Workshop - Engine Decoke

Details for this section are for the Lambretta 175 Li Series 1 and 2 however there are many similarities between these models and other models of Lambretta threewheelers/Lambros. This information and the pictures included are provided from the 'Instructions for Repair Shops' for these models published in 1963. It is intended for informational purposes and we would recommend seeking professional advice before undertaking anything described in the Workshop section.

The decoking of a two stroke engine is a very important operation and should be carried out with the maximum care, in relation to the damaging effect the carbon deposits have on the functioning of the engine.

The parts mostly involved in the decoke operation are:

- Cylinder exhaust port
- Piston ring grooves
- Combustion chamber in the cylinder head
- Piston crown

The cleaning of the cylinder exhaust port is of fundamental importance, as the deposits which form there bring about a reduction of the exhaust area, thus creating an increased resistance to the outlet of the combustion gases from the cylinder.

This resistance can be translated as a loss in the useful cylinder capacity and therefore a reduction in the power output at the moment of specific consumption. Furthermore, it will also create overheating with the danger of fragments of carbon becoming loose, thus scoring the cylinder barrel and causing piston seizure.

The cleaning of the piston ring grooves is also very im­portant, in as much that dirty surfaces facilitate adherence of the incombustible residues of fuel and oil.

Due to the temperature developed on the cylinder, these residues form deposits which can hinder the elasticity of the rings in so much as to cause them to stick.

These conditions also can bring about loss of power, overheating and possible seizure.

The cleaning of the piston crown and cylinder head is necessary the carbon deposits have a low coefficient of heat transmission and therefore affect the cooling of the head and piston and cause faulty spark plug functioning.

The overheating of the carbon deposits in ratio of their low heat conductivity, can bring about in certain zones a notable increase in the temperature, which through the reduction in the volume of the combustion chamber and therefore subsequent increase in the compression, can bring about preignition or auto-ignition, which manifest themselves by abnormal engine noises and increase engine stresses.

OPERATIONS
We give you some illustrated details on the operations necessary for the correct decoking of Lambretta engines, so as to facilitate and simplify the execution.

1	Remove head and cylinder
2	Dismantle piston by removing the two gudgeon pins, circlips, and push out the pin by pushing it with a rod or using a gudgeon pin extractor.
3	Clean out the cylinder exhaust port by scraping so as to dislodge the carbon deposits, with a thin scraper. Clean cylinder and port with petrol and dry out with air jet.
4	Clean the exhaust gasket and ascertain if still in usable condition. Otherwise replace with a new one to ensure perfect sealing between cylinder and exhaust tube. When fitting the gasket, be sure to fit the right way up, the hole in the gasket must correspond exactly to the outline of the exhaust port on the cylinder.

5

To clean piston ; check the efficiency and quality of the rings; remove rings placing them on the bench so as to be able to replace them into the original grooves. Wash piston with petrol, scrape and remove the carbon deposits from the piston crown, clean the crown with a thin blade, do not use emery cloth, as the emery dust and metallic dust caused by the emery can stick to the piston and work loose during running. By using a piece of an old ring (Fig. 77) scrape the ring grooves with its broken edge. Wash piston and rings with petrol and dry with compressed air. Check the play between rings and grooves by rotating each ring on its respective groove (Fig. 78). Check the piston ring stop pins To check the piston ring gap, slip each ring into the cylinder square to the bore. The gap in the ring should not be smaller than 0,3 mm (0,012). If the gap is more than 0,6 mm (0,024") change ring. Refit rings to respective grooves.

6	The clean cylinder head, remove carbon deposits from the combustion chamber and clean its surface with a curved scraper. Wash in petrol and dry.