Flat spots....

[S4Patrick]

OK...

Acceleration on an AF05E (completely stock) is nice & strong. But when you hit about 27mph, the RPM "levels off" or seems to have a power fade for a second or two. After that, the RPM's continue to ramp up again climbing to about 38 on the speedo.

I have adjusted the carb. air screw to all sorts of settings, but to no avail.

From my experiences with my "built up" engines, it's something in the variated transmission. I'm thinking the rear center clutch spring is worn.

I have been confronted with this before on other bikes and have been able to iron it the "flat spotting" out. I've never been truly able to put my finger on the exact cause so I've made this post to inquiry on others experiences/ knowledge.

Lets discuss this a little.

Thanks to all.


~Patrick

[Wheelman-111]

Greetings:

There may be an issue with the engine's power delivery, graphically represented as a Torque Curve. Almost no engine is perfectly linear except an electric motor. In addition to carburetion, the complex interplay of opposing pressure waves can result in a situation where the power dips at some RPM, then recovers as the engine crawls out of that "trough" and the revs climb. Four-strokes are not immune to this, but 'Smokers are particularly susceptible.

If it's not engine power, your idea of a soft Contra spring has merit. Such a spring would allow the variator to shift too soon, magnifying a Torque Curve low spot. Also, literally "flat-spotted" out-of-round rollers can cause this behavior too, by causing a "clunky", sudden belt shift that taxes the engine with a suddenly-taller "gear". Round off the sharp edges of your ramp plate to reduce "roller shaving" that seems to contribute to roller flat-spot development.

Finally, a worn or poorly-matched variator ramp curve or ramp plate will sometimes do the same thing. I have a very flat ramp plate that came with my Giant 97mm Brand-X. When I use that flat plate, the scooter takes off well as long as I'm on the throttle, but if I back off just a little bit, the revs drop suddenly to 6500 and there's just not much torque at that RPM. It's great for putting along, but not so good if you need quick acceleration in traffic. A steeper ramp plate solved this issue for me, but necessitated a different - heavier - load of rollers. A steeper plate causes the variator to "shift" at higher RPM but it also moves it a bit further along the crank boss, potentially permitting a greater "upgear".