Bystarters 101

A place to discuss matters related to Honda Helix scooters.

Bystarters 101

Postby charlie55 » Sat Aug 30, 2014 2:26 pm

Introduction:

The purpose of this discussion is to provide a user-friendly tutorial on the topic of the bystarter, an essential and often misunderstood device on most conventionally-aspirated (carbureted) scooter engines. It is not applicable to engines equipped with fuel injection. The photographs and illustrations contained herein are intended to provide a generalized concept of bystarter architecture and function and, as such, do not depict any particular make/model. Additionally, most are not to scale.

Background:

Before getting into an explanation of what a bystarter is and how it works, it might be a good idea to present a little info about fuel and conventionally-aspirated (carbureted) internal combustion engines. A good starting point is what's called "stoichiometric air/fuel ratio". In layman's terms, this is the ratio of air-to-fuel at which the mixture burns the most efficiently. For gasoline this value is approximately 14.7 to 1, meaning 14.7 parts air to 1 part gasoline. Now, this is all well and good once the normal operating temperature of an engine has been achieved. However, things change when we're talking about a "cold" engine. In that scenario, a "richer" mixture is required, and can be achieved in one of two ways: restricting the volume of air, or increasing the volume of fuel.

The "less air" approach was traditionally used for automobiles and motorcycles. It involved decreasing ("choking") the flow of air through the carburetor by means of a movable plate called a choke (or butterfly) built into the intake side of the carburetor. Controlled either manually or automatically, this plate would be closed for cold starts and then opened as the engine warmed up. Most basic lawn and garden equipment such as mowers, weed trimmers, and chainsaws still use this approach, with the choke plate operated by a manual lever.

The "more fuel" approach is where the bystarter, in conjunction with an extra passage in the carburetor, comes in. Rather than decreasing airflow, the "enrichment circuit" formed by these two components automatically and temporarily increases the amount of available fuel.

Bystarters are also referred to as "auto-bystarters", "auto-chokes" and sometimes just simply "chokes". The latter two terms are dead wrong as the device doesn't achieve its intended purpose by "choking" anything, and the former two seem to be based upon bastardized or faulty English. So I'll use the term "enricher" from this point forward.

The Ins and Outs:

The simplest parts of the enrichment circuit are the inlet port and the outlet. These provide the endpoints for a flow of air to draw extra fuel into the engine. The following two photographs depict their typical appearance. The first photo also shows the typical location and appearance of the bystarter (enricher) itself. It is usually black, held in place by some sort of removable clamping bracket, and connected to a pair of wires.

Image

Image

"Cold" Engine Mode:

The following illustration depicts the major components of the enricher, namely:

- Heater
- Expandable Material
- Plunger
- Needle
- Jet

Image

In "cold" engine mode, these components are in the following states:

- Once the engine has been started, alternating current from the stator starts flowing into the heater. How quickly the heater rises to its
highest temperature is a function of its resistance. For now, we'll assume that it is as cool as the ambient air surrounding it.

- The (thermally) expandable material below the heater is also initially the same temperature as the ambient air. At this temperature, it is in a contracted state.

- The plunger, which is spring-loaded to counteract the enlargement of the expandable material, is retracted.

- The needle, which is connected to the plunger via a shaft and spring mechanism, is also retracted.

- Due to the retracted state of the needle, the jet, which allows access to the fuel supply, is fully unblocked.

As the engine is started, the vacuum created by the intake stroke of the piston draws air through the outlet and inlet port. This flow of air, in turn, draws fuel up through the jet. Once the fuel enters the inlet port-to-outlet stream, it atomizes and forms a combustible mixture for use by the engine. This supply is in addition to the fuel/air mixture provide by the carburetor's normal idle circuit, and that's what the whole "enrichment" thing is about.

"Transitional" Mode:

In between the point where the engine is cold and the point where it is at normal operating temperature, the enricher "transitions" as the temperature of its heater rises. The expandable material responds to the change in temperature by gradually expanding and exerting downward force on the plunger. This force is transferred to the needle, gradually extending it such that it begins to block the jet and reduce the air flow/fuel supply in the enrichment circuit.

