Z1 air box on 78 kz1000

VTEC wrote: ............If you don't run it for 6 months (or sometimes even 3) or more, drain the tank and carbs. ...............
 

Yes and no.

DO either drain the carbs or run the bike for a few minutes with the petcock turned off (if yours has that option) so there is little or no fuel left in the carbs.  Fuel left in the carbs for an extended period will coat the inside of the carbs (jets and all) with a nasty coating that will need to be cleaned off manually or the bike will run like crap if it runs at all.  Having been through that exercise once on my KZ650 I now start the bike at least once every week during the winter and let it run  about 15 minutes to ensure any old fuel has been purged from the carbs.  After running for about 12 minutes I turn off the petcock and let the engine run for a couple more minutes to empty most fuel from the carbs.  I've done this with my KZ650 for the past 20 years and have not had any carb issues.  Summer is not a problem since I ride the bike at least once a week.

DO NOT drain the tank!  If you do the interior of the tank will rust unless you very carefully coat the inside with oil and periodically check it.  If you keep fuel in the tank it will not rust.  I say this having owned motorcycles for the past 53 years, so I have more than a little experience.  Fuel in the tank will not degrade over six months or even longer, and it will prevent rust from forming inside a sound tank.  However, I would not use the fuel if it is several years old. Ed

I disagree with the old theory of keeping a tank full to the brim to prevent corrosion. Unless the tank is in a highly corrosive environment. The biggest factor being garaged or not (surrounded by moisture or not).

Ethanol is what's causing the corrosion in today's fuel. And it's only happening while the metal is submerged. Just like dry carbs prevent corroded jets, same goes for tanks. 

My KZ is 35 years old (always garaged), and it's spent several years at a time sitting with fuel in the tank. The upper portion which is typically above the fuel line is spotless, but there are multiple corroded spots forming at the bottom seams which have always been covered with fuel.

My experience differs from yours.  I've owned my KZ650-C1 since April 1977 when it was on the showroom floor.  I've kept fuel in the tank for those 41 years and I have zero rust in the tank.  Before that, I owned a BSA for 11 years that was never garaged and had the same results. 

Water is heavier than gasoline, so if the rust formed in the bottom seams of your tank it's possible (and likely) that there was some amount of water in the fuel that sunk to the bottom and caused the rust.  Ed

Here's a good example. lol