I have a 20hp twin cyl. B&S engine,I was getting no spark so I bought a new MAGNETO . I had spark but still weak.I put a card between for gapbut still weak spark Help!!

Have you tried contacting the manufaturer of the machine and asking about a repair manual ?

This B & S link may help:
Spark Plug Gap | Briggs & Stratton (http://www.briggsandstratton.com/us/en/support/faqs/identifying-the-correct-spark-plug-and-gap)

You need to have the right gap first in order to do any proper troubleshooting. The card you used could be way under or over the design gap. Good luck.

Hi,
You may also want to disconnect the ground wire to the armature to make sure something else on the machine is not affecting the situation.
Peace,
Clarke