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Rough Idle When Cooling Fan Runs

Rough Idle When Cooling Fan Runs

By Chris Thomson
January 2001


I noticed after some fan maintenance on my ’86K75C, the bike would run poorly when the fan automatically activated. The engine would misfire, not a single cylinder miss, but apparent instant ignition source failure, causing the bike to stall in traffic, restart and throttled off idle, engine would run with persistent misfire, until the fan would stop.


Lifting the tank revealed the ignition source cable (pickups on the front of the engine) would loop around close to the back of the fan motor, then up to the connector and into the ignition control unit. All connectors were checked and cleaned with electrical contact cleaner, and the ignition source cable, routed and secured away from the fan motor and cable. The bike was tested fault free when the fan ran (regular in Bangkok). The following 6 days the bike traveled from Bangkok to Kuala Lumpur in Malaysia and back, covering 3500 km’s without fault.


LE Jet FI systems source all synch-timing from the ignition unit, hall-effect pickups on the front of the engine. The low level signals present on the pickup leads are cleaned up by the ignition controller, and used for spark timing, a clean synch-pulse sent to the FI computer for injection timing. The leads on my K75C from the pickups to the controller appeared to be unshielded, close proximity to the electrically-noisy fan motor inducing noise on the pickup signal, that couldn’t be processed by the ignition controller, causing rough running. NOTE: Japanese cars of the mid 80’s using the same ignition/injection system had heavily shielded and grounded cable between the pickups in the distributor body, and the FI computer/ECU.


Prior to my ride from Bangkok to KL, I had a number of problems to solve. This little ’86K had been sitting for approximately 10 years, outside in the tropical elements, unused and unloved, with only 4000 km’s on the clock. The bike cleaned up well, and showed remarkable resistance to deterioration over this time. The bike has been used for Bangkok city runs, trips to the North-East etc, bringing the mileage to around 7500 km’s before the trip South. During a check of the charging volts, a static run of around 4000 rpm off-load, with lights on for a few minutes had the header pipes glowing red-white. Ambient temperature of approximately 35 degrees Celsius, no wind or breeze around the bike. The engine was running happily, with neutral mixture, and dropped back to a perfect idle. At motorway speed at night, no glow is visible despite the engine being under load, so airflow across the pipes seems the key. As mentioned before, the bike ran without fault for the next 3500 km, delivering good performance, and economy, even in 40+ degree temperatures.

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