What Causes the Engine to Stall When the Choke Valve is Opened?
3 Answers
When the gasoline engine stalls immediately after opening the choke valve, it indicates that the engine temperature is too low to rise properly. In such cases, the choke valve should be partially closed to increase the fuel-air mixture concentration. If the engine stalls upon opening the choke valve, it suggests that the cylinder cannot heat up or that components are not operating smoothly, requiring a very rich fuel-air mixture to forcibly sustain operation. If excessively low ambient temperature is ruled out, check the lubricating oil and replace it if it has deteriorated. Below is an extended explanation: The function of the choke valve: Starting the engine in winter can feel very difficult. Especially in the morning, it can be quite perplexing. In fact, winter starting difficulties mainly occur in carburetor-equipped engines. During the design of such engines, consideration was given to the fact that gasoline does not evaporate easily in winter, making starting difficult. Hence, the choke valve was specifically designed. The choke valve reduces the amount of air entering the carburetor intake, increases the fuel pressure at the nozzle, and enriches the air-fuel mixture. This makes starting the engine easier.
Last time I fixed my brother's old motorcycle, I encountered this situation. The engine stalling with the choke on is basically a sign that the engine hasn't warmed up enough, especially noticeable in winter. It's like covering a cold engine with a blanket, then removing it when it's warm. You can tell by checking the spark plug color: if the center electrode turns white, the air-fuel mixture is too lean, possibly due to a clogged low-speed jet in the carburetor. For fuel-injected vehicles, focus on checking the coolant temperature sensor and idle valve, while carbureted vehicles need to check for vacuum leaks. Another small detail is the exhaust pipe temperature: after warming up, touch the muffler area—if it's not hot enough and you open the choke too quickly, it will definitely stall. I remember solving it by cleaning the carbon buildup behind the throttle body and readjusting the idle screw three and a half turns to stabilize it.
I've fixed this issue many times. Essentially, it's caused by sudden changes in the air-fuel mixture concentration that the engine can't handle. Opening the choke increases air intake, and if fuel supply can't keep up, the engine stalls. The most effective troubleshooting approach involves three aspects: check if the fuel system has insufficient supply (whether the filter is clogged, if fuel pump pressure is adequate); inspect the air system for abnormal leaks (whether the manifold gasket or vacuum hoses are cracked); finally examine the ignition system (distributor contact gap, ignition coil resistance). Additional tip: Some older vehicles experience this due to incorrect valve clearance - when the engine heats up, gap changes cause insufficient cylinder pressure. I recall one case where an owner adjusted the choke cable too tight, blocking the bypass fuel passage. In such cases, simply disassemble and clean the carburetor assembly.
