weak spark on mercury 05 15 hp 4stroke

[gofish1]

I have been having intermittent problems with 15 hp ignition. Tested coil and got .3 ohms across the primary.. should be 16 to 24 ohms. After removing the plug boots I got 4.7 k across the two plug wires. I could not find any common connection between primary and secondary. motor starts right up and runs fine. After 30 or 40 minutes at WOT it will miss and sometimes run on one cylinder. Spark is reddish orange, not blue. I regaped my plugs to .040 and there was an immediate miss. My compression is 180 on 1 and 178 on 2. Carb has been cleaned and all fuel lines have been replaced. The motor has about 300 hours on it and it has been well taken care of. Is this a known problem? Did Mercury redesign this, I have heard of weak or no spark problems on the internet

[jayman]

I would say the charge coil is breaking down. You can check the voltage to the coil when it is running, it should be 200 to 300 volts ac. Coils will start to break down at 160 to 180 vac. You should use a peak reading volt meter but a cheap analog volt meter will be close.

[gofish1]

Thank you for your response

I hope you are wrong, I just replaced the coil! It seems better but the real test will be next week on lake Champlain. It only occurs after running at WOT for 40 minutes.
Joe

[gofish1]

Success!

I ran my motor for 14 days all over Lake Champlain and it never missed a beat. The old coil measured .33 ohms on the primary and 4.7 k ohms on the secondary. The plug caps also read around 4.7 k. It was never hot or anything! It is actually a Yamaha coil, it cost me about $85.