Sportwin 9.5 power issue

[vabrad76]

I have a 68 sportwin I think. I have had it almost 20 years and haven't done anything to it except change the plugs in it. The last 2 times (previous years) I took it out the motor started up fine and ran at full throttle and then a minute or 2 later the power was cut in half. It's like I went from 9.5 to about 4 hp and it never opened back up to full throttle again.
Any suggestions? I thought I used fresh or treated gas when I used it those times.
It's been a good motor and I appreciate the help. thanks

 

[lckstckn2smknbrls]

Check to see if it running on both cylinders.

 

[krawler]

It's either the points or coils. Pull the flywheel and replace the points, pull off the coils and replace if their cracked.