Long since called an endangered species, the two-stroke engine model has been the catalyst for Japanese motorcycle manufacturers to make great strides in the global market. Lightweight, compact, and simple construction is a major characteristic of the 2-stroke engine. Although 2-stroke air-cooled models and even 2-stroke water-cooled models are rarely seen on the streets these days, 2-stroke air-cooled engines, in particular, are recognized by motorcycle tinkerers as a very enjoyable part of their lives because their mechanisms are particularly simple and it is easy to identify malfunctioning parts, regardless of whether they can be repaired or not. The engine is also recognized as a very enjoyable part for those who like to tinker with motorcycles.
World's smallest displacement, largest mass-produced 2-stroke twin model
A Yamaha Little Twin HS90 was stored in immobile condition and married to a staff member of this magazine. The front and rear fenders were fitted with HX90 fenders, but there was no noticeable rust or rot on any of the parts, and the vehicle appeared to be in relatively good condition. However, it was still an immobile vehicle. As production road motorcycle, the Yamaha 90cc Little Twin series was the world's most successful series model as the smallest displacement 2-stroke twin. The lineup began with the AT90 in 1965, followed by the HS-1, HS90, and HX90. The maximum displacement based on the same crankcase extended to the RD200, an export-only model. The Japanese model of the little twin was 90cc, but the main export version was the YL-1, a 100cc engine, both in Europe, the United States, and Southeast Asia and in the North American market it had the pet name the Twinjet. The Yamaha Little Twin series, with its unique and distinctive exhaust sound and acceleration, has numerous deep-rooted fans.
First step is fuel and ignition system maintenance
Most of the causes of immobility are due to dirt in the carburetor and clogged internal passages, as well as dirt in the ignition system point contacts. Some problems are caused by poorly adjusted oil pumps, where unburned 2-stroke engine oil stays in the muffler and clogs the baffle diffuser. When restarting a 2-stroke engine vehicle, it is an ironclad rule to make and use a gasoline mixture of about 20 to 30 gasoline to 1 oil when refueling the gasoline tank or supplying gas in a drip tank.
It is important to know the ignition system [what kind of ignition?]
The mileage on the meter totalizer is based on the overall condition of the vehicle, and it is up to the machine owner to believe it or not. It is up to the machine owner to check the ignition system when starting the engine and know "what ignition? is happening.
If the motorcycle has Flamag points or Flamag CDI, which ignites the spark plugs when you step on the kick, it is fine, but if it is a "battery ignition motorcycle," you must remember that a battery is essential.
Availability of basic data will also be important.
Since service manuals for Yamahari Tolzwin are not readily available, I found an English version of the service manual (SM) on an overseas website. I purchased the SM for the relevant model and checked the technical data. In this era (60's to early '70s), all manufacturers' SMs had more extensive descriptions and data in their overseas editions than in their domestic editions.
When restoring a motorcycle, it is very helpful to have a parts list (PL) to know what parts are available, regardless of whether the parts are in stock or not.
POINT
Point 1 - Start by inspecting the main parts of the engine, rather than starting it right away.
Point 2 - Fuel tanks must use drip tanks!
Point 3 - If it's a 2-stroke engine, use engine oil to make it a mixed fuel specification.
It has been 30 years since the license for large motorcycles was lifted. Since the end of the 20th century, a boom in large motorcycles has arrived. In that case, attention will be paid to small motorcycles that are easy to handle and lighter than anything else, especially the two-type moped model called "Hara 2" in Japan's road conditions. As you know, looking at the trends of motorcycle manufacturers over the past few years, it has become a “Hara 2 attention level” that can be called a return to the 1960s and 1970s.
On weekends and holidays, it can take two to three hours to get to a tourist destination due to heavy traffic on the expressway, but by taking the Hara 2 down the road and taking a shortcut, you can reach your destination in one and a half hours! There have been cases such as this.
Our editorial staff has had such experiences many times in the past. It is precisely because today's road and traffic conditions have improved and there are so many things about the moped model that we can reaffirm.
Here we report on our efforts to start the engine, hoping to get the old, immobile HS90 released by Yamaha in 1970 to run. The HS90 was a series model that was a late model of the HS-1 introduced the previous year in 1969. It is equipped with a 5-port cylinder which is an improved version of the AT90 series little twin engine that was introduced in 1965. It was known as a model that was loved by many young people at the time for its exciting performance that was very suitable for the word "lively".
This current engine was actually in a condition where it would not kick-off. However, in terms of total condition including exterior parts, it is not in poor condition. It seems to be because it has been stored indoors for many years. We thought that if only the engine could be somehow restored, we would be able to enjoy the unique sound and performance of the Yamaha Little Twin engine again.
What is the cause of the trouble? First, let's start with the cause of the lack of kick-off (see the next item for a detailed report), but first, there is something we need to know about the ignition system. That is, what is the ignition system? Generally, small-displacement models in the moped category use a flamag ignition system (flywheel magneto ignition system) in many cases. Electricity is generated when the flywheel rotates with the rotation of the crank when the kick is depressed. This is the system that ignites the spark plug using this power source.
You may have seen sparks fly to the plug electrodes by removing the spark plug, inserting it into the plug cap, and stepping on the kick while grounding it to the engine, but just because sparks are produced does not mean that the ignition system is safe.
In the case of a battery ignition system, the battery is, as the name implies, the power source.
Therefore, without a battery, the engine will not start. This HS90 is just such a case, employing battery ignition. However, even without a battery, when the driver steps on the kick, the electricity generated by the kick goes to the ignition system and spark the spark plugs, albeit weakly. This is the source of the misunderstanding. Even in a vehicle in good condition, the engine condition can be significantly degraded by the absence or weakening of the battery.
When starting the engine of a motorcycle that has been immobile for a while, the first thing to do is to check what the ignition system is before starting work. This is not limited to this HS90.
