outboard that runs with the cowl off but stalls with it on indicates an internal exhaust leak, where poisonous exhaust gases are trapped inside, forcing the engine to burn oxygen-depleted air. Common causes include a failed powerhead-to-adapter gasket, a leaking engine holder due to corrosion (especially on DF115/140 models), or damaged exhaust manifold gaskets.
Common Causes & Symptoms
Corroded Engine Holder/Plug: On older 4-stroke Suzuki models, exhaust leaks from the engine holder (exhaust manifold) area, often near the flush port, caused by saltwater corrosion.
Powerhead Gasket Failure: The gasket at the bottom of the powerhead (powerhead-to-adapter plate) fails, releasing exhaust inside the cowling.
Symptoms: The engine idles fine with the cowl removed because the exhaust dissipates, but with it on, it stalls, runs poorly, or runs very rough.