Intermittent No-Start After a Wet Commute
A 2016 650 cc parallel-twin commuter arrives after three no-start complaints in two weeks. Each time the owner reports the engine cranking briskly but not firing, and each time it restarted after sitting overnight in a dry garage. The last failure happened during a 40-minute ride in heavy rain, at about 4,000 rpm, and the dash briefly flickered. Battery rests at 12.6 V, drops to 10.9 V while cranking, and charging voltage at 3,000 rpm is 14.2 V. The customer wants the bike back today and has already replaced the spark plugs himself. Shop policy requires documented root cause before any part is replaced on a comeback.
1. The technician suspects the customer's own plug change. How should you sequence diagnosis, and what evidence would you need before touching the ignition system?
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The plug change is a plausible but unproven cause, so it must be treated as a variable, not a conclusion. First confirm the basics the customer may have disturbed: plug type and gap against specification, plug cap seating, and whether the caps were pushed fully onto the coils. Then reproduce the fault. Cranking voltage of 10.9 V and charging at 14.2 V are both acceptable, so the battery and charging system are not the primary suspect.
Because the failure is moisture- and vibration-linked, use a wet test: run the engine, mist the ignition and harness areas with a spray bottle, and watch for misfire or stall. Wiggle-test the harness while monitoring. Record findings on the job card before any part is fitted, since shop policy requires documented root cause on a comeback. If the fault does not reproduce, say so in writing and agree a monitored-use plan with the customer rather than guessing at parts.
2. The dash flicker and rain correlation point to a supply or earth fault. Describe how you would isolate it and what you would check first.
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A dash flicker plus a rain correlation suggests a high-resistance connection that goes open under vibration or moisture, most likely an earth or a main supply connector. Start at the battery earth strap and engine-to-frame earths: check for corrosion, tightness, and voltage drop under load rather than relying on a visual inspection. A cranking voltage-drop test across each earth path will show more than an ohmmeter.
Next, inspect the main fuse box and multi-plugs for water ingress and green corrosion, then load-test the ignition supply circuit while flexing the loom. Correlate the flicker with a specific circuit by monitoring voltage at the dash supply and at the coil feed simultaneously; if both drop together, the fault is upstream and shared. Only after the faulty circuit is identified do you repair or replace connectors. Document the voltage readings and the flex points that triggered the fault.
3. The fault will not reproduce in the workshop. What is the correct professional response?
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Non-reproduction is a real outcome, not a failure of the technician. The correct response is to avoid replacing parts on suspicion, because a guess may mask the fault and create a comeback. Instead, gather data: check for stored fault codes, note any historic codes, and inspect for evidence of water tracking, arcing, or chafing that the customer's description supports.
Then agree a documented plan with the customer: a road test in wet conditions if safe and permitted, or a temporary data logger or tell-tale on the suspect circuit. Explain in plain language what was and was not proven, and record it on the job card. If the customer insists on a repair, present the evidence-based options and their risks, and let them choose in writing. This protects both the customer and the business.