BSDenning
Petty Officer 1st Class
- Joined
- Nov 23, 2007
- Messages
- 271
A friend of mine has asked me to see if I can fix this 20hp Mercury. I have a few Evinrude and Johnson motors that I tinker around with, but I have never worked on a Mercury. From my research, I believe this is a 1974 model. The serial # is 3884074. The problem is that the motor will not idle. It will not crank in neutral. You have to put it in gear so that you can increase the throttle to about half throttle. Then the motor will crank and rev way up, but you have to throttle it down to get the RPM's back down. Then it is kind of a battle to keep the throttle high enough so that it won't die, but not too high to keep the motor from throwing a rod. It won't idle down without dying. My first thought was that the motor had a bad coil. I tested both coils with my multimeter as shown in this video.
http://www.youtube.com/watch?v=KT8rk5QWgS0
One coil was showing me about 12 holms resistance between the spark plug wire and the ground. The other coil showed 1.3 holms resistance between the spark plug wire and the ground. So, I figured that the 1.3 was probably too low to fire the plug. I cranked the motor with only the good coil's spark plug wire attached and the motor ran pretty similar to how it did with both both wires attached. I attached the bad coil's spark plug wire and detached the good coil's wire. No matter how much I pulled, I couldn't get it to hit a lick. So, I was then pretty sure that the problem was a bad coil. I then looked into ordering a new coil. I learned that a new coil for this motor costs $65 - $70. So, I decided that before I spent that kind of money, I would switch the coils and then if the opposite plug fired than before, I would know for certain that the problem was a bad coil. So, today I switched the coils. Now the motor will crank and run with either wire detached from the plug. But, it still will not idle down without dieing. So, I decided to check the coils again with my multimeter. The coils both showed about 1.3 holms resistance. Now, I am completely perplexed. Is this method of testing coils not applicable to this type of coil? Or do I have 2 bad coils? Or, are the coils good, and perhaps the problem is somewhere else?
The motor has good compression, 100psi in each cylinder. I think I may take the motor out for a test run on the water tomorrow to see if I can get full power out of the motor.
I will greatly appreciate any and all advice on this problem. Thanks in advance.
-Bret
http://www.youtube.com/watch?v=KT8rk5QWgS0
One coil was showing me about 12 holms resistance between the spark plug wire and the ground. The other coil showed 1.3 holms resistance between the spark plug wire and the ground. So, I figured that the 1.3 was probably too low to fire the plug. I cranked the motor with only the good coil's spark plug wire attached and the motor ran pretty similar to how it did with both both wires attached. I attached the bad coil's spark plug wire and detached the good coil's wire. No matter how much I pulled, I couldn't get it to hit a lick. So, I was then pretty sure that the problem was a bad coil. I then looked into ordering a new coil. I learned that a new coil for this motor costs $65 - $70. So, I decided that before I spent that kind of money, I would switch the coils and then if the opposite plug fired than before, I would know for certain that the problem was a bad coil. So, today I switched the coils. Now the motor will crank and run with either wire detached from the plug. But, it still will not idle down without dieing. So, I decided to check the coils again with my multimeter. The coils both showed about 1.3 holms resistance. Now, I am completely perplexed. Is this method of testing coils not applicable to this type of coil? Or do I have 2 bad coils? Or, are the coils good, and perhaps the problem is somewhere else?
The motor has good compression, 100psi in each cylinder. I think I may take the motor out for a test run on the water tomorrow to see if I can get full power out of the motor.
I will greatly appreciate any and all advice on this problem. Thanks in advance.
-Bret
