YAMAHA RD125 LC
Stan tunes this tiddler.
When the RD125LC came out in 1981 it moved the goalposts in the 125cc class. Let’s face it there wasn’t much opposition: the Suzuki GT125, the Kawasaki AR 125 and the Honda MBX 125! The LC was intended to conform to the new restrictions for 125s but it was simple to derestrict and anyway there was an unrestricted model. Its release coincided with the ACU announcing a new 125cc Production Race Class. At that time our business was all about racing and production racing was the most popular racing of all with our LC250S and 350s ruling the roost. One of our most colourful and successful sponsored riders in the LC racing classes was Curt Langan; I see Curt is out racing again in the resurrected Pro-am Series! Curt asked if I would prepare him an LC125 engine for the new class. I tuned the engine and its first race weekend was over a three-day holiday. Curt rode at three different circuits on his SS Tuned LC125, LC250 and LC350S in all the production classes and some open class races and won over 60 races that weekend! He also won some of the open 125cc class races on the little LC. You can see why I have soft spot for the LC125. The engine was based on the YZ125 motocrosser and the inlet reedblock was the same size as the TZ750! When I write tuning articles in CMM I usually say that the maximum you would normally raise an exhaust port to on a race engine would be 50% of the stroke; the LC standard was only 1mm short of that. Also the maximum width would be 72%; the LC standard was only 2mm less than that. For model recognition, the unrestricted model was the 10W and the 12A was the restricted model. On the DT125 LC trail bike which shared a very similar engine, the unrestricted model was the 10V and the 12W was the restricted model. When I write articles for CMM I have to write about an engine that I have in the workshop or I wouldn’t be able to take photos. A customer of mine in Ireland, Declan Foster, that I had just renovated a 350LC engine for, asked me to tune an LC125 for him. When the head, barrel and piston arrived a problem was obvious. I have written before in CMM that when a ring-peg comes out of a piston the ring turns in the ring groove and the ends of the steel rings catch in the exhaust port and gouge out the top and bottom of the port and liner. I haven’t had an example to photograph before but
here was a perfect example. Have a look at the photo, you can see where the peg has gone up the side of the piston and the ring has turned and chiseled out the liner squaring it off. This wasn’t going to clear with just one size of rebore. The bore was on standard size, I knew that it would not clear on fourth oversize which is 1mm but the sizes greater than 1mm were Taiwanese and I didn’t want to use one of those especially on a tuned engine. I ordered a 1mm Japanese piston kit from Grampian Motors and it arrived next day as always. I don’t like to bore 1mm in one cut so I bored it to 0.5mm and then to 1mm. The worst of the damage cleared and because I was going to be tuning the barrel I would be raising and widening the exhaust port which should, when it was chamfered, clear the rest of the damage. Now on with the tuning. Once again I went through my old tuning specs and found the spec I had tuned Curt Langan’s to. As I said earlier, the LC engine was produced in a fairly high state of tune so there isn’t a lot of work needed to make it a little flyer. With the huge inlet manifold and reed block and the scarily large holes in the piston skirt, the inlet port itself didn’t need enlarging – in fact with the small carb I think the reed block could have been improved with a flowed restriction. The only work I did on the inlet port was to lower the bottom of the main port from 86mm, measured from the top of the barrel, to 102mm. The exhaust port was 26mm high, measured from the top of the barrel and I raised it to 25mm which was virtually little more than matching the liner to the port. The port was 39mm wide as standard and I widened it to 41mm and flowed the port through making sure there was no necking in the diameter of the port. I raised the transfers from the standard 40.5mm from the top to 40mm, again almost just matching the liner to the port. A little bit of flowing to the transfer collection areas at the bottom of the barrel and chamfering the ports, especially the damaged exhaust port, and the barrel was finished. With an engine that is fairly well tuned to start with there is only a certain amount of tuning work to do to improve the power but also it is easy to do too much and lose power. Finally it was time to tune the head. There was some damage to the head from the ring peg but that would come out with the machining. First I machined the top of the plughole so that the head ran true when on the lathe. With the head on a simple mandrel screwed into the plughole, I machined 0.75mm from the face of the head. I then turned the cutter post to 12 degrees and machined the squish band to 57mm wide, the size of the bore. So, there you are Declan, as raced in 1981!
This shows ring peg missing and ring turned in groove and it also shows the huge piston windows.
Here we are boring the barrel.
Now we are machining the head face. As usual, precision is everything...
And finally machining the angle of the squish band.
This pictures shows the modified exhaust port and transfers.
This shows the exhaust port damage!
Look at this! A Tz750-sized reed block!
Here you can see the standard inlet.
And here we have a modified inlet.