Poulan lawn tractor model PD15538LT engin turns freely when started if no spark plug is in it but it will only turn once when spark plug is put in.Spark plug makes a spark when pulled out.

Hi,
Would seem to say you need to adjust the valves. Owners manual at:
http://www.poulan.com/ddoc/POUO/POUO2005_USen/POUO2005_USen_197053_.pdf
Parts Manual at:
http://www.poulan.com/ddoc/POUI/POUI2004_USen/POUI2004_USen_194599_.pdf
And YouTube video on valve adjustment at:
Lawn Mower Repair Valve Adjustment - YouTube (http://www.youtube.com/watch?v=NjzX-PUjKmo)
Peace,
Clarke