912 running rough with one ignition circuit

Check that the plugs are correctly torqued, plug caps are firm (making a good connection).

Try putting your old spark plugs back in, to see if the problem remains/goes away. If goes away you may have purchased one or more bad plugs.

Hi,

I have checked all plugs and replaced them without any result. As the plug caps were old I have replaced them all and also all cables are new

Hi Rick here, I have a similar issue. But my issue is that I have only one side of my Magneto has a 1000 rpm drop when I do my run up at 3700 rpm. So I originally did some electrical checks.
I can switch the plugs on my CDI boxes and the problem follows. So I thought I had faulty CDI box? So I replaced both of them. I also replaced all of the spark plugs And I still have the same problem. 
I also check the carbs., and if I keep my mag on the good one engine runs perfectly. 

Any help would be greatly appreciated.

Thanks Rick

Andreas Kuhn wrote:

Hi,

I have checked all plugs and replaced them without any result. As the plug caps were old I have replaced them all and also all cables are new

Andreas,

What is the RPM drop for both ignition A&B?  What type of plugs are you using, and what is the gap?  When you shut down one circuit during the run-up, and the engine becomes rough, pull out the choke, and ignore the RPM change, does the engine get better, worse, or stay the same?

When was the last time the engine ran good?  What has changed since then?

Rick Green wrote:

Hi Rick here, I have a similar issue. But my issue is that I have only one side of my Magneto has a 1000 rpm drop when I do my run up at 3700 rpm. So I originally did some electrical checks.
I can switch the plugs on my CDI boxes and the problem follows. So I thought I had faulty CDI box? So I replaced both of them. I also replaced all of the spark plugs And I still have the same problem. 
I also check the carbs., and if I keep my mag on the good one engine runs perfectly. 

Any help would be greatly appreciated.

Thanks Rick

Rick,

You said that you had switched CDI plugs, did you swap all four at once, or two at a time?

Assuming everything from the ignition module forward is good, I'd start looking at the connectors to verify that no pins are loose or pushed back.  Pay close attention to any connections, grounds, or damaged wires.  You need to check the ignition circuit including resistance as per MMH 74-00-00.

Also, when was the last time it ran well, and what has changed since then?  Did you do some work on the engine and didn't tighten down a ground wire?