Sudden, brief loss of power - 1987 K75C
Symptoms: When moving at speed (happened anywhere between 15-50 mph), sudden (VERY) loss of power, with no response to throttle. Bike slows, of course, but engine somewhat curiously appears to remain "on." No odd behavior of dash lights. After a few seconds, power and throttle response return. Occurred about 6-7 times in ~60 miles of riding, but "clumped" into three instances, where it happened once or twice a few seconds apart. (27-mile each way commute, with a little midday driving in between.)
My first guesses were: Fuel delivery problem. Suddenness of onset and resumption of normal response suggests electrical cause. Or maybe water in gas? That pernicious fuel-pump connector under the tank? Of course also suspect fuel filter/hoses/pump. Absence of funkiness with dash lights (and a much less catastrophic-feeling failure) suggests it's not a recurrence of my previously jiggly kill switch.
Testing today found a faulty throttle position switch, but not in a way that could obviously cause the observed behavior. The switch tests in the "open" condition. That is, on a normal switch (or so says the one on my parts bike), the outboard pin and the center pin should be closed when the throttle is closed, and open when the throttle is open. (The center and inboard connector should be closed at WOT.) The presumed-faulty switch tests open regardless of throttle position.
So, this shouldn't cause what I'm seeing ... or could it? We know some people remove this switch, which would leave the bike in the position I found in testing: circuit permanently open. If, however, the switch's internals are loose, such that the contacting bits could jiggle into a closed position, it seems to me it could cause what I experienced. Can anyone confirm an occurrence of this? Is anyone familiar with the internals of this switch? I'll pull it apart tomorrow; just ran out of daylight today.
Today's diagnostics also included:
- Check battery terminal connections: fine
- Check ground to frame: fine
- Check spark plug & fuel injector connections: snug
- Computer connector: snug, contacts clean
- Pull & inspect spark plugs: fine
- FPR vacuum hose: lookin crusty, replace next week
- Crankcase vent hose: also crusty; order & replace
- Check & reseat tank connector: Looks fine
- Check for spark: Fine on all three zylinders
- Check fuel pressure: 36-37psi (stationary, of course)
- Drain tank; no evidence of water or crud in gas (vibration damper newish; < 10k miles)
In progress, to be finished on return of daylight:
- Replace fuel filter
- Replace in-tank fuel hoses with appropriate new OEM hoses
- Replace fuel supply hose (tank to fuel rail)
- Check that tank vent pipe is clear
On receipt of parts order:
- Replace crankcase vent hose
- Replace FPR vacuum hose
- Replace fuel return hoses
(Edited to add other diagnostics performed.)
Classic symptoms of the under tank plug problem
So I had an 85, but unless the Germans dramatically redesigned that 4 pin connector they are problematic and reseating them hard accomplishes nothing permanent. This problem has been well covered here by people smarter than me, but the core design of the male tank plugs and the female 'tube' connectors in the wiring harness is at the root of your problems IMHO.
Sudden brief loss of power - 1987 K75C
During these episodes did the tach continue to show engine RPM or did it drop to zero ( even though engine is still being cranked over by the rear wheel ) ?
If the tach went to zero your hall sensor may be going intermittent . That happened sporadically to my K75 after I'd run it enough to get it hot . As soon as it cooled off for a few seconds as I coasted off the road , it would run fine again for a while ...
Sounds like you may have already fixed it , but if not, perhaps something to investigate ...
Sudden brief loss of power K75
When my engine cutout , I instinctively ( ingrained spanish 2 stroke dirt bike reflex ) pulled in the clutch and coasted off the road . It immediately restarted at the side of the road and continued on ( for a while ) before it happened again . Ambient temp was 70s . My first failure was probably 25 miles from home, then failure 15 miles after that, then 10 miles after that , then 5 miles . The last time it didn't want to start immediately , though after sitting for a minute fired right back up as if nothing happened ! This was to be a 300 miles ride day and at that point I was spooked about it cutting out in traffic.
Since it was dry day and I suspected that it was heat related ignition failure since failures were becoming more frequent .... and I wanted to continue , I removed the T shaped cover over the Hall sensor to cool it off then kept on riding . It did fail again , but this time after 150 miles or in traffic and then restarted immediately. To me that pointed to a heat relationship to hall sensor. I replaced it with used unit and it hasn't hiccuped in the past 25,000 miles . Fixed !