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Most common cause of intermittent spark loss?

P

Peter Devil Ears

Regular Contributor
Hi, my 175HP Mercury Black Max, 1979 model, runs great most of the time. Sometimes it just loses power, then runs well again after taking it easy on the throttle for a few minutes. Yesterday it didn't want to start after the third run, and after some time started but smoked. The problem now seems to be permanent. I can hardly start it now. What most commonly causes spark loss? I know it is spark, or rather I am 90% sure it is spark loss, since I sprayed "quick start" into the carb inlets, and still it wouldn't start. Quick start is highly flammable and is basically better than fuel, so if there was spark, and if the problem was fuel, the quick start should have made it fire right up. Thanks for any advice.
 
Step # 1 is to check spark and the proper way.--------Get a spark checker or make something with some wood and wires.------You need to have something with gaps of 1 cm or so to force the ignition to maximum output.--See if you have steady spark on all 6 cylinders.------Possibly one of the switch boxes is bad.
 
Thanks for the assistance guys. The spark is healthy and strong, jumps 1 cm with a bright blue spark. The spark plugs are also fine. So, must be fuelling then, even though I thought it was spark. Maybe it is flooding so the quick start process just added even more fuel and therefore seemed like no spark. Or could the exhaust be blocked? If so how can I check? For flooding I can check float level right?
 
I think they sound a bit weak due to the height above sea level here, 1500 meters. This problem is intermittent, so the fact that it really runs well most of the time means it cannot be a compression related problem right? Can it be bad reeds? Or can it still be a bad switch box, stator or coil, since some spark could be lost intermittently, or does the fact that all sparks jump a 1cm gap really stress these parts and prove them to be OK?
 
Use a timing light on one cylinder after the other and observe the flashing of the light.---------Black spots / intermittant light indicates ignition trouble .----If an ignition problem you will also now know which cylinder is affected.-Do this work out on your lake.
 
I do not believe there is such a thing as a " stuck reed valve "-------------A chipped or broken reed will be a constant problem and not intermittant.-----A chipped or broken reed will have a huge effect on idle quality because there is a lot of time for air to leak out.-----At high RPM there is little time for air to leak.--------Reeds are easy to inspect as they are right behind the carburetors.
 
Take a STRONG flash light and shine that into each carburetor bore at full throttle on the lake.--------Compare the amount of fuel going to each individual cylinder.
 
had exactly the same issue not that long ago with my 1979 50 hp, intermittent spark intensity, would run at wot then loose power and after reducing throttle for awhile would run at wot again for a very short while and then loose power. The last time it happened the motor was hard to start and was backfiring and smoking. I started the motor at home at night on muffs and after a few minutes i saw that 3 of the ignition coils were arcing.
 
That's exactly what mine did, I couldn't explain my symptoms better. Thank you, will start it in the dark (was suggested before, just have not had the chance to sort this yet). Going to take racerones' advice too, since I have found a leaking needle and seat, wonder if that could cause the same issue?
 
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