HFFS

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"How Far from Four Stroking" is a carb tuning method.

The Down Sides to Plug Chops

Plug chops known around moped army aren't true plug chops. They are plug reads. A true plug chop consists of starting in a high gear(higher than a moped has), letting out the clutch at low rpms(which you can't do with a centrifugal clutch) and running up a slight hill at WOT. You must also take out the variator weights of variated bikes for a true plug chop. Also cutting away the threads and looking at the last couple of millimeters of the porcelain is not common practice around here either. Even with a true plug chop or all the reading that one can do plug chops and plug reads are vague estimates at best. There is a better way.

How Far from Four Stroking explained

Carbs rely on vacuum to pull in fuel, vacuum is created by the venturi effect and by the vacuum of the engine. Slide carbs need certain amounts of vacuum for the size of jet it has, emulsion tube, and area of fuel inlet exposed around the needle. This vacuum is changed by the amount the throttle is pulled and hence how high the slide is and also by increasing or decreasing rpm. In this tuning method the vacuum will increase because of the rapid closure of the slide. This however drops the needle and therefore decreases the area of the jet exposed to the venturi which, if the vacuum where to remain the same, would decrease the fuel let into the engine. As it were this drop of the slide and it's affect on vacuum causes more fuel to be drawn into the engine than the decreasing area of the jet causes less fuel to be drawn into the engine. In simplest terms, a vacuum increase will pull more fuel.

In practice

Simply run up to top speed at WOT and then quickly cut the throttle down to 80% of WOT. The bike should start to four stroke. This is because the vacuum has increased causing more fuel to be pulled into the engine. If it doesn't four stroke you are running lean and if it will four stroke by only cutting it down to 90% of WOT, while at top speed, you are running rich. Once you get the hang of this you can substitute the 80% rule for a higher number if you want to run richer or a lower number if you want to run leaner. You can notch your throttle grip and use a marker or center punch on your hand control bracket to correspond to 1/4, 1/2, 3/4, and WOT so you can visually estimate the correct percentage of throttle cut down you desire. This same exercise can also be preformed at 1/4, 1/2, 3/4 or anywhere in between to tune your carb across the entire range and therefore all speeds. This is very effective because slide carbs demand that you "follow" the rpms with your throttle hand. Meaning, as the rpms increase you must increase how much the carb is open. Opening the throttle too much at too low of a rpm will lessen your vacuum and the engine wont pull in enough fuel. This is why you never go WOT except at close to top speed. To tune at other than WOT you must travel at any maximum speed any one throttle opening produces and then cut the throttle down to 80% of that amount of throttle opening. Simply put if tuning at 1/2 throttle, while at the top speed 1/2 throttle will give you, you would quickly cut the throttle down to 40% of WOT and you should four stroke.

Happy tuning.