Trying to get a K75 up and running

[Hubert.szcz]

I've perused this site for years now, and finally making a post.

A family friend had an old K75 (VIN: WB1057401L0133114) he was going to get rid of because it hadn't run in a few years, I told him I'd take it and fix it up (this was late 2023). I replaced a lot of things and got it to run, rode it around the block. It still had a lot of issues, but I had to let it sit. I am only in the same place as the bike for a month or two a year. I got back in late 2024 and started working on the other things that needed doing before it was road-safe. The battery had died, but I knew I wouldn't finish the rest of the repairs until I had to leave so I didn't replace it, but got pretty much everything else good. Now it's late 2025 and I was really hopeful that with a new battery and a few odds and ends it would be ready to go, but alas, once again mysterious issues.

Every time I leave, I drain and dry the tank, disconnected the battery, and put a sheet over the bike. When I get back I add 1 gallon ethanol-free to the tank to see if it runs.

Here's the history of my repairs:

Nov-Dec 2023:
-   New used tank (old had a small leak)
-   New everything inside the tank (fuel pump, fuel sending unit, lines/hoses, fuel filter)
-   New (used) battery
-   New fuel out of tank line
-   New fuses
-   New injectors
-   New spark plugs
-   New air filter
-   Clean mass air flow and airbox
-   Clean top of engine block between airbox and plenum
-   New vacuum tube from fuel pressure regulator
-   Plugged California crank case evap intake
-   New throttle body vacuum caps
-       New Crankcase Breather Vent Hose
-   Stripped the allen-head bolts going into the fork stabilizer. Got a hardened steel extractor broken off into one, called it quits

<got it running and rode around the block at this point>

Nov/Dec 2024:
-       Battery died, did not replace.
-   Removed and cleaned out radiator
-   Replaced fan with new Spal fan
-   Got lower fork stabilizer off, replaced bolts
-   Replaced bolts for middle trifold
-   New front Spiegler steel front brake lines, bleed and new brake fluid (have matching rear brake line to put in later)
-   Bought new forks, (old forks deeply scratched and with a bolt stripped inside left fork)
-   New tires
-   Greased driveshaft splines!
-   Emptied final drive oil, but couldn’t get the filler bolt to turn!
-   Cleaned rear brake calipers with brake cleaner – pads don’t need replacing
-   Copperease on bolts: 4 wheel securing, 4 final drive securing

Dec 2025:
-   New battery (Odyssey PC680)
-   New starter relay
-   New Crankcase Breather Vent Hose... again ... was seeing cracking on new one already
-   Added coolant to radiator

Bare minimum still need to do (other than mysterious new issue):
-       Get the new forks in
-       Get the final drive filler bolt out, add oil, new bolt

So, now I beseech this community to help me get it to actually run. Turning the key gets all the lights to turn on, horn works. It cranks but won't start. Couldn't get it going with a push start. What's left to fix? Could my new fuel injectors have gotten messed up some how without any use in 2 years?

[John Lang]

Does the headlight come on when you turn on the ignition? If not, there is a grounding problem: check the starter motor brushes.

[John Lang]

Oops. I see that it does crank so starter motor is fine.

[Laitch]

You’re learning useful lessons in this project from which others may benefit also! One is that buying replacement parts does not necessarily mean that a problem is being "fixed". Another lesson is: before replacing fluids in machinery, be certain the filler plug opens before opening the drain plug.

After some attempts at starting, remove the spark plugs and tell us whether they are wet with gasoline.

[Edward]

First on the list should have been locating and cleaning ALL the grounding points thoroughly. Use steel wool or very fine sandpaper. Please keep your battery on a quality trickle charger. If new, you need to initialize the battery first with a 24 hours pre charge. Make sure the battery has the correct specification. Do not underbuy quality. And yes, lets see the plugs. Hopefully you have the correct plugs. Personally, I use iridium on the K and my MV.

[Hubert.szcz]

Finally had some time to work on it. Plugs are dry. I detached the tube leading to the fuel rail, and unfortunately that is also dry when I try to turn it on - the pump isn't turning on. Next I'll check the various connections between the battery and pump with a voltmeter. The ignition coils give spark. I did notice when I pulled the plugs that the area inside looks a bit gunked up (see photo).

[Laitch]

Start by testing for voltage on both sides of the four-pin connector under the right side of the fuel tank when the ignition switch is On and the starter is spinning. Which wiring schematic are you using?

[Laitch]

While you're resting, probe that gunk in the spark plug holes and let us know the depth of the debris in there and whether it is sludge, granular or solid.

[Edward]

Given the visuals of possible gunk... Looking forward to more detailed photos.
You had stated the injectors are new as 2023, nonetheless it would be wise to use a 9 volt battery to test the spray.

[Smudgemo]

Is the fuel pump actually not working?  Make sure the fuel lines from the pump and filter in the tank are still attached as they should be.  I had the hose between the filter and the tank's hard line pop off not long after I replaced everything.  Not pushed on far enough or insufficient clamping force, I suppose.

When it happened, the bike didn't immediately die, but didn't run right or very far.  Residual fuel probably still working its way through the system. 

[Hubert.szcz]

Battery is at ~13V

Right two holes give 11.07V when key is turned, 9.8V with starter spinning

left two: 1.4V when key is turned, -1V with starter spinning

Opened up the tank again, all the tubes are secure (to/from the pump, filter, and out of the tank). None of the fuses are burnt out. Where is my missing voltage hiding? Should I replace the connector?

Not sure what you mean by wiring schematic.

I didn't get a chance to look into the gunk again, leaving for a trip. Will pick this up again in February.

[Laitch]

What are you trying to determine by probing contacts on that particular connector?