AmSnow.com is now SnoWest.com
Carb calibration tipsTo calibrate the carb to deliver the correct fuel mixture for best performance, you need a useful range of airspeed to create low pressure in the venturi and push fuel into the airstream. This becomes a case of brinkmanship as you want to have the largest possible carb to deliver more air at top engine speed, because the available airspeed would drop with a large venturi, meaning you wouldn't be able to transfer enough fuel into the engine. As a result the engine would seize due to the lean condition.
This situation was common in the Sno Pro days in the late 1970s through the 1980s. Most 340 twin race engines would then rev at 10,000 rpm and use two 44mm carbs. As a result, drivers were always looking for huge jets in the 700 to 900 range to transfer enough fuel. In reality, the correct mixture for the best and most consistent performance was hard to accomplish with this set-up.
Smarter guys switched to smaller carbs for more power. A good example was the Mercury engineers who insisted on using 38mm carbs on their 440 twin engines, while all others ran 44s. They didn't show any lack of performance on the top-end, but were always strong on acceleration.
Grand Prix motorcycle engineers had a similar philosophy in choosing carbs for their engines. They chose the smallest carbs they could without losing top-end, because this gave them more control of the mixture in midrange on twisty road courses. Likewise it allowed the engine to rev out on long straights without leaning out on top-end and seizing the motor.
In the late 1980s we did a lot of tests on our flow bench with different carb configurations. After much experimenting we found that flow would increase if we taper bored the carb's outlet side. By keeping the venturi at 38mm, but boring the outlet side toward the motor at 5 degrees to a 44mm final outlet, flow increased dramatically. As a matter of fact, this combination flowed almost as much as a straight bore 44mm round slide carb. The reason for this is that turbulence occurs along the carburetor wall, and if the bore is straight this turbulence actually restricts airflow.
By machining the wall to taper away from the venturi and increase in diameter, this turbulence was much less restrictive, thus increasing the total flow. This was a major improvement; we could now have our cake and eat it too. We could have a 38mm venturi, with good airspeed for calibration, but the carb would flow like a 44mm on top-end.
This meant that there was no "flat spot" due to a large drop in airspeed when you cracked the throttle open quickly, something often experienced on large venturi carbs. On the other hand the taper bore gave you the flow advantage of a bigger carb on top end. This modification made to a TMX flat slide carb makes it an ideal instrument for racing, and the quick throttle response and good top-end power has made it very popular on snocross race engines.
When you choose carburetors for your engine, you should not go by the "bigger is better" motto, but instead choose a size that will give you good acceleration and full control over the fuel mixture at top-end. If you go too big, and find yourself looking for jet sizes over 600, chances are you will also end up with a bucket of seized up pistons and cylinders because you will not have enough airspeed to create a pressure differential large enough to push the required amount of fuel from the float bowl into the airstream.
On the other hand, if you had an electronic fuel injection system and could just program in the fuel flow you needed, you could use larger throttle bodies, but that's a completely different story for another issue.