90 HP 4 stroke Mercury is not firing on the 2 bottom plugs

[bcribb52]

I have a 2005 90 HP carbureted mercury that's not firing on the 2 bottom plugs. New plugs and wires. Could the coils be bad or the ecu be bad?

 

[bcribb52]

Do I need more information to get a response. If so I can add more.I don't know.. I hate to buy coils or a computer and that not be the problem..

 

[QBhoy]

Hi. I think some more details might prompt more responses. So some might want to know;

what makes you think it’s not firing on these plugs ?
what was the voltage like ? Charging voltage ok ?
plugs been changed recently ?
mid there no visual spark when removed or just that they are black ?
or perhaps motor misfiring and leading you to this ?
compression numbers good on these cylinders.

hope to hear back soon.

 

[bcribb52]

I removed the number 1 and 2 plug and turned the engine over and got good spark. Did the same for # 3 and 4 cylinder and no spark. Has new plugs. Charging voltage around 14.. No visual spark. Even when I pull the wire off the plug it don't jump spark back to the plug like 1 and 2 cylinder does. Compression is good on all 4 cylinders. It has 2 coils and each coil supplies 2 cylinders. The first coil bottom wire and the second coil bottom wire are the ones feeding 3 and 4 cylinders and have no spark.

 

[217andy]

You could try swapping the top and bottom coils to see if the problem moves. If it does, you have a bad coil. If the problem stays on the bottom two cylinders, you most likely have a bad CDI unit. That particular coil sparks both 3 and 4 at the same time, it is basically just 1 coil with two spark plug wires coming out of it. Carb motors did not have an ECU.
Serial number of the motor would be helpful.

 

[bcribb52]

Serial number 0G960500 - T800999 . If its the cdi unit where can I get one reasonably. Thanks for your help. This is a friends boat and i'm trying to help him out because of his situation..

 

[217andy]

You are going to need the exact serial number off of the motor, because they made two different CDI units in that serial number group. There is only one number difference and they are the same price, but as far as getting one for a reasonable price? That is why it's so important to isolate the problem before buying parts that may or may not be bad.
The motor was made by Yamaha, so if you can find a Yamaha one (same year & hp and make sure it is for a carbureted motor) it should work and may expand your options. Good luck.

 

[bcribb52]

You are going to need the exact serial number off of the motor, because they made two different CDI units in that serial number group. There is only one number difference and they are the same price, but as far as getting one for a reasonable price? That is why it's so important to isolate the problem before buying parts that may or may not be bad.
The motor was made by Yamaha, so if you can find a Yamaha one (same year & hp and make sure it is for a carbureted motor) it should work and may expand your options. Good luck.

Changed the coils and CDI for $300.00. Done a little adjusting on the carbs and the motor is running great.. One of the problems with the bad idling was the large hose from the manifold was not hooked back up after I removed he manifold to clean the carbs..