I know this is about cars, but bear with me and hear me out. Used to do my own servicing on a volkswagen golf we had years ago. Lifted the air cleaner to get to the sparkplugs, jammed a rag into the carb intake to stop any debris falling in. Drained oil, replaced filters, including air filter, checked brake shoes and pads, adjusted handbrake, replenished oil. Fired her up. Went well for about 15 minutes then started misfiring like a good un'. Checked the hv leads and spark leads, yup, they're ok. Checked contact points , yup, correct gap. Jumped in fired her up a went for a test drive. Got about 5 miles from home and after spluttering and jerking for the last half mile came to a complete dead end. Called the AA and hour and a half later they arrived, told them what had happened. Mechanic scratches chin with puzzled look on face, lifts air cleaner, and hey presto.................rag is still in carb!!