according to one of the Briggs training videos, a cold engine requires about 8,000 volts to jump the sparkplug gap. When the engine is hot, only 4,000 volts is required.

If the cranking speed is slow, the ignition coil may not develop adequate voltage to start the engine.

bluemower,would grounding one plug give more voltage to the other plug. Thanks for reminding me to recheck plug gap and to try different settings.

I believe that grounding one of the spark plug cables WILL have an effect on the output of the other spark plug cable in these dual output magnetos, but I am at a loss to explain with certainty exactly how it does it. My opinion is that grounding one of the secondary coil windings causes an electrical "choke effect" that results in the opposite coil secondary output VOLTAGE being more robust or well defined.
But, as bluemower advised, cranking RPM is critical with Magnetron ignition. Minimum of 350 cranking RPM is required to ensure good spark output on the Briggs Magnetron ignition, and these old Opp Twins are very fickle about cranking.
And your estimate of 13 HP is a bit low. For this 42 cu in engine it will be more likely 16 to 18 HP.

Mownie,its a 18.5 HP. I used a .010 index card and set the coil gap and made sure I was on magnet. Plugs were .040 and .035,so changed them to around .027 and it started up ok and ran ok for not being started in over a year. While cold,started everytime,but when hot,had to use a car battery to start it. the manual states to only use starter for 5 seconds,wait one minute and then try again.(that helps,asÃÂ even the car battery doesnt always turn it fast untill you give it a minute to cool) Have noticed starter gets hot real quick,so probably need a new starter,but since its a back up,put up with starting it when hot. Now that I think back,it wouldnt start when it was hot,so I connected a wire to the plug to see if I was getting fire and it started,now me not being very smart,it took about 5 or 6 times this happening before I realized it started by grounding the plug. (then it got so it wouldnt start cold or anytime with out grounding the plug. So I guess gaps on coil and plugs and a hot starter all contributed to the trouble. So thank you and bluemower,as both your inputs helped me near get it fixed,as I aint spending $50 for a starter.

Be sure to check and clean ALL the connection points at both ends of each battery cable to rule out any high resistance.
High resistance ANYWHERE in a starter circuit can cause slow cranking RPM and will overheat the starter too.
If you have never cleaned and secured the battery cables at the solenoid and starter, and the ground cable ends, that is cheap enough to do and could yield some good results.