Complex sentence: Prior to the design changes, spark plugs would get covered in byproducts spewed by the engine and had to be manually cleaned every 70 to 150 kilometers for the vehicle to function effectively.
Split into simpler sentences: Prior to the design changes, spark plugs would get covered in byproduct spewed by the engine. You would have to manually clean them every 70 to 150 kilometers for the vehicle to function effectively.