TUNING THE RD250LC
Stan Stephens goes back in time.
Recently someone rang me and said he was a customer of mine back in the 1980s when he was racing a 250LC. We were chatting about the good old days of LC racing and how competitive it was and he told me that instead of building a classic original looking LC, he was building an exact replica of his proddy racer and would I tune it for him exactly as I used to tune them back in the day. At first I said no because I have inhaled enough alloy dust over the years and I don’t do any tuning nowadays, only engine rebuilds. He was insistent and said it would not be genuine if it wasn’t tuned by me; in the end I agreed. It brought back many good memories of the most fantastic racing you could wish to see, working on this engine. I had unbelievable success with the LCS in racing and so many of the riders went on to great success on the world stage.
The 350LC is always mentioned when people talk about LC racing but the first model had bad faults which took Yamaha a year to sort out and many riders’ bikes were late arriving. The 350LC only reigned for two years and then it was replaced by the far more competitive 350 YPVS, whereas the 250LC was competitive for six years. It was only when the Mk3 Suzuki Gamma and the Yamaha TZR 250s came out in 1986 that the 250LC’S reign came to an end: during that time I must have tuned hundreds of them! In the end I was getting over 50bhp from the engine in Production racer spec, i.e. standard exhausts, carbs and ignition. I dug out my old tuning spec sheets and started work. The bores were in a poor state and I re-bored them to 1mm oversize, which was the maximum permitted oversize back in the day. I always re-bore the barrels before I tune them because where the tops of the ports enter and leave the cylinder they are not at 90 degrees to the bore so that when you bore them, the tops of the ports are lowered. I modified the pistons as shown in the picture to remove the obstruction of the piston skirts. I weighed the pistons to check they were of equal weight and removed a little metal from inside the piston of the heaviest one until they were equal. The 250LC was low-revving and a lot of our success was by running a very high compression and a very weak mixture, I used to jet down a long way on standard and also run premix. To get the squish clearance tight I used to machine .5mm off the bottom of the barrels and had no step in the head. The reason for skimming
the bottoms of the barrels instead of the tops originated from the time when the rules said no machining must be visible from the outside and the scrutineers used to check the tops of the barrels to see if they had been skimmed. When that rule was relaxed I started to skim the tops instead because it was easier, but it lost power, I don’t know why, but I went back to skimming the bottoms of the barrels. With the head mounted on a face-plate on the lathe I machined 1.25mm off of the face of the head. To machine the squish bands the head has to be mounted on a mandrel in the spark plug hole. To make sure each combustion chamber is the same volume and to make sure the head spins true on the lathe I mount the head on the mill and machine the top of each plug-hole to the same depth. With the head on its mandrel on the lathe I machine the squish angle to 14 degrees and to 55mm diameter, the same as the cylinder bores and the bore of the head gasket. I did a huge amount of development with reed valves on the LCS, the best reeds I developed were a one-piece reed that was tapered, i.e. they were thinner at the tips and were thicker at the screws end. The reason for this was at low revs, where the reed didn’t need to open as much, only the tips would open and as the revs rose the reeds would open fully, this gave a very wide spread of power and was one of those tricks that took a lot of dyno work to perfect. The material I used to use isn’t available now so on this engine I just flowed the reed blocks like I used to do and opened the stops to 11.5mm at the tips.