North India is massive. Where do you mean by north India? North India has many different climates itself; e.g. Bengal, Orissa, the North-eastern states will be very humid around that time, the Indian planes absolutely bake in the extreme sun, from UP all the way to Gujarat and down to Maharasta, but in the Himalaya you will find quite nice weather in July, August and September. Again this varies between Kashmir, Nepal Manali, and Leh, because they are all at different altitudes, with the latter (Ladahk) receiving hardly any rainfall at all.

Indian Summers are extremely hot. During the months you'll be visiting, expect 40+ Celsius day in day, and this will be a very dry sort of heat. I've travelled extensively during in Indian summers and can tell you it makes an intense country all the more intense. I found it essential to wake up before 6AM and start my day then, because by 10 or 11am, it is already too hot! On the plus side, few other tourists brave the heat so you get places mostly to yourself and room rates are all the more negotiable.

Royal Enfields are fabulous to ride, tough machines, but can be rather unreliable and uneconomical compared to the Pulsars, Baja's, etc. You didn't say what your budget is; but I'd be tempted to buy/rent something that is brand new. Avoid the older Enfields unless you really know what your doing. Get a 500 if you want overtaking power on the highways 2-up, but generally they don't like to be pushed over 50/60mph... The newer ones have electronic timing and maybe even fuel injection now (not sure though!), with lower vibration engines shaking everything loose, of course Enfield purists/enthusiasts will tell you old is gold, but for touring reliability is essential.

You can rent or buy in Delhi, and places like Manali, Goa, are good to buy from foreigners, especially towards the end of the season. There is tones of information at indiamike.com -- I suggest you do some homework!

Good luck,

R.T.W.