How to lower the total number of map tasks

[Shing Hing Man]

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