1989 YZ250 Boyesen Boost Ports

The 1989 yz250 is a good bike but there are some things that can be done to improve it. One of the best according to Eric Gorr, is the installation of Boyesen reeds and ports. I had previously installed the reeds and now I am going to do some porting. According to Eric Gorrs book "Motocross & Off-Road Performance Handbook", the 1989 should have one port already. I pulled the jug but found no ports that seemed to fit his description. I did some searching on the web and finally found a small picture of the Boyesen ports that he speaks of. This was all the info I needed and I set about doing some porting.
This is a picture looking through the intake. The Boyesen ports should be 1/2 inch holes on either side of the main port. As you can see my 1989 has none. The only initial path for the mixture is through the floor, barely visible and under the reed cage.

I started by making marks inside the intake where the ports would go. I spent a lot of time looking around at both sides to make sure that the hole would fit. I had never done any porting before and I only had the one cylinder. I was using a Craftsman All in One Rotary Tool with a flex shaft and I wanted to make sure that the chuck would clear.

I drilled a small hole through and checked both sides to make sure that I wasn't screwing up. I was happy with the hole and I switched to a router bit to start opening it up. I was surprised at how easily the aluminum cut. I also had some problems with bit chatter that threatened to drive the chuck against the reed cage gasket surface.

This is the nearly finished port on one side.

This is the other side of the port where it meets the transfer ports. Notice that I also cleaned up the transfer passages with a rotary sander.

The finished cylinder with the Boyesen Ports. I thought that it came out pretty good for my first attempt at porting. I have since ridden the bike and it definitely broadens out the power band. I will need to spend some time getting the jetting correct since I am sure that this will require a change. I don't really know which way to go so if I figure it out I will post it here.