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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Atabeach on June 18, 2025, 11:24:37 AM
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Ok So I am the proud owner of 2 K75 bikes, My daily driver..a '90 K75S 'bout 40K...& a 94 Standard...less than 9K on clock. They both have no OOMPH below 2k and then suddenly smooth out...
The bikes feel like they are "holding back" like a no torque condition...The 94 smoothes out at EXACTLY 2000 RPM...The 1990 at about 2500. I dont think this is fuel related. I'm thinking Valve related since there is a smoothing out of engine sound. I find myself downshifting to 2nd gear around modest turns to avoid this. Post script, I saw the video from Chris Harris...he seems to think the valves rarely need adjusting.
TIA...If it aint broke...fix it?
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Which gear are you using at 2000rpm when you notice this condition?
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2nd & 3rd. I feel like I’m constantly downshifting..like I have no torque. The issue is with both bikes…at the correct RPM ( slightly different on each bike) it smooths out….To be honest…even first gear when taking off ….bike has that smooothing out feeling suddenly at 2k ish. Thanks in advance
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Have you checked for air leaks? They will give you a lean mixture at idle and small throttle openings and then smooth out at wider throttle openings as the normal airflow overwhelms the leak to bring the mixture into balance.
The crankcase breather tube AKA "Z" tube is notorious for cracking at the clamps. Microscopic cracks that allow just enough air to make the bike run like crap. A lot of experienced owners just replace them when they get a bike.
Also, the bushings above and below the throttle bodies are known to develop leaks. Check these with propane or carb cleaner when the engine is idling. Leaks are found when the engine speeds up.
Last, check the rubber caps on the throttle body balance ports. They are also known to crack and allow air leaks.
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Valves are crazy easy to check and not hard to adjust, compared to some Jap bikes I've worked on. Since they don't need adjusting often many never get checked. Good advice about the z tube, and it could be injectors not spraying properly, maybe try a couple tanks with some Techron or Seafoam added.
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Bingo Griff! Quick propane check has RPM rising when idle. I will swap them out..along with Z hose when I get a chance. Thanks! Responding to Chaos…that was my first thought…fuel related like it was starving. Went through the system and sent injectors out to be serviced in both bikes. No change. Thanks for the replies guys!