Author Topic: K75C Rehab  (Read 2428 times)

Offline johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #25 on: February 06, 2019, 03:02:40 PM »
Last night I put +12v on the green pin of the male tank connector, and -12V on the brown pin, and of course the fuel pump works. So, to recap: power at the female end of the 4 pin when engine turning over, when 12V applied to the male end pump runs = bad 4 pin :( Luckily have some waterproof connectors arriving today. The dreaded 4 pin connector strikes again!

Also, K75C it is! Drum brake isn't so bad after all.
  • Aurora
  • 1988 K75C

Offline johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #26 on: March 27, 2019, 11:37:52 PM »
Hello Flying Brick Friends,

Since we're in a bit of a warm patch here in CO, I've been spiritedly riding my K75 the last couple of days. Today, about halfway through a 10 mile ride the bike started surging/sputtering around 6k RPM. It does it in gears 1-3, but it seems more pronounced in gears 2-3. I thought it was maybe a fuel starvation/clogged vent line issue since I can hear some faint bubbling coming from within the gas cap after I'm done riding, so I turned the bike off and unlatched the cap. That worked for a couple minutes but then the sputtering came back. Also the idle is higher once the bike is warmed up (goes from ~1000 to around 1500 rpm). Any suggestions? The bike runs great up to right around 6k rpm.

I've replaced:
z hose
throttle body caps
air filter
battery
all outside fuel lines
vacuum line to FPR
fuel pump
fuel sender
4 pin connector
  • Aurora
  • 1988 K75C

Offline daveson

  • ^ Proficient Motobricker
  • Posts: 445
Re: K75C Rehab
« Reply #27 on: March 27, 2019, 11:50:24 PM »
I spose you replaced the fuel filter. Was the tank full before you went on the ride, and was it the first ride of the day. Maybe the vent is blocked or the fuel filter.
  • Victoria, Australia
  • Current;'86 K100RT Past; '97 Yamaha V Star 650 Yamaha V Star

Offline johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #28 on: March 28, 2019, 12:05:20 AM »
Yeah I’ve replaced the fuel filter as well. Tank is over 3/4 full. It was the first ride of the day. Seems odd for it to start out of the blue in the middle of a ride.
  • Aurora
  • 1988 K75C

Offline daveson

  • ^ Proficient Motobricker
  • Posts: 445
Re: K75C Rehab
« Reply #29 on: March 28, 2019, 12:20:12 AM »
Does the exhaust have a hissing sound at idle or revs like maybe it's blocked? Maybe check to see if removing the cap consistently fixes the problem, if so it might be time to check fuel pressure if there is nothing conspicuous in the tank or fuel pressure regulator.
  • Victoria, Australia
  • Current;'86 K100RT Past; '97 Yamaha V Star 650 Yamaha V Star

Offline Martin

  • ^ Quintessential Motobricker
  • Posts: 3805
Re: K75C Rehab
« Reply #30 on: March 28, 2019, 02:08:12 AM »
If you have a non OEM fuel return line they can crimp where they go into the tank this can cause running problems. It's hard to detect because as you lift the tank and pull it back it can uncrimp. Look for signs of crimping and collapsing. If it has collapsed you can remedy the problem by fitting a Unicoil.
Regards Martin.
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  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.
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Offline Laitch

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  • Posts: 6737
Re: K75C Rehab
« Reply #31 on: March 28, 2019, 07:10:37 AM »
I've replaced:
z hose
throttle body caps
air filter
battery
all outside fuel lines
vacuum line to FPR
fuel pump
fuel sender
4 pin connector
Check that the fuel lines and their couplings within the tank are tight and in good condition. Consider replacing the fuel filter. OEM filters are dated on the outside. Alternatives are available. Check the site's Alternative Parts section.

How is the tank being vented—into a cup mounted on the frame, or by a hose from the underside of the tank to behind the right foot peg plate? Check that the tank vent pipe within the tank is unobstructed.

Check for tightness and clean all ground connections, battery post connections and the battery ground strap attached to the transmission.
  • 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 Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: K75C Rehab
« Reply #32 on: March 28, 2019, 08:36:21 AM »
What's the average elevation of where you regularly ride?
  • 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 The Mighty Gryphon

  • ^ Quintessential Motobricker
  • Posts: 4541
Re: K75C Rehab
« Reply #33 on: March 28, 2019, 10:34:57 AM »
For reference:  Fuel filter=NAPA Gold 3032.

If you don't have the plastic cup under the tank that the vent and filler cap water drain run to, then you may have a pinched tank vent hose where it goes behind the right side footpeg plate.  Easy check is to take the bike out for a ride with the cap unlocked.  You might want to have about half a tank of fuel or less when you do it to prevent sloshing out. 

