What is the bystarter and what does it do?

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What is the bystarter and what does it do?

Post by noiseguy »

Excellent article from Mousewheels about the bystarter and what it does.

The bystarter is the little black tube with two wires that is mounted on the carb of all Honda 50 scooter carbs.

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Bystarter Circuit - 3 functions - primer, cold enrichenment, fast idle.

The bystarter is a 2 section valve. The needle at the end controls fuel enrichment, the top section controls an air bypass around the throttle slide.

When the bystarter when cold, is in the retracted position (needle is open, air bleed is open) extra fuel and air are allowed into the engine. When the engine starts, the alternator provide power to the bystarter. Inside there is a heater, which warms up the wax or oil inside the bystarter, expanding it. This gradually closes the bystarter valve, leaning out the mixture, and lowering the idle speed.

Primer function - Inside your carb bowl, notice that there is a seperate tiny bowl for the tube from the bystarter. It is filled from the main bowl via a small hole from the main bowl. Why the bowl in a bowl?

The first few cranks, all the fuel in the little bystarter bowl is sucked into the engine. This is like giving a pre-measured primer spray. This is better than a choke because you can't flood the engine. Once that small bit of fuel is emptied, the limiting factor from the enrichenment system is the small hole that refills the 'bystarter bowl'. That's roughly sized to provide the desired enrichenment after the engine is running.

Most of us have ~20+ year old scooters here. Lots of things get out of factory stock. That little hole filling the bystarter bowl can clog, taking the bystarter out of action. Sometimes the bystarter valve gets stuck by old fuel from the last owner, and is always in the enrichen position. As Kenny mentioned, if the idle jet is clogged, folks often find their bike runs better with the bystarter unplugged from electrical connection - thus in the 'always rich' posistion.

But if the carb's all cleaned up, the bystarter is working, no air leaks etc,
these 20 year old scooters start consistently in just a few revs.
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