Most of the oil leaks into the cylinder which will cause blue exhaust gasses and a greasy spark plug.
The main cause is worn out piston rings: these rings are on the piston and their purpose is to scrape the oil of the cylinderwall so the oil leaks back into the carter. They are made of hard metal.
Piston and piston rings
After a while these rings are worn out and they don't scrape enough oil from the cylinderwall, causing a high oilwaste and a decreasing compression.
Replacement is the only solution. To do this, the cylinder has to be dismounted.
In most cases (these mopeds are about 25 years old!) the cylinder, the piston and the piston rings are worn out. To be sure, you must measure these parts; the dimensions must be smaller then the maximum dimensions given by Honda: 42 mm + 0.02mm for the cylinder.
The cylinder is worn out at 42.1 mm.
The standard size of the piston is 41.98 - 42.0 mm at the bottom.
However, there is an easier way. If you feel an edge inside the cylinder then the entire cylinder must be regrind, giving the cylinder a larger diameter. The piston and the piston rings must then be replaced by bigger ones, the size depends on the new size of the cylinder.
Standard size is 42mm (1.6535") for the piston.
The standard size and the overmeasures for the piston rings and the piston are:
| Overmeasure | Ordernumber piston rings | Ordernumber piston |
| +0.00 mm | 13101-081-000 | 13010-044-000 |
| +0.25 mm | 13102-081-000 | 13020-044-000 |
| +0.50 mm | 13103-081-000 | 13030-044-000 |
| +0.75 mm | 13104-081-000 | 13040-044-000 |
| +1.00 mm | 13105-081-000 | 13050-044-000 |
If you do this, keep in mind that there is a possibility that your moped is no longer legal, here in Belgium the maximum content of the cylinder is 50CC (class B).
The cylinder has to be dismounted, starting with the cylinder head cover to the gearbox.
Cylinder parts
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Loosen al the bolts that you encounter. Do not loosen the nuts of the rocker arm, unless you want to check the valves. Before you remount the cylinder, it is preferable to clean the piston. Remove all the dirt without scratching the piston.
Important: if you dismount an engine, always re-adjust the valves and replace all the gaskets, Honda has a set of gaskets which aren't mentioned in parts lists: ordernumber 06110-081-000.
If the cylinder is dismounted, pull the old rings off the piston, you don't have to take the cylinder of the piston rod for this. Be careful not to scratch the piston: a piston has a very narrow fitting. Scratches will cause leaks, loss of compression and damage to the cylinderwall.
Now mount the new rings, the oilring - that's the biggest one - is placed closest to the carter, the other two small rings have to come on top of the piston.
Before you shift the cylinder over the piston, put some oil on the cylinderwall, otherwise you'll get scratches. Placing the cylinder over the piston should go easily. Make sure that that the ring gaps are shifted 120�, otherwise oil will leak through the gaps.
Put everything back together and tighten all screws in diagonal order: first the bottom left, then the top right etc.
Be careful not to overtighten the screws: although the cylinder is made of cast iron so the head is not extremely sensitive for warping, it is still advisable to tighten them with the prescribed force:
the six bolts: 0.9 � 1.2 Kg/m or 8.8 � 11.7 Nm
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Kris Maeswith the co-operation of Hans Hartman