"Hot" Mode:

The following illustration depicts the state of the enricher components once the engine has achieved normal operating temperature:

Image

- The heater, now fully warmed up, has caused the expandable material to achieve its maximum length.

- The expandable material now exerts maximum pressure on the plunger.

- The plunger transfers this force to the needle which is now fully extended.

- This results in the complete blockage of the jet, cutting off the supply of fuel and air through the enrichment circuit.

Failure Modes:

1. If there is a clog in the inlet port, the outlet, or both, no air can be drawn through the enrichment circuit. As a consequence, no additional air/fuel mixture can be supplied to the engine. This will result in a hard starting condition for a cold engine.

2. If there is a clog in the jet, or somewhere in the path leading to the jet, then only extra air will be supplied to the engine while the enricher is in "cold" mode. This will result in an even harder starting condition for a cold engine since it is not just being deprived of the additional air/fuel mixture, but is actually being supplied with a greater volume of (cold) air. This effectively "thins" out the mixture supplied by the normal idle circuit, making it even more difficult to get it to fire.

3. If a fault in the heater, expandable material, plunger, or needle causes the enricher to get "stuck" in "hot" mode (air/fuel flow totally blocked), the effect will essentially be the same as that described in the clogged inlet port/ clogged outlet scenario.

4. If a fault in the heater, expandable material, plunger, or needle causes the enricher to get "stuck" in "cold" mode (full air/fuel flow through the enrichment circuit), then the engine will be constantly running in a rich mixture state. While cold starts will be easily achieved, the maintenance of an over-rich mixture (long after the point where it is actually beneficial to starting) can lead to spark plug fouling, backfiring, and increased carbon deposits in the combustion chamber and on the valves.

Final Point:

In my opinion, and only my opinion, the primary shortcoming of the enricher is that it does not rely upon any feedback mechanism to determine when the engine is actually warm enough to run reliably without enrichment. Once the enricher itself has reached "hot" mode then, as far as it is concerned, the engine is good to go without any additional "help". Unfortunately, there may be rare times/circumstances when such is not the case.
Last edited by charlie55 on Sat Jul 14, 2018 2:22 pm, edited 2 times in total.
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Re: Bystarters 101

Postby sctr199 » Sat Aug 30, 2014 4:24 pm

Very helpful information, thanks!
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Re: Bystarters 101

Postby 600dan » Wed Sep 17, 2014 8:40 pm

Outstanding description of enricher operation. Thank you
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Re: Bystarters 101

Postby Motomanic » Sat Jul 14, 2018 9:33 am

It would be great to be able to manually command the enricher. Would this be possible by installing a switch to cut off electricity to the heater?
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Re: Bystarters 101

Postby charlie55 » Sat Jul 14, 2018 12:14 pm

Motomanic wrote:It would be great to be able to manually command the enricher. Would this be possible by installing a switch to cut off electricity to the heater?


I don't see what's to be gained by doing so. No power means that it will never warm up and thus never stop "riching up" the mixture. I know that's not super awful, but why burn more fuel that you have to? On the other hand, if you could somehow always have it on (fully extended) you'd be fine, except for starting in cool/cold weather.

Personally, I'd prefer a totally manual choke - no worrying about whether or not the enricher is shot or not.

BTW, I don't know what happened to the diagrams I included in my post. I'll check later and see if I can get them to be visible again.
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Re: Bystarters 101

Postby charlie55 » Sat Jul 14, 2018 2:26 pm

Fixed the problem. Images are now visible.
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Re: Bystarters 101

Postby Dan_Lockwood » Sun Jul 15, 2018 9:40 am

So the heater is AC powered from the flywheel... Didn't think about that.

I always figured it was DC, but wondered why they would not heat up with just the key on. The AC power makes sense as it requires the motor to be turning to create the voltage for the heater. This way the key can be on for many minutes without "heating" up the heater.

Most carbs like Mikuni and other Asian carbs have used enriching mechanisms for many decades. This is why opening the throttle when trying to start somewhat defeats the purpose of the enrichment circuit. So don't open the throttle when starting cold, you'll drop the low air pressure flow through the bleed ports of the bystarter circuit.

