Bystarter: What It Is, What It Does

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Bystarter system

The bystarter system provides cold start and warm up assistance. It came to Honda 50 scooter's in the early 1980's, arriving with the Aero 50. An improvement in functionality and reliability for Honda's, it continued into 2001 Honda scooter models and beyond.

Bystarter system provides:

  1. Initial fuel prime
  2. Cold running mixture enrichment
  3. Fast Idle


Electric Bystarter valve

The bystarter valve is an electromechanical device which controls 2 independent circuits.

  1. Needle controls fuel enrichment
  2. Brass collar controls an air bypass around the throttle slide.

Inside the bystarter is a heater. Power for the heater is supplied by AC from the engine once it is started. Heating causes gradual expansion of the valve, much like the thermostat in an automotive cooling system. Expansion over a several minute period extends the bystarter needle and air valve. This tapers off cold start enrichment and idle speed elevation.


Carb Bowl

Contains a separate section for bystarter fuel A small orifice between the main bowl and the bystarter bowl limits fuel.

Carb Bowl Small.jpg Bystarter MeteringOrifice.jpg

Orifice size

Orifice is close to size of the carb pilot (idle) jet.

First pic: Wire size not recorded. Second pic: a 0.010 wire demonstrates a clear path to the bystarter fuel bowl

Bystarter Metering Orifice with wire 1.jpg Bystarter Metering Orifice with wire 2.jpg

Fuel mixing tube

Fuel may drawn from the bystarter bowl until it is emptied. Mixing tube features air bleed holes. Air is drawn from the carb bowl above the fuel level. Once the bowl has emptied, only the amount metered by the orifice feeding the bystarter bowl can pass through the fuel mixing tube.

Carb Bystarter Mixing Tube.jpg Bystarter mixing tube.jpg

Carb body

Cast in passages to allow airflow around the throttle slide. The airflow and vacuum activate the fuel enrichment detailed above.

Carb Bystarter Air Path.jpg

Theory of Operation

Cold Start

Initial Condition: Bystarter valve cold. Cold puts the valve in its retracted position(needle is open, air bleed is open). Extra fuel and air are allowed into the engine. The cold condition is when the prime, cold running enrichment and idle speed increase are enabled.

Primer Circuit

Inside the carb bowl, is a small well for the fuel mixing tube controlled by the bystarter valve. The size of the 'bowl' is sets the amount of fuel available for a prime.

The first few cranks, all fuel in the bystarter bowl is drawn into the engine. This provides a pre-measured prime and is an improvement to a choke. A choke may flood the engine if cranked too long. In the bystarter design once the fuel in the bystarter well is withdrawn, fuel is limited by the orifice in the bystarter bowl.

Warm up

Once the primer bowl is emptied (see above), 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. At first, there is full air and fuel flow allowed by the bystarter valve.

Once started, the alternator provide power to the bystarter. Internal to the bystarter valve is a heater. This warms up the wax or oil inside the bystarter, expanding it. Expansion gradually closes the bystarter valve (over a several minute period), leaning out the mixture, and lowering the idle speed.

Hot Restarts

With the Bystarter valve hot, thus the bystarter circuit turned off no extra air/fuel are available at restart.

  1. No prime occurs
  2. Once started, no extra air/fuel is supplied

The engine takes longer to cool down than the bystarter. This would lead to the bystarter circuit becoming enabled while the engine is warmer than normal. Since too much fuel can lead to starting difficulties, Honda has put plastic covers to help retain heat in the bystarter. Some models even have 2 covers. Keep the covers in place. But if missing, it may be helpful to open the throttle a little for re-starts to give a little more air.

Photos detailing air valve and bystarter operation

Air flow diagram showing path around the throttle slide, when bystarter valve is open

Carb Bystarter Air Path.jpg

Cutaway view details air valve operation

BystarterAirPath 2.jpg

Cutaway view details complete bystarter assembly in cold and hot positions.

BystarterAirPath 3.jpg

From cold to hot, Bystarter extension of approx 0.1" will close the air circuit. At about 0.15", the air valve would bottom out.

Bystarter Extension estimate.jpg

Animation of bystarter valve operation as it extends (may take extra time to load)

Bystarter time lapse 10s frame rate.gif

How the bystarter valve is extended

Bystarter extension detail.jpg

Bystarter Testing and troubleshooting

Bystarter problems

  1. The orifice to the bystarter bowl can clog, taking the bystarter out of action. See photo below. Symptom: hard starting, runs lean during warm up.
  2. Stuck bystarter valve. Usually the valve sticks in the cold position. Symptom: Stuck in enrich position. Sprees may show bump sensitive engine dying.
  3. No electrical connection, or bad AC output from stator. Symptom: Stuck in enrich position

Related troubleshooting

  1. For the bystarter circuit to work effectively, the throttle should be at idle. If the throttle is open, it reduces airflow through the bystarter circuit, thus its effectiveness.
  2. If the idle jet is clogged, folks often find their bike runs better with the bystarter unplugged from electrical connection. With out the electrical connection, bystarter stays in the 'always rich' position. Connect the bystarter back up, and see if the scooter will idle when warmed up. When the pilot jet is clogged, once the bystarter system shuts of extra fuel, the engine will die.