Another possibility is the Hall Effect Sensors that trigger the ignition and the injectors.  When they start to fail they misbehave when the engine warms up, and work properly again when the engine cools down.  I'm not sure that is your problem, but it is something to consider when everything else has been checked.

I agree with Laitch that you should check electrical connections, especially the grounds on the engine and frame backbone.  Make sure the connector on the ECU is tight while you're at it.

It's also possible that you have a bad spark plug or plug wire.  Do you have the terminal nuts on the spark plugs?  Bosch has them, but NGK doesn't and you have to get them and install them yourself.  They are necessary if you have OEM plug wires.

Last, I have had problems with dirty ignition switches on two bikes.  They would cause the engine to quit when running.  It's not a likely cause of your problem, but with these bikes getting older, it's something to consider.  There are cleaning tutorials here.
  • 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 johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #34 on: March 28, 2019, 10:47:49 AM »
Thanks for all the replies, guys. Forgot to mention in the original post that the fuel filter is new as well (NAPA unit). I do most of my riding around 5280' but don't use the high altitude plug, and can't find where it would go either. I have not balanced the throttle bodies or checked the valves yet. FOR SHAME.

@Daveson- the bike idles well and revs up to around 6k rpm well. No exhaust hissing or anything like that. I'll look running it w/out the cap and see if that makes a difference.

@Martin-is the fuel return line the front left hose? If so, I used a length of non-OEM hose for that. Good call on the unicoil spring device. I will check that as well, as there is a pretty sharp turn into the top of the FPR also.

@Laitch- I will admit I have paid zero attention to the ground points. They could undoubtedly use a cleaning. As for the tank venting, there is a hose that runs from the rearmost tube to the footpeg plate, and a check valve on the other line that runs to the crankcase. I will investigate all the vent lines, and probably just buy the cup.

@Gryphon-I'm hoping it's something as simple as a pinched tank vent hose. I've checked the spark plugs (they're NGK with terminal nuts and in great shape). Ignition switch and HES crossed my mind also. I'll take that under advisement!

  • Aurora
  • 1988 K75C

Offline The Mighty Gryphon

  • ^ Quintessential Motobricker
  • Posts: 4541
Re: K75C Rehab
« Reply #35 on: March 28, 2019, 11:15:56 AM »
The high altitude plug is just above the coils below the tank, usually behind a frame tube.  The jumpers are something like $9 from the dealer.  I run with the jumper installed all the time when I'm in your part of the world.

It's not all that critical as far as I can tell.  I have forgotten it and been up to 9000 feet without it, and left it in down to almost sea level.  I suspect that it's more of an emission thing than performance.

If you haven't seen the plug, you may already have it connected by the previous owner.
  • 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 johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #36 on: April 07, 2019, 09:53:51 PM »
Update:

I've cleaned the spark plugs, ground points, checked the tank vent tube (can blow through it), and replaced the fuel filter again. I also removed the airbox to verify that the fuel return line is unkinked. All of that helped a little, but after awhile the bike would continue to sputter/surge above 6k RPM. Another point: I have the vent stub by the fuel cap, but my fuel pump screen doesn't have the little vent fitting, so I've just removed the vent hose. Then I cleaned the fuel injectors, and while doing so may have stumbled upon a clue.

After removing the fuel line at the most forward part of the rail, gas was continuously pouring out. I remember doing this in the past and having a very small amount of gas come out, but not in a continuous stream. Now I'm thinking that it must be a hole in the submersed fuel line within the tank? Planning on removing both fuel lines and submerging them and blowing into them to see if there are any splits or cracks. What do you guys think? It couldn't be related to the fuel pressure regulator could it? HALP
  • Aurora
  • 1988 K75C

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: K75C Rehab
« Reply #37 on: April 07, 2019, 10:07:46 PM »
Planning on removing both fuel lines and submerging them and blowing into them to see if there are any splits or cracks. What do you guys think?

It couldn't be related to the fuel pressure regulator could it? HALP
Checking those lines was recommended in Reply#31 and I still think it should be done. As far as the fuel pressure regulator is concerned, try starting the moto, shut it off then remove the fpr's vacuum hose from its port at the #3 throttle body. If it smells like fuel or leaks fuel, a new fuel pressure regulator is needed.
  • 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 johnclaude

  • Motobrick Curious
  • Posts: 28
Re: K75C Rehab
« Reply #38 on: April 07, 2019, 10:16:34 PM »
Thanks for the reply, Laitch. The fact that gas pours out of the feed line makes me think that it has to be one of the submerged fuel lines, which would maybe explain why it surges at speed when the fuel pump is working overtime (if gas under pressure is escaping the fuel lines). Does that sound plausible? But I will also check into the FPR vacuum line for gas stank as penance for not checking the lines earlier.
  • Aurora
  • 1988 K75C

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