" The motor shakes a bit as if it is misfiring (if it had more than one cyliner - does it?). " You need to furnish more information such as the model number of the engine to start with since there are more than one 17 hp OHV B&S engine. Walt Conner

Well, the mower is anything but new, but it was given to me with a handle that was falling apart, a blade that was really, really dull, splayed front wheels... Basically, used hard, often, and put away wet. The old air filter was years old and probably didn't do a great job of filtering. The gas can is clean, and the gas is about a week old. If time permits, I'll take the bowl off again and see if any other sediment has gotten into it, as well as blow out the main jet with compressed air. If it makes it to winter, I'll treat it to a full carb removal and clean/rebuild. vince

The 6 horsepower engine is most probably the 12 cubic inch Quantum series - not the smaller engine with the diaphram carburetor. Since you have already disassembled and cleaned the carburetor, try starting the engine with 5 - 6 drops of gasoline poured directly in the venturi area of the carburetor. If the engine fires up - then dies, changes are the problem is still in the carburetor. The main fuel jet is located in the bolt that holds the float bowl to the carburetor. This must be cleaned with spray carburetor cleaner. Other solvents are not effective. There is no problem running this engine with the air filter removed - just don't mow your yard until the filter is installed. If the engine starts - and continues to run, inspect your primer system. Install a new gasket between the plastic filter base and the carburetor. Do not install the filter or filter cover. Use a flashlight and observe the venturi area of the carburetor while depressing the primer bulb. You should see some fuel discharging. If there is no fuel, please ask for additional information.

It is very hard to imagine the cap coming off the valve while the engine is running and not having an extreme affect on the engine running if it would run at all. I am not sure but what the thickness of that valve cap would be such that it would exceed the depth of the indention in the rocker arm, which holds the push rod in place, letting the push rod fall out and rocker arm dislocate. I would come nearing thinking that it was dislodged when someone was working on the valves and left out when the valve was adjusted. Also, a dislodged valve cap is not going to cause the engine to "makes an awful clanging noise from the engine." It would be noisy of course. Sure, put the cap back on, adjust the valves and see what happens. (Don't try to "pop" it back in place) To answer your question, it is held in place by the close fit of the parts, not enough room for it to come out. Walt Conner