1 K75 Low power - missing on one cylinder Wed Jun 24, 2020 3:30 am
MikeyD
active member
Hi all! I offered to help a friend with her K75 so I could ride it as I had never been on one before. I picked it up last year because it wouldn't start. I'll make a separate post about that. After I got it running it had really low power on the test ride. Wide open throttle would only gradually cause acceleration and it barely wanted to exceed 60 mph. Having never ridden one before I wasn't sure what it was supposed to feel like so I didn't know what to attribute the low power to. Thanks to someone on here, I read about using the exhaust temperature as a quick method to check cylinder combustion. I determined the front cylinder was not firing. Below are my notes I took as I tested and made assumptions about the results:
Idled engine and discovered exhaust pipe from #1 (front) cylinder is cold. Assume no combustion on #1 cylinder. Checked resistance values on all three ignition coils - all checked good. Pulled spark plug cap from #1 plug and inserted a spare plug, grounded it on engine, cranked starter. Engine seemed to run as before removing plug wire from #1 spark plug. Repeated same steps for #2 cylinder. Engine barely started and died after a few rough revolutions. Assume no combustion on cylinder #1. Removing #2 plug wire left the engine to run only on #3 and it wouldn't idle.
Tested spark plug wire resistance. Should be about 6,000 ohms. #1 = 1.1 M Ohms (very high resistance), #2 = 350 K Ohms, #3 = 800 K Ohms. All high resistance, though #1 is extremely high. Later attempt resulted in open circuit, no current through #1 plug wire.
Installed 3 new spark plugs. Still runs poorly as before. #1 was black and dry, #2 was black and wet, #3 was black and oily. Though the engine hasn't run enough to warm up, which might result in cleaner plugs.
Replaced all 3 spark plug wires with new wires and it still runs poorly as before. Though #1 plug wire was dead and the other two had high resistance, it wasn't the only thing causing a lack of combustion in #1 cylinder.
Checked resistance across terminals of fuel injectors. Target value = 16 ohms. Each of the 3 installed injectors read 18 ohms. Assume good internal windings for the solenoids. Tested voltage across female terminals of fuel injector wire harness while cranking starter. Each showed voltage, at least 10V. Performed combustion test again as before by pulling spark plug from #1 and #2 cylinders (after installing new spark plug wires) to confirm still no combustion on cylinder #1. Assume fouled injector on #1.
Prior to ordering new injectors I did a quick, cold compression test on cylinders #1 & 2 just to see if compression on #1 is grossly low compared to #2. #1 = 137 psi, #2 = 145 psi. Assume enough compression in cylinder #1 for combustion.
Sanding, preparing tank for patches, patch tank holes with epoxy putty, remove injectors, clean deposits and sediment from fuel rail. Sand and paint epoxy patches on tank, clean injector ports and surrounding intake on cylinder head, install new fuel injectors. As a side note - when I first got the bike the fuel in the tank looked like Italian salad dressing; it was cloudy with different colored chunks. Even if none of that went through the filters, I suspect the fuel that remained in the injectors suffered the same fate after 3 years of not running.
Installed fuel pump into tank, installed tank and fired it up. Engine now runs on all three cylinders. Runs well on test run. It hesitates off idle when cold but that quickly goes away as it warms up. Something to deal with later.
So there was a double whammy on cylinder #1 - the bad plug wire and a bad injector.
Idled engine and discovered exhaust pipe from #1 (front) cylinder is cold. Assume no combustion on #1 cylinder. Checked resistance values on all three ignition coils - all checked good. Pulled spark plug cap from #1 plug and inserted a spare plug, grounded it on engine, cranked starter. Engine seemed to run as before removing plug wire from #1 spark plug. Repeated same steps for #2 cylinder. Engine barely started and died after a few rough revolutions. Assume no combustion on cylinder #1. Removing #2 plug wire left the engine to run only on #3 and it wouldn't idle.
Tested spark plug wire resistance. Should be about 6,000 ohms. #1 = 1.1 M Ohms (very high resistance), #2 = 350 K Ohms, #3 = 800 K Ohms. All high resistance, though #1 is extremely high. Later attempt resulted in open circuit, no current through #1 plug wire.
Installed 3 new spark plugs. Still runs poorly as before. #1 was black and dry, #2 was black and wet, #3 was black and oily. Though the engine hasn't run enough to warm up, which might result in cleaner plugs.
Replaced all 3 spark plug wires with new wires and it still runs poorly as before. Though #1 plug wire was dead and the other two had high resistance, it wasn't the only thing causing a lack of combustion in #1 cylinder.
Checked resistance across terminals of fuel injectors. Target value = 16 ohms. Each of the 3 installed injectors read 18 ohms. Assume good internal windings for the solenoids. Tested voltage across female terminals of fuel injector wire harness while cranking starter. Each showed voltage, at least 10V. Performed combustion test again as before by pulling spark plug from #1 and #2 cylinders (after installing new spark plug wires) to confirm still no combustion on cylinder #1. Assume fouled injector on #1.
Prior to ordering new injectors I did a quick, cold compression test on cylinders #1 & 2 just to see if compression on #1 is grossly low compared to #2. #1 = 137 psi, #2 = 145 psi. Assume enough compression in cylinder #1 for combustion.
Sanding, preparing tank for patches, patch tank holes with epoxy putty, remove injectors, clean deposits and sediment from fuel rail. Sand and paint epoxy patches on tank, clean injector ports and surrounding intake on cylinder head, install new fuel injectors. As a side note - when I first got the bike the fuel in the tank looked like Italian salad dressing; it was cloudy with different colored chunks. Even if none of that went through the filters, I suspect the fuel that remained in the injectors suffered the same fate after 3 years of not running.
Installed fuel pump into tank, installed tank and fired it up. Engine now runs on all three cylinders. Runs well on test run. It hesitates off idle when cold but that quickly goes away as it warms up. Something to deal with later.
So there was a double whammy on cylinder #1 - the bad plug wire and a bad injector.
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1985 K100RT (0052390), 1985 K100 (0030066), 2003 K1200GT (ZK01448), 1984 R65, 1984 R80ST, 1974 R90/6, 1976 R90/6, 1997 R850R, 1991 Honda CB250, 1998 Sportster 883