Hot No-Start That Comes Back After Cooling
A 2019 150 cc single-cylinder commuter arrives with an intermittent no-start that appears only when hot. The owner says it always restarts after 15 to 20 minutes of cooling, and that a forum member told him to replace the CDI. The battery rests at 12.6 V and holds 10.8 V while cranking; the terminals are clean and tight. Cold, the plug produces a strong blue spark. After a 10-minute ride, the same check gives a weak yellow spark that then disappears. Plug cap resistance is 5.1 kohms and plug gap 0.9 mm (spec 0.8 to 0.9). Ignition coil primary reads 0.4 ohm (spec 0.2 to 0.5) and secondary through the cap 12 kohms (spec 8 to 15), both measured cold. The exciter (source) coil reads 340 ohms cold against a spec of 120 to 200 ohms; the pickup coil reads 190 ohms (spec 180 to 220). Cranking speed sounds normal, valve clearance was checked last month, fuel flow is good, and a known-good CDI from an identical unit is available in the shop.
1. Which measurement points to the fault, and what is your next test?
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The exciter (source) coil is the outlier: 340 ohms cold against a 120 to 200 ohm spec. Everything else is inside range, and a battery holding 10.8 V while cranking is acceptable. A winding that is already out of range cold will usually drift further as it heats, which matches a spark that dies after 10 minutes of riding and returns after cooling. Next tests, in order:
- Repeat the exciter resistance immediately when the fault appears, without cooling the engine, and record the hot value beside the cold one.
- Measure exciter AC peak output at the CDI connector while cranking to the manual figure. Resistance can look marginal while output collapses under load.
- Inspect the stator for burnt lacquer, chafed leads at the grommet and a loose flywheel, and check the engine-to-frame ground path. Applying gentle heat to the stator with a heat gun while watching the meter will confirm thermal breakdown before you commit to a part.
2. Why is the CDI a poor first suspect, and how do you prove the fault before quoting the customer?
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The CDI is a sealed module with no meaningful static test; the only field test is substitution. It is also the most expensive part in the circuit, so guessing with it is expensive guessing. The evidence here points upstream anyway: the spark is weak rather than cleanly cut, and it fails only when hot, which is the classic signature of a winding breaking down rather than a switching module dying. Prove it methodically. Verify battery voltage at the CDI supply, then check the kill switch, side-stand switch, ignition switch and every ground. Measure pickup pulse and exciter output at the connector while cranking hot, and flex the harness near the steering head to rule out a chafe. Only when the inputs are correct and the output is still missing do you fit the known-good CDI as a test, recorded on the job order as a test and not as a sold repair.
3. What do you tell the customer, and what goes into the record?
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Tell him plainly: the CDI is not the part that has failed. The exciter coil measured 340 ohms against a 120 to 200 ohm spec when cold, and the spark disappears only once the engine is hot, so a new CDI would leave the fault in place. Give two numbers, not one: the stator replacement as the recommended repair, and a separate note that if a hot no-start somehow persists afterwards, the CDI will be re-checked without a second diagnostic charge, but the part will not be given free. Document: cold and hot resistance values, the manual spec, battery rest and cranking volts, spark quality cold and hot, the heat test if used, and any test CDI fitted and removed. Have him sign the authorization for the stator job and initial the declined items, and keep the removed stator available for inspection.