So how do these things work?

This section of cyclemaintenance.com will be dedicated to 4-cycle engines, which covers most all of the street bikes out there that aren't dual purpose.  2-cycle (or 2-stroke) owners, click HERE for a message. 

The Basics

You need three things for an engine to run:  FUEL, AIR, & SPARK

Just understanding this simple concept can help to significantly simplify your troubleshooting efforts.  Let's expand a bit.

  • FUEL - usually in the form of gasoline.  How does it used?  Let's pretend you are a molecule of gas.  You start out your journey from the gas tank, obviously.  When the fuel tap is set to "ON" and the engine isn't running, you just sit in the tank.  It takes a small vacuum to open the fuel tap so you can flow into the carbs.  Once the engine is started though, that vacuum opens up the fuel tap.  You then would flow through the fuel tap (which sometimes can have a fine screen inside of it - especially on older bikes).  From the fuel tap, you likely head into the fuel "rail" or fuel distribution pipe.  This varies from bike to bike, but usually goes to the very middle of the carbs, and then branches out to each carb from there.  After in the fuel rail, you will travel to a carb (assuming the bike isn't fuel injected).  You flow from the rail into the carb, but only if the float is down (the fuel level in the carb is lower).  When the float is down, it opens a "needle valve" that lets you flow into the carb bowl.  Think of the carb bowl as a second mini-fuel tank.  From there you will be sucked up into one of maybe three different "jets" or nozzles.  Inside the jet, you are combined with a much larger volume of air and you will have to pass the throttle (butterfly) valve and the carb piston.  After you are past there, it's on to the engine! 
    You head into the intake boot (which is usually rubber) and then wait until your cylinder's intake valve(s) opens up.  Once it opens up, the piston will be moving down and will suck you with all that air and a few other of your friends into the combustion chamber.  The intake valve closes behind you, the piston compresses you until it gets to the top then....BANG - the spark plug fires.  You convert most of your stored energy into heat and pressure and force that piston down.  The exhaust valve(s) then open up, the piston reverses again and blows you out to the header pipe.  From there, you head through some baffles and out the muffler/tail pipe.
  • AIR - To come.
  • SPARK - To come.

Can't wait for the rest of my explanation...try the HowStuffWorks page on engines, which is a really good place to start.

Thanks for your patience! 


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Last Update: Wednesday, December 14, 2005 11:53 PM