Re: HELP MERCRUISER 3.7 170 hp low spark
Well the problem seems to be one of two things. Spark or fuel. So after reading above, I am going to say try this.
Pull a plug and hook the spark wire to it. Turn it over for 2-5 seconds. Look at the spark color. Is it blue/white? If it is not, it could indicate bad spark which then work backwards. Swap the wire with another (new) one and see if any better. Then look at rotor and distributor cap for bad connection. After a inspection/cleaning of those and still a yellow/red spark, look at the coil. Try a different one as the coil could be going bad.
If all that was fine, then look at fuel. So grab the manual at the adults section and go to the fuel test section. You grab a jar, turn the motor over for so long and you should get so much fuel. That will tell you if the pump is fine. Then open the choke and look down the carb. While looking down the carb, fully open the throttle all the way. Did fuel squirt into the carb? If so then the accelerator pump is working. Do you smell fuel while looking down carb after turning it over a bit? If not, that could indicate a bad float or soemthing else bad with the carb.
If that is all good, pull each spark plug and look for color differences, defects, or wetness. While plugs are out, turn over the motor for about 2 seconds and look for anything comming out the plug holes. There should be nothing (no water vapor or mist of fuel as this would be bad and possible flooding of engine). The only thing left to do (that I can think of) is a compression/leak down test. I recommend the leak down as you can then see if you have a single bad cylinder and where it is leaking (ring, valve, head gasket). If you have low compression, it is not uncommon to not start.
If that is all good, you are in some magical twilight zone or skipped something thinking it should be good when that was infact the problem.
Good luck.
