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How to lower the total number of map tasks

I am running Hadoop 1.0.3 in Pseudo  distributed mode.
When I  submit a map/reduce job to process a file of  size about 16 GB, in job.xml, I have the following
mapred.map.tasks =242
mapred.min.split.size =0
dfs.block.size = 67108864
I would like to reduce   mapred.map.tasks to see if it improves performance.
I have tried doubling  the size of  dfs.block.size. But the    mapred.map.tasks remains unchanged.
Is there a way to reduce  mapred.map.tasks  ?
Thanks in advance for any assistance !  
Shing