REPLICA YAMAHA YZ623

David and I became friends recently through business, he is a very talented carpenter and its his attention to detail in his craft that most impressed me, however it soon came to light that he too shared a love of two wheels and I was invited me to view a project he had been working on. The end result is very impressive and I asked David to put pen to paper so he could share his story with you all. This he kindly agreed to and below is his story which I hope you will enjoy.
Back in 1995 I felt like having a go at building another competition bike having a few years earlier put together a pre-65 trials Tiger Cub using a Bantam frame and an engine from 1959/60 Cub. The bike was ok but the jockey was s..t, and having fallen off too many times whilst taking part in events, I decided that it was time to do something safe and stay in the shed.

The inspiration for the Yamaha goes back in time to the 1960's when my hero was Phil Read. At that time he was racing British singles in the form of AJS and Matchless machines along with a sprinkling of others. He was good at his job and the then fledgling Yamaha Company took him on as their number one rider and supplied him with 250cc and 251cc machines for him to campaign in national meetings in the UK. The machines were RD56 works racers, the 251 an overbored 250 with which he could take part in 350cc races. Both machines were fast but reliability somewhat erratic but on their day they would bring home the champer’s. It was these screaming two stroke machines that really excited me but finance in those days prevented me from any sort of entry into the sport. However, I would travel to various circuits to watch the racing and was lucky enough to marshall in the Isle of Man at the TT on three occasions in the late 60's and early 70's. I have included in the photo’s some taken at Oulton Park during those early days when Read would turn up in his Citroen estate (that was before he came in a Roller), the works transporter was a Commer van with Yamaha painted on the side, a far cry from the palaces that the GP riders are provided with today. It was following the purchase of a book in 1995 chronicling Japanese Racing Machines where I saw a photo of the Yamaha YZ623 that I decided to have a go at making a replica racer and a trip to Gary Burton’s place at Cawston saw me rummaging through piles of bits and coming home with the basic parts needed for the project. Although the YZ623 is a 125cc machine, I decided to stick with the 200cc engine as this was with the bits and is virtually identical in size to the RD 125 and it seems parts are more readily available. Although most of the main bits were to hand I made dozens of phone calls to various contacts to locate the things I still needed, but in time I was able to put them all together and have made the machine you see in the photo’s.Frame is RD/RS 200, All bits not required have been removed, extra tubing has been bent to shape and added to mount the footrests, seat and fairing. The frame is painted with Hammerite smooth black and looks very like the stove enamel on the YZ623. Forks are of un-known origin but may be Honda, they have been modified to fit the Yamaha head stock and are of a heavier construction than the somewhat skinny RD items, clip on’s of course have been used along with alloy levers.Engine and gearbox are RD200. The engine and box etc, had been sitting somewhere and water had been allowed to get into the crankcase, it was at a bit of an angle and consequently the big end that side had picked up some quite serious rust damage. The engine was completely stripped and cases, barrels and heads sand blasted to try to make them look like the sand cast items fitted to the factory racers. Again all un-necessary bits such as electric start, generator, posi-lube and carbs etc. were ditched. The crank was overhauled, barrels were re-bored and new bearings, rods, pistons and rings etc, fitted and combustion chambers polished. Motor was re-built carefully with new gaskets and seals but was left otherwise un-modified. Carburation has been altered by fitting modified reed valve inlet mounts together with 28mm, Mikuni’s. It was suggested by Fahron Engineering that these would be required for a race modified RD engine of this type (when and if I ever got around to doing it). Racing plugs are fitted with the original points ignition being retained at present, electronic set up was to be fitted when funds allowed, twin coils take care of the spark. When running, I use either leaded or un-leaded fuel with synthetic racing oil as pre-mix. Exhausts have been made by Andy Bacon of ABCON exhausts who has the technical and manufacturing details of those specified by Fahron Engineering for their successful race modified RD200s, they are superbly crafted and make all the right noise that you would expect from a two stroke machine from the early seventies. Brakes have been modified also, the front item is Honda twin leading shoe, CB72 possibly and air cooling has been improved by the addition of a perforated vent one side with holes drilled on the other. The racer that I have replicated has a double sided 4 leading shoe arrangement but I guess I would have more chance of finding the proverbial rocking horse stuff than picking one of these up at the local jumble, unless you know different! please search your shed for one just in case you put one on that shelf. Rear brake is the original Yamaha item. Wheel rims are period looking narrow-ish alloy and have been re-spoked, tyres are quite narrow by todays standards but they look similar to the triangular racing Dunlops used during the sixties and early seventies, they may not be contributing to my personal safety but look the part. Fuel tank and seat are made from fibre glass and are identical to the 125cc. YZ623 works machine, they were made by Norwich’s Tony Milk from moulds taken from a TA 125 that he owned, this was Yamaha’s over the counter 125cc. racer which shares a number of similarities to the YZ 623. The fairing was purchased new from Hersham Racing down in London and is allegedly from a mould taken from a works 125cc disc valved racer, having spoken to Ken Inwood from Hersham at Snetterton circuit, he said that he thought that he had one that might fit, off I went to London and came home with it, as you can see, it fits perfectly. The belly pan that it came with was also fibre glass but this was discarded in favour of an aluminium panel carefully hand rolled just like the works machines had fitted, this is held in place using Dzuts fasteners. Painting of all components was done by myself using brush-on enamel for the frame and cans of spray-on for the fibre glass bits. I suppose you are wondering how it goes, well I have no idea as I have not had it on a track. It seems to run ok and makes a lot of the right sounds but is totally for track use so I can not even venture on to the road outside my house. I have ridden it relatively slowly on some of the runways at RAF Sculthorpe during a classic sprint meeting (I was not taking part), but the organisers asked me not to venture too far away from the paddock as their insurance was limited. Maybe I will get sorted and take it to a track day sometime before the Lord makes other plans for me. If you would like any more info regarding the bike David would be very happy to share his experiences and contact can be made through Adrian at the bike club or by leaving a note on this post.