1973 115 No spark when humid

[JOFO17]

I have a 73 Johnson 115 that I was troubleshooting for running with no power and breaking up at higher rpms. When checking spark with a gap tester, 2 of the 4 cylinders seemed weak. I swapped coils on the port side cylinders to see if the problem moved with the coil or stayed put. before I could get another good spark test the battery was starting to tire so I decided to charge the battery and start again next chance I got. That was on Monday. It was rainy and humid ever since, and has been raining all day today as well. I just went out to see if the weak spark traveled with the coil, but amazingly had NO SPARK on ANY cylinder. I had to close up the gap to about 1/4" to get spark. I was wondering if no or very weak spark is a sign of something that is a common issue? Any help is appreciated.

 

[emdsapmgr]

Re: 1973 115 No spark when humid

A gap tester is a good way to check for spark. Also an inductive timing light. Swapping the coils is also a good troubleshooting technique, though generally, those 4 coils are pretty reliable. The battery should be well charged prior to starting. The starter must crank the flywheel at 200-250 rpm in order to fire the ignition system on that engine. A weak battery may not be cranking at a high enough rpm... There is a website which has some excellent troubleshooting info: cdi.com. There is a section on your engine that may be of assistance.

 

[JOFO17]

Re: 1973 115 No spark when humid

The battery had a fresh charge today and was cranking just fine. I did do another test. A resistance test on wires 2-4 and 9-12 after disconnecting them from the power pack. Both had identical 22 ohms of resistance. From what I read that should have been in the 6-10 ohm range. Resistance between wires 7-8 was within specs @ 625 ohms. I'm curious as to whether the resistance will change when the weather dries. I did have a nice strong continuous spark on 2 of the cylinders on the last dry day, and a strong intermittent spark on the other two. I just lost spark AFTER all of the rain. The motor was covered in case u were thinking along those lines.

 

[emdsapmgr]

Re: 1973 115 No spark when humid

I've had better luck checking output voltages than resistance. You'll need a special peak-reading voltmeter. I'd check the voltage output on the both the stator and the timer base. If they check out ok, the power pack becomes the prime suspect. Check out this web page for voltage ouput specs:
http://www.cdielectronics.com/downl...77 85-140 HP (Screw Terminal Power Packs).pdf For some reason this website may be truncating the website address. sorry for the problem.

 

[JOFO17]

Re: 1973 115 No spark when humid

Thanks, that link is a lifesaver. I found the problem in less then 5 minutes. As soon as I took off the terminal cover to access the rectifier I saw the badly corroded connection. Once I disconnected the yellow wires and retested for spark, I had a nice strong spark. I'm going to borrow one from another motor I have while I wait for the parts house to get me a new one in. I'll let you know if that solved all of my problems. I'm hoping a good solid spark will take care of everything.