Thanks for the tutorial, very good.
Dan

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'98 Red Helix
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Re: Bystarters 101

Postby kenyrider » Thu Jul 19, 2018 1:35 pm

i think this is exactly the problem my scooter has been having. i think im going to replace the autobystarter since its only 30-40 bucks.
i though this piece was mostly just about starting. mine starts fine, colder mornings.. but then bogs bogs bogs. i think it gets stuck in cold mode and its overloading the carb with fuel. im burning through gas noticeably faster, but the worse part is loss of power.
the kicker is after work this summer, 95 plus degress, it runs FINE. get on the highway at 4 and it rips right up to 72 and runs smooth as can be for the ten miles to pick up my kid. turn it off for ten minutes, go back out, starts and stalls or stops completely. then the rest of the ride wastes gas, backfires, and dogs. its been doing this for the better part of a year now so it CAN'T be the fuel pump, it wouldve had to have died by now! i replaced the spark plug, carbuerator insulator boot that holds carb to block, and the ignition coil, all to no avail.

the other weirdo part that sometimes i feel like im imagining, is hitting a bump when it runs like crap and all of a sudden it lurches and takes off with normal speed and acceleration. do you suppose because that needle faces downward into the carb im literally knocking it loose and into place, blocking the port? feels likes thats whats happening.

good tips here
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Re: Bystarters 101

Postby charlie55 » Thu Jul 19, 2018 7:01 pm

kenyrider wrote:i think this is exactly the problem my scooter has been having. i think im going to replace the autobystarter since its only 30-40 bucks.
i though this piece was mostly just about starting. mine starts fine, colder mornings.. but then bogs bogs bogs. i think it gets stuck in cold mode and its overloading the carb with fuel. im burning through gas noticeably faster, but the worse part is loss of power.
the kicker is after work this summer, 95 plus degress, it runs FINE. get on the highway at 4 and it rips right up to 72 and runs smooth as can be for the ten miles to pick up my kid. turn it off for ten minutes, go back out, starts and stalls or stops completely. then the rest of the ride wastes gas, backfires, and dogs. its been doing this for the better part of a year now so it CAN'T be the fuel pump, it wouldve had to have died by now! i replaced the spark plug, carbuerator insulator boot that holds carb to block, and the ignition coil, all to no avail.

the other weirdo part that sometimes i feel like im imagining, is hitting a bump when it runs like crap and all of a sudden it lurches and takes off with normal speed and acceleration. do you suppose because that needle faces downward into the carb im literally knocking it loose and into place, blocking the port? feels likes thats whats happening.

good tips here


A couple of questions:

- How much has your fuel economy dropped off?
- Is there a strong smell of gas, and have you seen any drips near the carb overflow tube?

Unless the needle has broken off from the rest of the bystarter, it shouldn't be able to move on its own. And if a bump did make it drop into place, how would it subsequently lift itself out of place? I'd do the following myself:

- Remove the bystarter and test it with a 12V battery as outlined in the service manual.
- Check if there's varnish or gummy crap either on the working end of the bystarter or in the socket it sits in.
- Drop the bowl off the carb and see if the float sticks.
- Remove the float and the inlet cutoff needle. Check all around for varnish, crud, etc. I'm not sure, but I think the tip of the cutoff may be rubber and that could degrade over time.

It's a long shot, but maybe your float isn't fully closing the cutoff. The sudden shock of a sharp bump could be temporarily getting to work correctly. My gut tells me it's the carburetor innards and not the bystarter.
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Re: Bystarters 101

Postby scootcat » Wed Jul 25, 2018 1:51 pm

There was a company that made a manual conversion for the by starter, maybe check with some of the parts suplyers.
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Re: Bystarters 101

Postby scootcat » Wed Jul 25, 2018 6:19 pm

You tube shows you how to make one by Jake Westergren.
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Re: Bystarters 101

Postby scootcat » Wed Jul 25, 2018 6:24 pm

And the beautiful thing is if you won't more fuel for some reason you can manually open a little.
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