In this article I am going to assume that some of you who read them got enthusiastic with your new-found knowledge and carried out some tuning work on your classic two-stroke. Maybe your next step was to buy some race exhausts and larger carbs. If the port windows have been modified/tuned but the diameters/areas of the inlet ports and exhaust ports have remained the same, then with the race pipes and bigger carbs it will be like trying to get a quart into a pint pot, or the metric equivalent. I am going to cover enlarging the inlet and exhaust ports.
With a tuned engine, you will have a higher and wider exhaust port window (the window is the actual port in the liner in the cylinder bore), combined with maybe a standard exhaust port (from the window to the exhaust pipe flange). If this is the case the gases are not going to be able to get away as quickly as they want to, there will be a bottle-neck in the port. The engines and ports I am describing here are in classic road-bikes, not race engines.
First of all measure the internal diameter of the flanges of the race exhausts you have bought/fitted. Most times the exhaust maker will have made the exhausts with a larger flange internal diameter than the standard exhausts with the idea of the port diameter being enlarged. Usually they will have made the flange the same diameter as the exhaust gasket if the gasket is larger than the port (which it usually is).
If this is the case with your exhausts and gaskets, you can use the gasket as a template for the next part of the job. If the exhaust flange is a different diameter to the gasket you will have to make a ring of the correct diameter as a template. Mark inside the exhaust port where the gasket sits with a felt-tip pen, then fit the gasket or ring and run a scriber around to mark out where you are going to enlarge the port to. Although I scribe mark it, I leave the gasket in position and use it to grind to.
I use my porting tools to remove the metal but if you buy a long series carbide burr from a motor factor you can use it in a hand-drill. Grind the metal from the exhaust port to match the gasket or ring all the way around. When finished you will have a large port window and a large port outlet but a narrow port all the way through, which obviously will have to be enlarged. Do not try to remove too much metal too quickly. Use a pair of callipers or something to measure and keep checking the internal diameter. Basically what you will be doing is removing metal from the port carefully from window to outlet so that there is a straight passage with no necking from the window all the way through. When you are satisfied with the work you have done with the cutter/burr, make a split mandrel to wrap some emery cloth around and finish off the port smoothly.
To make a mandrel, get a piece of metal rod approximately 6mm in diameter and saw a slot in the end about 30mm long and round off the end. You can use progressively finer grades of emery cloth to improve the finish.
With an inlet port on a piston ported engine, that is one without reed-valves, the work is similar to that on the exhaust port. If you have fitted or are fitting larger carbs, measure the internal diameter of the carb and enlarge the carb rubber to the same size. Use the carb rubber as a template to mark out the diameter with a felt-tip and scriber that the port has to be enlarged to, if it is only a small amount just match the rubber to the port. As with the exhaust port grind the inlet port to the scribed line and use the straight edge and callipers to make the port as straight as possible with no necking in size. Finish off with the mandrel and emery cloth.
With the inlet ports of the Yamaha reed valve range things are a little more difficult. Once again I am assuming the port window has already been enlarged. To make the inlet port larger entails machining the barrel to take a larger reed block, usually a block from another Yamaha in the range. For example some of the small 50cc and 80cc machines can use the reed block from the RD125DX or an early 50cc or 80cc moto-crosser. The RD125DX can use the blocks from an RD250. The RD400 and 250/350LC can use the blocks from the 350YPVS. The example I am showing is fitting a V Force Banshee reed block to an RD500LC. I really rate the V Force reed blocks, they can be bought from Hard-core Racing.
Yamaha reed blocks are tapered from side to side so that when you measure the width, measure it at the widest point. Mark out the new width that you want with the felt-tip and scriber. Carefully grind the inlet to the marked out diameter. You will find as you try to fit the reed block into the barrel that you need to keep enlarging the port a bit at a time as the block is entering the port, it's a time consuming job and you need to be careful. If you do grind through the side of the port it's not the end of the world you can always have it welded, as I have found on this job! The V Force Banshee block will not quite go in the RD500 without a little welding: it is almost dead size and leaves the port too thin.
When you have ground out the port so that the larger block fits the next step is to make sure that the reed petals open without catching in the port, this something that most people forget to check. How does that sound?