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The exhaust/transfers/intake port open periods of the 48cc Chinese HT engine are 110/125/113 degrees which are not quite ideal. The exhaust needs to open later and the intake needs to open earlier for good low-rpm power. The compression is also pretty weak at 5.5:1 which should be raised to 7:1. To acheive correct intake porting you can cut the piston intake skirt, and to raise the compression you can grind off .5-1.0mm from the head mating surface. Or to accomplish both of these goals and also correct the exhaust port timing you can just shave off 1.5mm from the bottom of the cylinder (at a machine shop). The following paragraphs further explain it all: Cutting 1.5mm from the bottom of the intake
skirt of the piston permits the air/gas mixture to enter the engine 4
degrees earlier in each engine cycle. The period of intake port opening
will then be 121 degrees as this chart shows is the best for both low and
high rpm's. Or if you want the best climbing power and don't care about
losing 1-2mph in top speed then cut the skirt 2mm for an intake period of
125 degrees. The engine compression of air/gas needs to be corrected by reducing the compressed combustion area. This can be done by not using the head gasket. But to prevent any compression leaks both mating surfaces need to be leveled by moving them in a circular pattern on sandpaper on glass. Also you can increase the compression even more by having the head mating surface milled down by .5 - 1.0mm.  Even better than the previous two steps is to
cut 1.5mm from the bottom of the cylinders mating surface for the best low
rpm porting. With this it is unnecessary to remove the head gasket or
remove .5mm from the head surface which only serves to increase the
compression without lowering the cylinder ports for the best port timing.
With this mod the port open periods for the
exhaust/transfers/intake change from 110/125/113 degrees to
115/131.5/122.4 degrees. This mod also increases compression
significantly.  |
