Rich, as found in the often-used phrase running rich can mean one of two things:
- your mixture of air and fuel is off such that there is more fuel in the mixture than needed
- you are using a more oil-rich mixture of fuel and oil
Rich FUEL/AIR mixture
Richness and Starting
When cold starting a moped, the mixture needs to be more rich (or have more fuel in the mixture than it normally does) for it to start effectively. Most carburetors accommodate this need by incorporating some sort of choke device to restrict airflow. This allows you to temporarily increase the richness of the mixture to allow for easier starting. Once the moped is running smoothly, the choke needs to be turned off (simply increasing the throttle will take the choke off on many carburetors).
You can correct a rich running condition by adjusting the fuel delivery in your carburetor. In Bing and Dellorto SHA carburetors, this usually means changing the main jet. In more complex carburetors such as the Dellorto PHBG and Mikuni flatslide, there are several places where you can adjust the mixture to correct richness.
Identifying a Rich Running Condition
Running rich can occur in different parts of the throttle range, but is most easily recognizable at top speed when four-stroking occurs. The more complex carburetors mentioned earlier include devices to adjust mixture for different throttle ranges.
An overly rich mixture at idle can also cause a moped to die.
Rich FUEL/OIL mixture
Refers to the relative amounts of fuel and oil in your premix.
Generally, the proper fuel/oil ratio can be found on a sticker on the moped or in the owner's manual. Using the correct amount of oil will help prevent seizure and/or reduce the carbon buildup. Common fuel/oil ratios are 50:1, 40:1, and 32:1, with the latter being the most oil-rich mixture.
When breaking in a new speed kit, it is wise to premix with more oil than recommended by the manufacturer.
This article offers an in-depth discussion about both kinds of rich running conditions and performance.