The STL designation is for the "hi perf" version of the 150. Puts out about 10% more hp than the basic 150. Also, this usually means that the ignition system is the high-output version. Instead of a 10 amp charging system, this 150 version outputs 35 amps. Usually this flywheel has the visible "vented" flywheel. There is a free website that has some excellent troubleshooting for the ignition system. Check out this site: cdielectronics.com. When you get into this website, check out this company's ignition troubleshooting guide. It's excellent and covers all of the crossflow years. Read through the various models and compare to your engine to match up the guide with your engine version. Also, your engine is stamped with your model number on the top of the block. Check out this casting plug on the top of all blocks. The model and serial number are stamped with the same ID info from the serial tag which is riveted to the transom bracket. See picture.
the model number is the year identifier, not the serial number.
