[Blindstitch]

May be as simple as this.

http://www.webbikeworld.com/Motorcycle-tires/tire-data.htm



http://www.steelthundercc.com/tiresizes.html



Any details on the bike? Year, style, cc....

 

[Blindstitch]

Looks like the bike in question is a



1984 BMW R100RT



Found this.

http://www.webbikeworld.com/r100rs/bmw-r100rs.htm



BMW R100RS Tires: Tire sizes for the R100RS are the 3.25-19 front and 4.00-18 rear. Modern size replacements are 100/90-19 front and 120/90-18 rear, which will fit the '84 and probably other model years. Tires are available in these sizes from Avon (AM18 F/R); Bridgestone (BT45); Continental (Blitz, TK11/TK12 F/R); Metzeler (ME33 front and ME99A rear); Michelin (Macadam 50 & 50E). Metzeler recommends a 100/90-19 front and 4.00-18 rear. I use the Conti's on my '84 and they work great!

 

[Blindstitch]

What's the starting problem? Remember this new gas turns to crap pretty quickly so if you haven't ridden it in a while drain the carbs and start over.