Dirt Bike Channel Podcast

2 Stroke Oil Mixing and Carb Jetting 101 - The Basics - Ep 73

February 14, 2021 Kyle Brothersen
Dirt Bike Channel Podcast
2 Stroke Oil Mixing and Carb Jetting 101 - The Basics - Ep 73
Show Notes

Fuel/Oil Ratio vs Fuel Air/Ratio

  • Fuel/Oil ratio is the amount of oil you are mixing in the gas.  It's in your gas can
    • 32:1 is a lot of oil - thick fuel
    • 50:1 is less oil - thinner fuel
    • 80:1 is even less oil - thinnest fuel
    • Pick something that works for you and stick with that.  We'll fine tune the mix with our fuel/air Ratio
  • Fuel/Air Ratio is fine tuned inside the carburetor with Jets and needles. 
    • I've got oil running down my silencer and I"m running my bike 32:1. I guess that I'm running it too Rich, right?
      • Well yeah, but it's not because of your oil mix.  You are too "rich" but you need to take fuel away from the bike by allowing less of it to enter the cylinder of your bike.

Denser air (more of it) makes your bike fuel lean (less fuel) because now you have more air/pressure but the same amount of fuel

Density altitude is what we are really tuning for.

  • Aviation - Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases.
    • Air pressure is reduced at higher elevations - Elevation goes up - bike runs " fuel rich"
    • temperature effects  our carbs - temp goes up - bike runs "fuel rich"

Pilot Circuit - idle up to about 1/8th 

  • air screw for fine tuning
    • 3/4 of turn out to 2.5 "turns out" is a fine range
    • If you have to run the bike at less than 3/4 of a turns out, you will need a bigger pilot jet
    • If you need to run it more than 2.5 turns out, then you need a smaller pilot jet

Needle Circuit 1/8th to 3/4 throttle - Where it's at!

  • uses clips to fine tune here
    • Moving the clip UP the needle will allow it to drop down further in the carb which restricts fuel (makes it leaner)
    • Moving the clip DOWN on the needle keeps the needle higher which add fuel (makes it richer)

Main Circuit 3/4 throttle and up

  • there is certainly some cross over here and I'm not saying that it will ONLY effect you at 3/4 throttle, but this is where it mostly comes into play.

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