Carb Size Calculation

“Bigger is Better” is the common theme when it comes to carb size but in reality there is a sweet spot for size, and other than trial and error the only way to determine it is by calculations. Changing the carb size is changing the air velocity and the air velocity past the needle is what determines the fuel droplet size which is what affects performance. Different droplet sizes (and the % that is vaporized) affects the rate of combustion, how fast the flame front expands. When you select a size you are basically just selecting the air velocity and how atomized the fuel becomes. There is a perfect size for each engine/RPM and you can go too big or too small and miss out.

Factors mostly affecting burn rate are carb air velocity, engine compression ratio, gas octane, and squish velocity. A formula to help determine the optimal carb size would also require inputting the cylinder bore mm since time for the flame to reach the cylinder wall should be around the same degrees of crank rotation as other successful engines. Of course also needed is the RPM that you want to target for power by the carb size whether it is peak power RPM or lower.

Don’t forget the ignition timing to match everything which should be adjusted also. (go by the clean band on the end tip of the plugs center electrode).

So the gist of this all is that it can be computed without having to guess and be out a lot of money buying different carbs. Here’s my video on the subject:

This looks pretty handy for determining if your carb is a good match for your engine. I assume this was developed more for MotoX and Trail Bike. I am do not know the Rule Set for Bike, but in Karting most if not all classes are required to run whatever came stock from the Manufacturer with no Modifications. This includes any modifications to the Bore, Venturi, Intake Cone/Cup, Fuel Discharge Ports and Throttle Body. Sometimes to the point of specifying square or round head screws where the Throttle plate attaches to the shaft.

The only exception to this rule I have heard of is in Outlaw/Unlimited Classes. I have yet to see any such classes run in my region.

If I cannot effectively change my carburetor, then how can I use this Calculator to Maximize the Carb I have?

  1. Change compression Ratio? I am allowed 1 of 2 cylinder based gaskets which would effectively change the combustion chamber volume, but am limited to a minimum Squish. I would have to order both gaskets and measure the change is squish making sure I do not drop below minimum spec.

  2. Change the Ignition Timing? My Motor uses the Selectra CDI system and the Poles are fixed. Some Motors have the ability to vary them slightly.

  3. Change the Fuel? My class has a Spec Fuel. No go there.

  4. Change the Cylinder Bore? Unfortunately that would require changing cylinders as once you Hone it for a Top End rebuild, you can’t Un-Hone it to decrease your Bore.

  5. You spoke of Fuel Droplet Size affecting the Burn Rate. In the video, you stated larger droplets burn faster than smaller droplets. This seems counter intuitive. One would think smaller droplets would have and increase surface area and better dispersion in the air/fuel charge so the burn would propagate faster compared to that of larger droplets. You also state that Air Velocity affects the Droplet Size. How can you tune a Spec Carb to alter this?

I do not mean to sound pessimistic. I would like to understand how to use this information to maximize my gains.

1 Like

yes you can change compression ratio, fuel octane, ignition timing, and squish velocity. I want to incorporate the latter into the formula but I have to ask around on forums to get the needed data first.

Again, I can only change one of these items. Compression ratio by way of raising or lowering the cylinder height with one of two base gaskets making the combustion chamber larger or smaller. Doing so also alters the port timing slightly (degrees they open and close, not duration). Everything else is fixed/spec’ed! Can you explain Squish Velocity? How can this be altered and what effect does it have. Is it relative to the propagation of combustion through the fuel charge? I know for a given charge and temp, you can alter the plug temp to regulate combustion rate for a given octane.

What Greg is saying that that many (almost all) of these variables are set in regulations in karting. Carb size/model, ignition timing. Cylinder head volume, combustion chamber shape, exhaust, fuel etc.

There are only a handful of classes that are “open” In Karting really. Unlimited outlaws, superkarts

1 Like