Backfire thru carb and exaust.. points and timing problem

Okay, I will tackle a tune up on a 36 year old Yamaha. Warning: I am 59 and haven't touched that bike since I was 23 and a mechanic at Lynns Yamaha in D.M. Iowa. Your bike has CV carbs and I know very little on the carbs.

First let me suggest you get a service manual. Go to www.repairmanuals.com and input " 1973 Yamaha 650 ". You can choose a Clymer or a Haynes manual. I don't care for Clymer and have never used a Haynes. The Haynes is $23.77 which is really a pretty good price considering not many of the manuals still exist.
Also, for future reference, go to the following site where you can see a parts diagram for your specific bike. You will select the brand, year, model, etc., once you go to the site. Part numbers and prices are also shown. Parts can be ordered if the price is shown > no price = the part is not available.
http://www.babbittsonline.com/pages/parts/viewbybrand/1/Yamaha.aspx

Also Google " motorcycle salvage yamaha " and Google " vintage yamaha motorcycle parts ".

Start with new NGK B8ES spark plugs. New points can be gotten at the Babbitts site. Set the points at .015 max opening. Hook a timing light up to the left cylinder. Remove the left case cover to expose the flywheel. Start the bike and point the light at the static reference mark on the casing. IF I RECALL CORRECTLY, rotate the base plate that the points are mounted on to adjust the timing. The timing should advance when the engine is reved up. If it doesn't, the spark advancer needs service. Now put the light on the right cylinder. IF I RECALL CORRECTLY, changing the right cylinder point gap is the only way to set the timing for that cylinder.

I hope this helps. Please rate my extra long answer. Thanks.