After carburetor cleaning or long storage, a twin-cylinder engine (V-twin, parallel twin) that idles only on choke, stumbles under acceleration, with one wet plug and one dry — that's a classic misdiagnosed scenario. The reflex "wet plug equals too rich, I'll turn the mixture screw" is almost always wrong here. Here's why.
Reading Both Plugs Together
On a single-cylinder engine, a wet plug can signal excess richness. On a twin, what matters is the difference between the two cylinders, not the absolute value. Asymmetry (one wet, one dry or fouled) betrays a mechanical difference between the two sides — not a global adjustment issue.
Cause #1: Air Leak on One Cylinder
The rubber intake manifold between carb and head hardens and cracks with age (often after 10–15 years), or its clamp loosens. The affected cylinder then draws air parallel to the carb → lean mixture → misfires → dry or fouled plug. And if you've opened the mixture screw to compensate, the other cylinder (healthy) gets flooded → wet plug. Hence the asymmetry.
The test that confirms it:
- Engine idling, spray a little brake cleaner around each intake manifold. If the idle changes when you spray one specific manifold, the leak is there.
- Alternative: block the air filter inlets with a cloth for a few seconds. If the engine runs better, there's an air leak upstream of the carb.
Other Causes of Asymmetry
- Out-of-sync carbs (independent carburetors): check with a vacuum gauge, both cylinders should read the same at idle.
- Idle jet clogged on just one carb: incomplete cleaning, that cylinder stumbles.
- Float level too high on one carb (needle valve): that cylinder is constantly flooded, wet plug persists.
