YAMAHA RD125 LC

Stan tunes this tid­dler.

Classic Motorcycle Mechanics - - CONTENTS - WORDS AND PHO­TOS: STAN STEPHENS cmm

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.

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