Originally Posted by bozla
I'd like to know this too. From what I understand, a smaller venturi will restrict airflow making the engine run richer. Is this correct?
How does the smaller diameter affect the standing wave set-up from the carb to the intake port?
Internal combustion engines require three things to produce power - fuel, oxygen and heat -- and they need them in specific ratios. Running in situations in which the air is denser (contains more of oxygen) an engine produces power with ease. In situations in which the air is less denser (less available oxygen), an engine will not be able to burn as much fuel as it does at higher density air situations. Less fuel means less power, and that means slower speeds.
What can you do to minimize the effects of less denser air ? Compression creates power, and when the air is less dense, there is less fuel/air mixture available to compress.
To recover some of the lost compression, you can use a fuel containing a higher percentage of nitromethane, or reduce engine-head clearance to increase the compression ratio.
To compensate for less density air, you need to adjust your needle valve to a leaner setting to maintain the proper fuel/air ratio. The opposite is true for higher density situations. The idea is to maintain the optimum ratio of fuel and oxygen by adjusting the fuel volume to compensate for ambient air pressure.
Now, if you have the appropiate compression, and still need some more fine tuning for lower density situations, that is where the different sized Venturis come in handy.
A venturi such as a model engine carburetor's works on the vacuum principle. As air rushes through the venturi, it accelerates and creates a vacuum at the spraybar. This vacuum draws fuel from the spraybar and fills the crankcase with a mixture of air and atomized fuel; at very low density situations, less air accelerates through the venturi and past the spraybar. This reduces the vacuum at the spraybar, so less fuel is drawn through it, and the mixture becomes too lean.
There is a way to compensate for this: Switch to a venturi with a smaller area.
When you have a smaller venturi area, the airflow velocity through the venturi is increased. This also increases the vacuum at the spraybar, which, in turn, draws more fuel.
Obviously, you have to experiment to learn which combination size of Venturi best suits your engine, and ambient situation, and you have to take into account the rules and regulatios of your association or club. IFMAR / ROAR staes 5.5mm for .12 engines, and EFRA states 5.4mm as maximum dia. of venturi at the slide side of carb.