The current motor im using is very stock in regards to cylinder squish (its about 1mm which is very high), it has no boost port and the exhaust port is still stock size. The modifications to it are mostly bolt on so it should remain reliable for the season altho down on power compared to the new one.
This motor should be very high powered and high strung. So i pulled out a few very old and worn out barrels to test a few techniques ill be using to make it before i ruin the genuine article :)
First part to be modified is the reed cage and reed valve. These engines introduce the fuel air mix into the engine via a flexible fibreglass/tin/carbonfibre sheet known as a reed valve. The bikes come stock with a thin tin reed that flutters badly at high RPM and just doesnt work well at all. The aperture is also very small so the air flow is heavily restricted as you can see. To get more power the engine needs more air mix!.
Milling the center bar out and opening the cage by about 3mm on the top 1mm on the bottom and 2mm on each side increases the flow rate potential up by almost double. You just need to be careful you dont ruin the sealing rubber on the sealing side of the reed cage.
To replace the stock tin reed im using a BZM .45mm thick fibreglass reed. This will allow the engine to rev much higher due to the reed being stiffer. The thick fibreglass ones work well. Ive found they allow the same revs as the carbon ones (which are generally regarded as better in watercooled engines) but give more bottom end. That and they are much cheaper!
Ive detailed the work on boost ports in a previous entry so i wont go into it here, aside from saying it will be shaped a little different from before.
The biggest difference from my other engines will be the main transfer ports will recieve some attention. Now im using a huge 16mm carburettor compared to the 12mm stock one and the reeds are modified the rest of the engine needs to flow an equal amount more.
The stock ports are pretty small and they arent shaped well at all. To aid in schnurle loop scavenging (look it up on google) and make the engine work more efficiently i enlarged and reshaped them with a ball nosed end mill to look like such. Excuse the finish, it was done fairly rough to see if it would work. The actual engine will use a much less agressive feed so the finish will be heaps better.
This directs air up towards the combustion chamber and away from the exhaust port before looping it back out the exhaust port due to the radiused port roof compared to the old flat one that just directed the flow into the center of the engine due to the old ports having a flat roof.
Next job is to port the crankcases out equally. They will be seriously modified so i will document that soon :)
Enjoy
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