll verify that along with the filter date code, which I haven't checked. I'm not exactly sure how to quantify how much fuel the injectors sprayed. Each sprayed maybe 0.2mL per pulse? The fuel pump runs as you described.

@Chaos, no the gas is maybe 3 months old at most, and has had fuel stabilizer in it for the bulk of that time.

@The Mighty Gryphon, Ah thanks! Chris's videos have been super helpful. We just ordered extra spark plugs too, so that's a few things to try.

Thank you everyone for your input!
  • Durham, NC
  • K75

Offline Laitch

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Re: '91 K75 fires but won't fully start
« Reply #8 on: March 08, 2019, 02:06:28 AM »
. . . would a weak battery significantly shunt current from a healthy car battery connected in parallel? . . .
The healthy battery is trying to supply the weak battery with enough charge to start the engine. The weak battery might not be able hold enough charge to do that and even if it does start the engine, it might not be able to run it for long, or restart it. If the engine doesn't operate at high enough revs, the alternator won't be activated to take over both running the engine and also recharging the battery. A discharged battery could be one of the sources of your present challenge.

Connect the engine to a 12V battery verified to have ~13V output and be in good condition. If the engine starts and idles, the battery's condition was part of the problem.

Before making an attempt to run a used bike made from variously-sourced used parts, valve clearances should be checked, coil output should be metered, coil primary and secondary connections should be verified, all electrical connections including electronic fuel injection control and ignition control plugs should be cleaned then tightly connected, and a battery in good condition should be used.

The condition of everything should be verified rather than assumed.

Good luck with your project.
  • Along the Ridley in Vermont.
  • 1995 K75T 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: '91 K75 fires but won't fully start
« Reply #9 on: April 03, 2019, 12:33:58 PM »
Sounds to me like it is at least part of the problem too. Check out:
Be careful, you can fuse the starter relay with low voltage.
I didnít think I heard the fuel pump either before or after, can you hear it running?
Sounds a lot worse than mine, and itís been sat since 2008.
  • Down in Selwood Forest
  • 1986 K75C, 1982 Honda CX500EC Eurostreetbrat
One More Of These,One Less Of Those...

Offline daveson

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Re: '91 K75 fires but won't fully start
« Reply #10 on: April 24, 2019, 05:52:55 AM »
A quick check.

Disable the injectors and see if it starts with starter fluid (and stops when the fluid stops) That would be a pointer to a fuel problem.

For some reason I cant open the video, but I do get the k100 one on the bottom.
  • Victoria, Australia
  • Current;'86 K100RT Past; '97 Yamaha V Star 650 Yamaha V Star

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