I found oil leakage gasoline leakage mixture leakage from the cylinder base gasket. This is the cause of engine contamination, and since this is a 2-stroke engine, it is obvious that a decrease in primary compression will lead to a decrease in engine power. In any case, replacing the gasket was a must.
So, the work was put into practice. If we remove the cylinder, we should check the piston condition and the wear condition of the piston rings at the same time.
If you want to enjoy your old motorcycle for a long time
Older 2-stroke motorcycles have more wear parts than older motorcycles with 4-stroke engines. One of them is pistons.
Although the pistons are no longer sold by the manufacturer, we were told by a Yamaha Mini Fan user that pistons for the Mini Toray could be used for the Bovi, which were sold by a manufacturer specializing in external pistons. I was skeptical, but I bought them. If you want to continue to ride your motorcycle for a long time, you should secure spare parts not only for pistons but also for gaskets.
Decided it's still usable. But it needs care!
The bore of the piston that came out of the engine was exactly Φ47mm, so it was STD size.
The engine was in excellent condition for having been ridden more than 20,000 km, with no seizure marks or scratches at all. The motorcycle must have been ridden gently. I decided that with such good condition, the motorcycle could still be used with plenty of time to spare. I cut off the bottom of the PET bottle with a cutter knife, turned the piston upside down, sprayed carburetor cleaner chemical on it, and waited for a while. The carbon that had accumulated on the top of the piston would dissolve as it soaked up the solvent that had accumulated at the bottom of the bottle.
Piston rings are checked for mating gap
The removed piston rings were set in the cylinder and the ring mating gap was measured with a sickness gauge.
Rings that have been used and worn out are the main cause of compression loss, but the old rings were not worn out as much as expected. We also compared the ring to a new ring, but unfortunately were surprised to find that there was no significant difference. I wondered if the piston rings had been replaced relatively recently, but since they were not worn out, we decided to keep the new piston rings as spare parts. The data on the gap between piston rings differs from engine to engine, but for a 2-stroke engine, 0.3 to 0.4 mm is the standard size, and 0.7 mm is the limit of wear.
Engines that have been run at high RPMs far exceed the wear limit, and it is not uncommon to find engines with a gap of 1.2 mm.
Valve cleaning for reed valve engines
The reed valve is responsible for keeping the primary compression. In a piston valve engine, the piston itself plays the role of a valve that maintains temporary compression. If this reed valve becomes open, the primary compression drops and engine power decreases. I disassembled and inspected the valve plate and body, and found no dust bites or carbon deposits. In any case, if the cylinder is removed, the reed valve should be disassembled and cleaned. Remove the valve plate and gently polish the contact surface on the body side using an oil stone.
The base gasket was replaced with a new one in order to eliminate leakage and seepage from the cylinder base gasket, which was the cause of the engine waist disassembly in the first place. After carefully removing the gasket using a scraper, the cylinder base surface and crankcase gasket seating surface were polished with an oil stone soaked in oil.
Both sides of the new gasket were restored while a thin layer of the gasoline-resistant liquid gasket was applied.
POINT
Point 1 - Pistons in 2-stroke engines are easily reduced due to the structure of the engine, so they should be considered consumable parts.
Point 2 - When securing spare parts, we also secure peripheral parts at the same time. For example, if you secure a piston, secure piston rings and gaskets at the same time.
Point 3 - Disassembled piston skirts are not polished. Only deburr and chamfer the edges.
The reason for disassembling the piston and its surrounding parts was that I felt that it was slow to run with no punch in actual driving. When we test-rode a friend's motorcycle equipped with a similar engine, it was torquey, fast, and punchy. In comparison, the Bobby 80 was noticeably slower. It was also a lump of sluggishness. So, I took out the piston and checked the piston wear and piston ring wear.
Surprisingly, the piston that came out of the engine was in remarkably good condition, and the piston rings were not at the limit of wear, but rather at near-standard values with minimal wear. The condition of the reed valves was also checked during disassembly, and they were also fine.
After much research, we determined that the cause of the lack of power may be the exhaust system. I tried burning the carbon in the exhaust system and exhaust pipes (we burned them with a gas burner for a while), but we could not seem to burn off the carbon that had accumulated inside. So I bought an expansion chamber (for Passol) for another model. I welded together the stock exhaust pipe for the Bobby 50 and made my own original chamber. When we test-drove it, it was a complete change from the way it had been running! The feeling of power, speed, and most importantly, the punch has changed! The result was a very satisfying driving experience.
When disassembling an air-cooled 2-stroke engine with a simple and easy structure, for example, do not forget to prepare peripheral parts in addition to the pistons and piston rings, which are the main replacement parts.
It is easy to forget to purchase various gaskets and other parts, so when disassembling engine parts, whether it is the engine waist up or the clutch, do not forget to get various gaskets.
