The Black Magic of Carburetors (basics)

The operation of the carburetor can be confusing to some, especially the more complicated ones, like Mikuni and Kiehin.  I guess lets start with the way ALL carburetors operate.

Carburetors function on a principal of pressure difference. When there is a higher amount of pressure in one area, it will always try to take the easiest route to the lower pressure area, to neutralize the difference. Moving air over a surface creates low pressure.  The faster the air moves over the surface (higher the velocity) the lower the pressure.  For example, lets say you put a sheet of paper up to your bottom lip (don’t get a paper cut) and blow across the top of it.  You will notice that the paper raises.  the paper raises because the air on the bottom of the paper is has higher pressure and is trying to fill the low pressure area you are creating on the top.  The harder you blow across the top of the paper (increasing the velocity), the higher the paper will rise.  We can create a pressure difference inside a tube that flows air into the intake of your engine by placing a venturi into the throat of the carburetor.

Carburetor Venturi

As a whole the carburetor is a complex mechanism but can be easily understood when each system is broken down.

When air moves through a tube, it creates a kind of momentum (because air has mass and volume).  when a small  hole is inserted into this air-stream at a 90º the air moving over the hole produces a negative pressure in the hole.  This small amount of negative pressure is what causes fuel from the float bowl to enter the engine using the atmospheric pressure pushing down on the fuel in the float bowl.

Carburetor Air flow and fuel flow

To increase efficiency of this system most carburetor manufacturers put a streamlined restriction into the throat of the carburetor called a venturi.  The venturi causes a low pressure area that is usually referred to as vacuum. The main fuel tube is usually located in this low pressure high flow area.  The faster moving air over the tube is more efficient at getting the fuel into the air-stream.

Carburetor venturi fuel pulled or pushed up into venture due to low pressure area

So this is all fine and dandy if you have an engine that is set at one
RPM or speed.  So what happens to a variable speed engine when that throttle is snapped open from an Idle? Well pressure
in the  carb equalizes for a moment, causing fuel in the fuel pickup
tube to slow or stop flowing.  This causes engine hesitation. Other systems are put in place to help keep this from happening, like an accelerator pump.

Standard round slide carburetor how the fuel molecules flow past needle jet

So here is your standard slide carburetor.  These are generally used on recreation vehicles such as motorcycles, snowmobiles, 4wheelers, etc. So where is the venturi in this picture?  The throttle slide is what makes the venturi.  The less throttle the smaller the venturi and faster the airflow. The amount of fuel that is allowed to flow into the engine is controlled by the needle.  The needle on this type of carburetor is tapered.  This type of carburetor has a slew of parts that can be changed so a guy/gal can get them adjusted correctly. The individual tuning operations will have to be on another post or Mikuni also has a great tuning book for all their carburetors.

Mikuni throttle valve openings  Pilot system includes Pilot air and fuel jets pilot fuel screw

So why do they need so many adjustments?  Motorcycles, snowmobiles, 4wheelers, etc. all run and operate at different altitudes and temperatures (air density changes with these factors, therefore the engines that are tuned for performance changes). The higher performance an engine gets, the more critical it is to tune the carburetor to use this performance to its max.  This is also referred to as jetting your carb.  The thing is, more than the jets can be changed for tuning purposes.

Basically, if you know how a carburetor should operate, this knowledge can be stretched and adapted to all carburetors.

The most important thing is, if you have questions about something ask.  You wont be the first to ask, and you will not be the last.  A whole lot of time and money can be saved if you ask or reference the manual.

If your working on snowmobiles I found it was best to get the repair manual if I was unsure about how a particular carburetor was tuned, how it worked, or how the carburetor went together. I found that the Clymer Snowmobile Service Manual – SMS11 was the most useful when it came to finding information on snowmobile carburetors when I couldn’t find a manual for the older snowmobiles I was working on. (this was back in 93 – 94 when I didn’t have the internet)  Now-days we have it easy, if you belong to a specific forum for what you are working on, more than likely someone will come up with the manual you need, just search the forum or ask.


Riding motorcycles and wrenching since my preteen years, I have moved from motocross to street bikes. Being a teen back in those days it was tough to get me off of the bike. Now days even though I am very busy being a dad I still have my weekends and go to the track to race and on occasion will do a track day or two.

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