TOTALLY CONFUSED NOW !! Define rich/lean !!

Now I'm confused, been arguing with a friend about rich and lean and what an air mixture screw does and after a week of this, I'm all confused and turned around. So, this is what I think I know and not even sure if I'm right anymore so please jump in and correct me.

Rich: too much fuel, too little air, runs cooler but fouls plugs.
Lean: too little fuel, too much air, runs hotter, can burn a piston.

Air mixture screws: I always thought that turning then in made the bike run leaner. Reason I say that: I once had a Suzuki Marauder 800. Eliminated the air injection, drilled out the baffles in the exhaust and the guru's on the Marauder sites said to turn out the air mixture screws another 1/2 turn because the bikes run too lean. So, I've always thought that turning out the screw changed the air mixture and made the bike run richer. My buddy says that's the opposite, turning out the air mixture screw adds more air to the mixture and makes the bike run leaner.

I have a '75 KZ400 that's running too rich on one cylinder, I keep turning the screw in a little at a time and it's not getting better...kinda supports my buddies theory but is totally opposite of what I've always thought so..please...someone set me straight, for the record, HOW DOES IT WORK ???

your rich and lean analogy is correct. HOWEVER not all pilot screws are the same. some are fuel screws and some are air screws. This can depend on different model carbs.

Here's a link to a good KZ400/450 site that has a down-loadable service manual plus loads of other info.

Click > www.kz400.com/

A rule of thumb is that when a pilot adjustment screw is located on the air box side of the carb (such as a side-located pilot air screw), it screws out counter-clockwise to add more air, which provides a leaner pilot circuit mixture.

And when a pilot adjustment screw is located on the engine side of the carb, such as a bottom-located pilot mixture screw, it screws out counter-clockwise to add more mixture, which provides a richer pilot circuit mixture.

Here's a comparison of manual slide carbs:




May be wrong, but thinking '75 KZ400 has CV carbs (not manual slide), with top-located pilot mixture screw on engine side of carb, akin to the BS34 carb illustrated below, which would screw out counter-clockwise to add more mixture, and thereby provide a richer pilot mixture:



Good Fortune!