The engine starts, holds for a few minutes, then stalls as soon as it heats up or you release the throttle at a red light. It's one of the most frustrating symptoms—and one of the most misdiagnosed. The reflex of "it's the choke" or "it's the ignition" is often wrong. Here's how to think it through.
What the symptom tells you
An idle that collapses when hot almost always points to a disturbed air/fuel mixture: either too much air (vacuum leak), or not enough fuel (clogged idle circuit), or a richness setting gone wrong. Heat expands parts and intensifies a vacuum leak; oxidized fuel clogs the small idle jets first. This is why "when hot" is such a valuable clue.
1. The idle circuit (carburetor)
On a carbureted motorcycle that's sat through winter, suspect number one is the idle jet clogged by oxidized fuel. Idle jets have tiny passages that clog first. Symptom: it starts on choke, holds at mid-rpm, but dies at idle. The solution isn't to force the richness screw but to remove and clean the circuit.
2. The richness screw
After cleaning only, refine the richness screw with the engine warm. A common starting point is around 1.5 to 2 turns out, but the exact value depends on the model and year—it's typically the kind of number your mechanic AI pulls from the factory manual for your specific bike.
3. The vacuum leak
A cracked intake manifold hose, hardened gasket, loose clamp: the engine sucks unmeasured air and runs lean. When hot, the rubber expands and the leak worsens—hence the stall that only appears once the engine is warm. Simple test: spray a little brake cleaner around the manifolds at idle; if the rpm changes, you've found the leak.
