Interesting: Jetting and plugs

[ericalm]

Checking the spark plug has traditionally been the way to determine if a scooter is running too rich or lean (i.e., whether the mix of air and fuel fed into the engine is optimal for your riding conditions).

A knowledgeable friend recently told me that this isn't so reliable anymore. Because of the way gas is formulated now, with varying mixes and amounts of ethanol, it burns less carbon. That means that running rich (too much gas) may no longer result in a blackened, fouled plug.

This leaves performance as the only useful measure, but as well all know, that's rather subjective. Fortunately, jets are cheap (<$6).

[Tocsik]

Huh.
Unfortunately, if you tune just based on performance, it generally means you're running a little lean. That's just how small engines roll. But too lean for too long (especially at high temps) and you won't be rolling at all!

[agrogod]

There is the other side of the issue of wasted fuel through excessive burn off. With carbon fouling you at least knew if you ran too rich and could adjust your carb jetting accordingly. Even though the engines are small, if your burning too much fuel, trying to judge distance through mileage just got harder.

[JettaKnight]

I'm sort of at this point already. If I go one size up on my main jet it sputters when opening up the throttle at that point.