Hi Debbie - I've been doing some googling to see historic rainfall...
Last year, there was 1mm of rainfall in Delhi between the 14th and 25th September and that came on one day. From historic averages, at the start of your trip there is a 25% chance of rain, but by the end this has fallen to 20%.
Sadly, the weather is an uncontrollable and unpredictable force. I would urge you not to be put off by the threat of bad weather. There are beautiful places in this world and some of them simply sparkle after some rainfall.
Kindest regards,
Julia