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Chapter X: Fuel, What Kind, How Much & How to Deliver It



This component of the project is far-reaching.  The decisions here impact areas way outside of mixing fuel, oil and air.  How much fuel will I need to carry with a given setup?  How much will that weigh?  How will it impact ergonomics and suspension?  What will the maintenance burden be for the motor or for the fueling system itself?  How will environmental variances impact setup and maintenance issues?

This is where systemic thinking is required.  All components of a system interact, and one must look at the entire composition when evaluating a single point.

So…first thing is first, but it may not be what you’d expect.  Where am I riding?  Here in Central Texas, we have extremely rapid, unexpected changes in temperature, barometric pressure and humidity.  Mid-summers will be in the high 90s-100s during the day with super-low humidity.  Spring and Autumn are volatile.  I’m writing this in April…yesterday was a humid 85 degrees.  This morning it is 46 and dry, with the temperature expected to drop all day to an overnight low of 36.  Tomorrow, the high is expected to be 70 again, and we’ll be in the mid-eighties and dry for the remainder of the week.  I’ve seen 30-degree temperature swings in a single afternoon over less than three hours.  What one jetting setup will work in all of that?  How ready are you to re-jet, sometimes mid-day?  If compromising, rich is safer than lean, and that may mean a lot of fouling or blubbering.

I’ll be riding off-road, not on a track, with loops that can reach 50 miles.  I’m on a 250 two-stroke, so fuel consumption needs to be accounted for.   Either I need to increase capacity or I need to somehow increase economy.  Also, off-road riding means far more partial-throttle riding at lower revs…only occasional bursts into the peak of the power curve.  Oiling needs at partial throttle and lighter loads are not as extreme as track riding, so I can likely go with a lighter premix ratio to avoid smoking, pipe spooge and load-up…which will also cause a revisit to jetting specifications.

The 1997 CR came with an early Keihin power-jet.  The servos had been known to fail, and jetting can be inconsistent as the performance of the electronics degrade over time.  For the sake of simplicity and maintenance, a new carb is in order.  I could go with a Keihin PWK Quad-fin Air-Striker, which would set me back a few hundred before jetting experimentation.  Man, though…a Lectron or SmartCarb look like really sexy options.

Where does this fit in the bling equation?  The Lectron is supposedly self-adjusting for temperature and altitude.  The High-velocity variant is designed for better partial-throttle performance.  Most important, non-endorsed riders have regularly reported 30-50% improvements in fuel consumption numbers.  I have better ways to spend time than fiddling with jetting.  Increasing my fuel consumption by 30-50% would be like adding an extra gallon of fuel to the tank, but without the ergonomic compromises or impact to suspension setup.  That is all far from bling.

However…price…man, that is huge.  The price tag alone sent me back to the forums, and I read post after post of how good the Keihin PWK Air Stryker did once properly set up.  A review of online OEM parts sources allowed me to find a new-in-box OEM Keinin PWK Air Stryker for the 2000 CR250R for right at $200.  Smaller diameter than many are putting on aftermarket, but I’m looking to trail ride and run partial-throttle in off-road events while living in the bottom and midrange.  Winner, winner, chicken dinner.  I’ll still wait to see how things work with the stocker at first, but if a replacement is a must-have, or if I end up with some bonus cash, this will be a quick purchase.

Regarding air quality and filtration, Central Texas is far dustier than what I was used to in Michigan.  In the mitten, it was loamy dirt, mud, sand or water-crossings.  Here, it is far drier, rockier and dustier.  I was floored by the amount of dust on my little boy’s JR50 after only two hours of riding in the Austin area.  I’m getting ready for far more regular filter exchanges and cleanings.  I can tell right now that I’ll be using a fresh filter skin with each ride to try to safely extend that service window a bit.

When the current Boyesen reeds start to age and fail, I’d consider a Moto Tassinari V-Force-3 cage setup, based on reports of improved low-end response with no jetting changes.  However, if I’m satisfied with performance before then, I’ll likely stay with the stock cage and quality reeds.


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