California Carburetors

Hello, and thank you for reading "The Fast Track". This is a column that I will be writing in every issue of Inside Karting, which will cover technical questions and concerns that racers might have. The purpose of the is column is to try and inform the new racers of information that their competition would never tell them, and clarify data for the veteran racer. In the following column, I will attempt to inform you about the Honda 4-Cycle carburetor. There is nothing quite so simple, and at the same time complicated as a carburetor. The general principles behind the device are relatively simple, but to maximize performance is all together different. This "maximum" is achieved by a perfect balance between all limiting factors within the carburetor.

Honda Carb - What Happened?

Honda carburetors used to be much simpler, but unfortunately the government stepped in and forced Honda to produce "California Emissions Carburetors", which lean out the fuel mixture and in turn produce less harmful emissions. This is fine, except the Honda motor is designed to run at 3500RPM, not 6500+ RPM like we pull in racing. Therefore, we must correct the problem of the California carb. These models can be identified as having (4) transition ports located on throttle bore, near the throttle plate. (Note: this is one of the reasons you may hear a backfire going into a turn. If your engine is set to idle high, one of the transition ports may not be covered when you let off the gas for a tight turn. Raw fuel spills into the engine causing it to backfire.) Anyway, to correct the "California problem, we are allowed to add an air correction jet by tapping the main air hole jet (See Picture) to 6/32". Take a 6/32" allen set screw and drill through with a #57 minimum drill bit. This will correct the problem of your "California Carb". The air correction jet that is installed simply lets air to an annular opening around the spray nozzle (emulsion tube). As pressure drops, the differences in viscosity cause airflow from the correction air system to rise more rapidly than fuel-flow from the spray nozzle thus stabilize the mixture (makes it richer).

Emulsion Tube:

This is a neat little jet. This tube takes in fuel at its lower end, and admits the correction air through a pattern of holes drilled in its sides. At low fuel, the well is almost entirely filled with fuel, and air passes only through the topmost holes. As overall flow increases, the fuel level in the well drops to uncover more holes and the correction air component of the froth delivered to the top of the tube becomes larger (It atomizes the fuel in other words). An emulsion tube with large holes at the top and smaller ones down will tend to deliver a richer mixture at high revs - the opposite is found when the holes are smaller than those in the rest of the hole pattern. As part of your maintenance - make sure the emulsion tube holes are kept clean. The tube should fall out after removing the main jet with only a slight push from inside the carb throat. Keep the passage clean that houses the tube, as any buildup in this area will alter the fuel curve.

Idle Mixture:

At very low engine speeds - air velocity through the carb is not sufficient to pull the fuel from the emulsion tube. At the side of the carb, you will find a small chamber that is supplied with fuel and air. The air is supplied at the throat through a single hole located at the carb entry. The fuel is picked up from a chamber off the emulsion tube. The idle system keeps the engine running at low speeds as well as supplies fuel for the transition between closed throttle, to the point where sufficient air is flowing through the system to pick up fuel from the tube.

Float:

The float height should be measured from the carb body (remove the o-ring) up to 13.7mm. It is also very important to check the float valve needle and seat. Any dirt or grooves here will cause the carb to flood and decrease low-end pull. Give the float a shake to check for leaks. Adjusting the spring will not give you any extra performance so don't waste your time fooling with it. Simply keep all parts in correct working order.

Main Jet:

The main jet in your Honda takes control in conjunction with the tube at about ¼ to 3/8 open throttle. Before that the jet is practically useless, and could be taken out and thrown away. What we are doing is matching the jet, to the emulsion tube, to the air correction jet (the one we added our self). From my experience with shifter karts, the main jet is not the end all jet. I could run a 175 or a 200 Main jet on the same day, by changing the other jets. On long tracks we want the system to keep pulling fuel, while on shorter tracks we want it to cut out and pull hard in mid-range.

Air Filter Adapters:

These are very simple pieces. They allow a smooth airflow into the carburetor, without causing secondary resonance. How does your adapter do? Well, from my experience, the intake should be a cone diverging style, at a min. of 15 degrees or a parallel wall tube having about 400% of the throttle bores cross sectional area. Both of the above devices will provide the same condition as a pure, atmospheric inlet. The diameter of the filter adapter also plays a major role in the proper tuning of the carburetor. The smaller the diameter of the bore, the richer the mixture (more fuel to air). When running a restrictor plate class, it is sometimes useful to run a smaller bore adapter. This better matches the amount of air entering the carburetor, to the amount of fuel allowed to pass through the restrictor plate. If your air filter adapter is too big for your particular application, you may notice a slight hesitation when applying the throttle.

In Closing:

I hope that you now understand more about the working of the Honda Carburetor, and are better informed to diagnose problems when they should arise. Remember, the carburetor is just on of hundreds of components on a kart that have to de dialed in correctly to make the package fast.

Yours in Sport,

Michael Romeo

Romeo Motorsports