These can be tricky to get right. FWIW I always pull the head.
After setting the ignition timing, I put the flywheel back on hand tight and spin it over by hand to check for spark. Just connect the spark plug and rest it on the top of the block.
That way if there is no spark, it’s easy to disassemble and make adjustments, rather than putting everything back together and have to pull it all apart again.
In the flathead service manual (available on this site, link below) there are specific instructions to set the ignition timing.
If you follow those instructions carefully, you should be fine.
http://www.smallenginesuppliers.com...cumseh_L-Head_Engines_Service_Information.pdf
Good luck, you can do this. Plenty of great people here to help