Testing the bystarter

The Honda Service manual outlines a test

  1. With the engine off and cold, connect a vacuum or pressure pump to the bystarter air passage. Air should freely flow.
  2. Apply 12v to the bystarter yellow and green wires. Wait a few minutes, then activate the pressure or vacuum pump. The air valve is not expected to be air tight, but flow should be well restricted.

The external link below is provided by member GSX1400. It is a video showing the leakdown of a properly working air valve. [1]

Photos of bystarter test setup from GSX1400

Bystarter Test setup.jpg

Author: mousewheels

Spree - Seized Bystarter air valve

Failure: Air valve seized in the open condition

Symptom: Bump sensitivity, engine dies

Models: Spree 1984-86

  • Fuel splashing in the main carb bowl, may pass through an air vent to the bystarter bowl.
  • When the bystarter air valve is open, fuel will be immediately drawn into the engine, which can cause it to die.
  • After restoration of free operation of the bystarter air valve, splash sensitivity is only during warm up.

Honda/Keihin recognized the splashing problem. Late model (1987) carbs changed the carb top casting to shield the air inlet.

Photo Air passage to the bystarter well on early Spree carbs, and the mixer tube.

Spree Carb bystarter vent.jpg

Spree Carb bystarter vent comparison.jpg

Author: mousewheels

Cold/Hot Measurements

Type bystarter length*, cold bystarter length*, hot

CH80 2.727" 2.902"

CH125/CH150 3.285" 3.442"

CH250/Helix 3.480" 3.652"

Author: Smooth

Air valve and needle measurements, cold only

94 - 2001 Elite SA50 (left) 87 - Aero NB50 (right)

Bystarter AF16 AF05 Comparison.jpg

Author: mousewheels

Bystarter Re-assembly

Every once in awhile, someone will come across a taken-apart bystarter. Since there are no service parts for this, Honda does not publish a blow-up diagram of this part. So here it is.

This is my tutorial on how to put a by-starter back together. If you look at this picture


you can see an exploded view of the by-starter. It's actually quite simple to put together once you know what part goes where. First, push the small silver rod(notch side first)into the hole on the golden looking piece with the o-ring in the middle and the spring attached to it. Then, push the bigger spring over the top of the small black cylindrical piece with the small needle valve piece on it. After that, push the two pieces together as shown in this picture

[ed: picture missing from documentation]

Once that's done, place the very small metal-like disc piece inside the top plastic assembly(with the two wire leads attached to it) as shown here


Then, place that middle assembly you just put together inside the bottom round plastic piece (with the threads on the outer rim) as shown here


All you have to do now is push that bottom assembly up into the top assembly, being careful the metal disc doesn't fall out or the springs pop lose. While pushing the two assembelies together, screw the bottom piece into the top one as shown here


and keep screwing in until you come to this point here


Now that it is complete, push it back on the carb, screw on the clamp, and try it out! One way to test it before you install it on the bike; hold the yellow lead on the positive terminal and the green lead on the negative terminal of the spree's battery. It is better if you do this carefully with some alligator clips, but if you are patient, you should(very slowly) see the needle valve move outwards as long as you hold it there(to an extent of course). If it moves, you know it's operating good; if it doesn't, then either you didn't install it correctly, or you need a new by-starter.

Chinese Bystarter

This is an exploded view of a Chinese version which fits our Honda's:



  • I started by cutting the upper plastic portion that holds the wires, in half to look for another possible resistor. Nope nothing just crimped wire ends, which showed no resistance when tested. So no more heat there. Just the ceramic disc sandwiched between them, also tested this and couldn't find any large resistance as well, so must just act as a heat transfer to the copper cap that is held against it and the wires by a spring.
  • The copy by-starter uses a solid ring terminal on both wires to hold the ceramic disc vs. the Honda one that uses 1 solid and 1 ring type with a hole to fit the ceramic disc, then a spring over the copper cap and tensioned to the terminals.
  • Decided to cut the copper cap off of the brass part to see what kind of spring was hidden inside. This is where the surprise came in. There was no spring, only a white themal grease inside of the copper part, then a rubber cap that separated the thermal grease inside the copper from Some kind of black gel type stuff inside of the brass section. Can only assume that this is also some kind of heat activated substance. Now this stuff is sealed into the brass portion by a small rubber plug inside of the nose of the brass piece. Against the rubber plug sets a small thin nylon disc that rests against the steel needle inside of the brass parts nose.
  • The steel needle is the first mechanical movement in the by-starter, it is pushed against (what i am calling the lower needle holder) by the expansion of the black gel.
  • Inside of the brass thing. which in turn sends the needle into the enrichment tube inside of the carb, while at the same time it makes the small brass cylinder (that that needle goes through) seat against the flat portion of the enrichment tube. ( another copy vs. Honda difference.) the brass collar that the enrichment needle goes through: The Honda one has a rubber seal that seats against top of the enrichment tube whereas the copy is brass only, no sealing surface. Could this cause a slight vac leak through the enrichment holes from air filter side to intake side in carb??? It seems that the small spring that attaches to the enrichment needle only allows the needle and slide to move slightly so they can align with the enrichment tube when extending.
  • Although it would seem that since there are no springs to weaken, then barring any thing plastic breaking or themal compounds going bad, that there would be nothing inside the by-starter that would need anything but a cleaning to make it work.

Author: Mousewheels, JF, dun rite

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