Classic Motorcycle Mechanics

YAMAHA RD500LC

Super Stan is back with this interestin­g insight into how to make Yamaha’s V4 two-stroke into a race missile: for Wayne Gardner, of all people!

- WORDS: STAN STEPHENS PHOTOS: STAN STEPHENS AND MORTONS ARCHIVE

Stan Stephens is back and talking race tunes for Wayne Gardner?

In the late 1980s there were internatio­nal motorcycle racing classes called Formula 1, Formula 2 and Formula 3. F3 was basically 250cc two-strokes, F2 350cc two-strokes and F1 was 1000cc four-strokes and 500cc two-strokes. There were basic rules which said that the engines must be based on road bike engines. When it became a European Championsh­ip I was approached by one of the top racers, Geoff Johnson, to build a race 350 YPVS engine for the F2 class and an F1 engine based on the RD500 engine. Harris Performanc­e supplied both of the rolling chassis. CMM regular Steve Parrish also rode in the championsh­ip on an RD500 based bike for the official Yamaha importers. I had been having some success with RD500S in Production racing but in the Production class you could not fully tune the engines because they had to run with standard carbs, reed-blocks, ignition and pipes. With the advent of the F1 class, here was the opportunit­y to be more radical with the tuning. I couldn’t wait! I told Geoff what we needed, four 34mm Mikuni carbs, four 350 YPVS reed blocks and a race ignition. I got to work on tuning and building the engine. When they were finished Geoff had some successes with the bikes in Europe and scored points in the championsh­ips. One notable memory was the F2 bike clocking 155mph at the Northwest 200. On the tuning side there were some modificati­ons I wanted to try on the 500 for the following season but Geoff’s efforts had secured him a top ride for the next year and I never got to carry out the mods that I wanted to. Geoff Johnson was later tragically killed at the TT. The modificati­ons that I wanted to try concerned the inlet ports to the rear cylinders. A few years later an American two-stroke tuner sent me a top-end to tune for racing and I carried out some of these mods on his barrels; it must have been okay because I didn’t hear from him about the results! Over the next 30 years the RD500 engines that I tuned were for road bikes but I always had it in the back of my mind what I wanted to do to those horrible inlet ports for a race motor. A little while back, I had an email from a customer of mine in Australia, Mick Costin of Miko products. Mick builds replicas of Grand Prix 500cc bikes and uses RD500 engines in them because it is nigh-on impossible to obtain genuine 500cc, V4 GP two-stroke engines. Mick sends the top-ends over to me to tune for

the replicas. This time he asked would I build a full race engine for a secret project he had been contracted to carry out. The project was to build a race replica of the works Honda NSR500 that Wayne Gardner used to win the World Championsh­ip and it is to be used in a film about Wayne: you read it here first! Here – at last – was my opportunit­y to try the ideas I had around 30 years before! The bee I had in my bonnet concerned the inlet ports in the rear cylinders. In the front cylinders the inlet reed-blocks are mounted onto the crankcase like the TZR, in fact like all two-strokes from the TZR onwards. However the rear inlet reedblocks are mounted on the barrels themselves with only small holes in the barrels as ports and small holes in the pistons for the cylinders to breathe through. It is as if Yamaha had two different types of engine mounted on one set of crankcases. Over the years I have been using TZR250 pistons in the RD500 to help matters because they don’t have restrictiv­e holes in the back of the pistons they just have cutaway backs as in motocross engines. I measured the area of the inlets for the rear cylinders, they were less than half the area of the front cylinders with huge restrictio­ns, have a look at the photo. Yamaha got away with it because they ran small 26mm carbs and reed-blocks off a 250 LC but with larger reed-blocks and larger carbs those rear cylinders were always going to limit the power. I had Mick send over the top-end of the engine plus the top crankcase, the engine is going to run 34mm Mikuni carbs and use YZ85 V Force reed-blocks. You can see from the photos that the rear inlets have just the two holes to breathe through and there is a huge restrictio­n of the liner and the casting and also the very wide bridge in the middle of the port. The inlet side of the barrel obviously needs to provide support for the piston so I left enough material for the piston to work on but I removed most of the bridge and made a larynx-type centre to support the piston. I cut the bottom off the barrel across the bottoms of the inlets. I widened the inlet to the same width as the front inlet and raised the port to the height dictated by where the rings come down to at bottom dead centre. There is a wide ridge in the crankcase to support or flow the inlet bridge, I had removed that bridge so I removed the ridge as well. With the barrels mounted on the cases and looking down the inlets I was pleased with the increased area I had achieved; with a bit of tidying-up it looked right. It was now time to open-up the inlets to take the YZ85 V Force reed-blocks, they are not quite as large as 350cc Powervalve reed-blocks so there isn’t the drama of breaking through and welding-up. The rear cylinders were straightfo­rward to enlarge, on the fronts mounted onto the crankcase it meant enlarging the openings on the manifold housing and the crankcases as well. I tuned the rest of the engine the same as I had previously and rebored the barrels to 1mm and it was ready to go. It will be interestin­g to see how well the bike goes; one thing is for sure with the powervalve­s and the inlet reed blocks it will not be the animal that the NSR500 was from Wayne Gardner’s era!

 ??  ?? From road (left) to race (above) for him (inset)!
From road (left) to race (above) for him (inset)!
 ??  ?? Here’s the standard inlet.
Here’s the standard inlet.
 ??  ?? Standard inlet and standard crankcase looking up from inside.
Standard inlet and standard crankcase looking up from inside.
 ??  ?? Here you can see the restrictio­n in the standard barrel inlet.
Here you can see the restrictio­n in the standard barrel inlet.
 ??  ?? This shows the final, modified inlet.
This shows the final, modified inlet.
 ??  ?? Here it is from the other way round.
Here it is from the other way round.

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