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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Los_gatos on July 29, 2025, 10:07:40 PM
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How does this look for flow from the fuel return line on a 92 k75? I'm currently having some rich running issues. Saw the discussion on another post and thought I'd grab some video.
https://youtube.com/shorts/PtcQt7yBF6A?si=4SA_QE39N66Jw7JN
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Looks good.
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92 k75 running rich based on all three plugs being wet. Idles fine, but stumbles and dies at 1/2 to full throttle off idle. Wot is fine above 3-4k rpm. I pulled the injectors to find this (pic below). My question is whether I should just replace the injectors or get the 'injector repair kit' from MotoElectrics. I've cleaned them up, but suspect they're sticking open. All the black stuff is sticky which I assume came from the fuel pump dampener which has since been replaced with a new fuel filter too.
Air temp, water temp sensors, and AFM all tested in spec. I do have an air leak after the AFM and the new top air box/pipe arrive tomorrow. The injectors were also leaking when I sprayed them (increased rpm).
Thanks!
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https://www.mrinjector.us/
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If RPM increased with spray then you probably need to replace the throttle body boots.
https://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/k75pa.rehab/k75parehab.htm#fuel.rail
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Thanks. Maybe. I was spraying the carb cleaner on the bottom of the injectors with the little straw to specifically look at the injector orings. I'll focus a spray on the boots though soon.
Unmetered air wouldn't cause it to run rich right?
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Adding un-metered air makes the mixture more lean.
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Received the injectors back from Monsieur Injector, none were leaking and one had a 'fair' flow. The other two were 'good'. Good peace of mind though. Still the same issue of falling on it's face with 1/4-WOT off idle. It idles great though. It ran fine before I starting tinkering with it.
Tinkering done: got the weeping tank welded, replaced the fuel pump diagraph that had degraded in ethanol, replaced fuel filter, replaced most fuel lines except the return line, replaced both handle bar controls, replaced top air box and tube, cleaned up the ground cable, and lubed splines at the clutch and rear diff.
Not sure what could be making it run rich. TPS switch clicks just off idle like it should, 'choke' seems to do what it's supposed to when cold, coolant temp tested fine cold (I may let it warm up and test it), intake air temp tests okay, air flow meter tested okay. If I had to throw a part at it, it would be the coolant temp, but I never tinkered with it. It does have a dumb cafe exhaust on it which I had to remove and reattach to do the spline lube. Would the exhaust back pressure affect anything?
Any thoughts? Thanks!
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If I had to throw a part at it, it would be the coolant temp
Any thoughts? Thanks!
Of course you know you don't just throw parts at a problem; that can wind up being both costly and ineffective. You test them first. There are two ways to test the coolant temperature sensor. You can test it with a multimeter at ambient and operating temperature, or it can be removed and tested in hot water to determine its resistance at various stages of cooling.
Find instruction in the method on pages 21, 22, and 24 of the flow chart/troubleshooting guide found in the link at the bottom of this post (https://motobrick.com/index.php?topic=11014.msg95717#msg95717) in the Service Manuals section.
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Based on what did you conclude it was running rich?
Of your bike dies on you opening the the throttle it could be a few issues.
At least from what was written, it reads like an assumption.
Stalling at opening the throttle could also be caused by:
A faulty MAFS( flap ),
Vacuum leaks,
Bad TPS,
Bad throttle body (springs),
Valve clearance,
And a bad coolant temp sensor.
On top of what Liatch stated therse a third way, just for testing purposes and that is to paperclip connect the 2 connection points. Seeing if the sensor fully functions and activites the fan wil require a heat gun because it activates at 103 celcius (you yourself may convert it to freedom units)
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All three plugs were fouled and the exhaust smells rich. The coolant temp sensor tested fine yesterday. The throttle did seem a little better after I let it warm up but still died at probably 1/2-wot. I'm going to driving it tomorrow evening and see if just running it might help. Thanks for the input.
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Rode it and it did okay once warmed up. Hesitates a bit off throttle and it's backfiring a bit. Will fill it up with gas and ride it longer this weekend. I might get a fuel pressure gauge and see how that's doing.
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Ran it for about 10 miles and it all seems fine. Nice to finally ride it. It's slightly too tall for me. A low profile seat is hopefully in the future. Just a few things to tidy up and it'll be ready to daily.