Classic Motorcycle Mechanics

Stan again on his wide torque tune for the TDR!

Stan even became the man in hovercraft racing circles with his two-stroke tuning.

- WORDS: STAN STEPHENS PHOTOS: STAN, MORTONS ARCHIVE

Back in the early 1980s I was doing some dyno developmen­t work on our slave 250LC motor. I was trying a new direction, looking for more mid-range power. The main part of the work was to lower the exhaust port and increase the transfer timing. I machined the bottoms of the barrels to lower the porting overall and raised the transfer ports, I widened the exhaust ports but didn’t raise them and raised the compressio­n. The result was a massive increase in mid-range but we lost a lot of top-end revs, I shelved the idea. Quite a while later a customer came to see me and asked if I would tune his racing hovercraft engine. He showed me the engine; it was a purpose-built American made single cylinder 250cc two-stroke engine designed for hovercraft racing. It was so agricultur­al and I told the customer it was no good for any performanc­e. He then told me that they produce 22bhp and everyone uses them! I asked him if twins were allowed and he said there was nothing in the rules which said they had to be singles. I told him that I could provide him with well over double that power with an LC250 engine. He said that because they didn’t have gears they had to have a lot of mid-range power and they must not rev too high or the tips of the huge driving fan became supersonic and they would break up. I thought: lots of mid-range power and low revving. This sounded very much like my failed dyno slave engine! I sawed the gearbox off the engine and the guy fitted it to his hovercraft. That season he won the British Championsh­ip and the next season the European Championsh­ip. I did a lot of those LC engines with a wide power tune for hovercraft racing after that and they went on to win the championsh­ip every year. When the TZR250 came out it was a very revvy engine even though it had powervalve­s which lowered the exhaust port at low revs. I did the same work on the TZR that I had done earlier on the LC by lowering the exhaust port and raising the transfers, my TZRS took over in hovercraft racing and were still winning championsh­ips years later. When the TDR250 came out my work went full-circle; the TZR engine in the TDR was too revvy for the trailie type TDR so I

did my wide power tune and it transforme­d the bike. Rupert Paul the well-respected editor of Performanc­e Bike magazine tested my own tuned TDR and he loved it. In fact years later when he did a question and answer interview, he was asked which, out of the thousands of bikes that he had ridden, he found had given him the most fun and he said: “Stan Stephens TDR250 with the wide power tune.” The work I did on the LC and the TDR has been of great use over the years, I converted a lot of MX 125 engines into engines with more mid range power for Enduro use and MX 500 engines for Supermoto. The reason for writing now about the tune on the TDR is that I have just had to do a wide-power tune for a customer and I am sure he is going to love it. I will share the secret of how this is achieved with CMM readers. To carry out the tuning work you must have right-angle porting tools because the main part of the tuning is the raising of the transfer ports. The transfers and the boost port have to be raised by just over 1mm, to 39mm measured from the top of the barrel. The top of the exhaust port remains standard but has to be widened to 40mm wide. The powervalve has to be modified to match the exhaust port and flowed until the long screw which holds the two halves of the powervalve together is just visible. Some flowing work around the transfer collection areas at the bottoms of the barrels is beneficial. The barrels have to be shortened by 1mm, this will make the exhaust port 1mm lower and the raising of the transfer ports by just over 1mm is where the mid-range comes from. To shorten the barrels I mount them in a three-jaw chuck and machine 1mm off the bottom gasket face. The head remains standard. With 1mm machined from the bottoms of the barrels the pistons stick out the top of the barrels by 1mm. Use a rear head gasket from an RD500LC. The TDR gasket is 0.15mm thick and the 500LC gasket is 0.65mm thick. This gives the right squish clearance of 0.8mm and also raises the compressio­n at the same time! With the tuning articles I try to come up with a big-bore idea for the engine I am writing about. With the TDR and the TZR all that is available is the 2.5mm over-size Mitaka pistons which bring out the capacity to 275cc. When over-boring the barrels to 2.5mm o/s, you will have to trim the powervalve­s to miss the pistons and with the TDR wide power tune you will need to open up the head gasket. It didn’t take long to tell you how to do it but believe me that info has been gold-dust over the years! cmm

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 ??  ?? The TZR/TDR parallel twin two-stroke. A bike ahead of its time and now much-loved.
The TZR/TDR parallel twin two-stroke. A bike ahead of its time and now much-loved.
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 ??  ?? The state some parts arrive in!
The state some parts arrive in!
 ??  ?? Here we are machining the bases of the barrels.
Here we are machining the bases of the barrels.
 ??  ?? Here is a standard exhaust port.
Here is a standard exhaust port.
 ??  ?? And now tuned transfer ports!
And now tuned transfer ports!
 ??  ?? Here are standard transfer ports.
Here are standard transfer ports.
 ??  ?? And now the tuned exhaust port.
And now the tuned exhaust port.

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