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Discussion Starter #1
I guess I should show this build too. not a big deal, just a bike I'm throwing together for my 12 year old nephew. I live in Riverside CA and there is a track a few miles from me, old Perris raceway, where they race flat track. I was talking at a family gathering (before covid-19) and my nephew said he wanted to do it so that's all the excuse I needed to start this. No big deal here, just a 1970 Sl100 I picked up of Craigslist last year, I always go for old bikes under $100. so I picked it up. It was a roller with motor and gas tank and some loose parts. I picked up a nice CB200 front end and wheel on e-bay, had the cylinder bored to .75 OS to fit the new piston, removed the kick starter parts and freshened up the motor. I got a new (import) 24mm carb, built the pipe from a CBR600 header I cut up before and made the muffler from a stainless water bottle ($9.00 from Dollar Tree). The seat, I'm makeing out of fiberglass, I put some thick sign cardboard over the frame and fiber glassed it, them pulled a tail piece, molding off of a GS650 tail I had laying around and bonded them together. After I got the shape I liked and squared it up (by eye) I put more layers on it to make it strong enough for racing? After I got it to fit and be strong, I filled in the low spots with fiber glass reinforced Bondo. I'm waiting for more parts to come from e-bay, I will ding out the gas tank and paint it soon. If the nephew sticks with it, I will get nice rims and make it nice, but for now I just want to make it function as a track bike? I use the tin foil with a no stick coating over my molds, so the glass releases easy, also so I can keep the weight down. here's some photos;
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Very cool.
Honda 100s were very popular in the mid-70s for dirt track. Among the many trick mods, was to use the transmission gears from a CL 100, as I believe they were closer in ratio than the XL, CB, or SL.
Your nephew will surely have lots of fun.
Saw in another post, that you are building a YDS2 to race, yourself. A compression release would be a great addition to your bike. Big help starting a slide into a turn.
I would encourage you to see what kind of knee brace you can come up with, especially given your age. You only have a few years on me. I remember hobbling around the college campus for weeks on crutches, after tearing up my left knee in a hard crash while short-tracking my '82 RM-125.
Do either of you have a steel shoe, yet? The old-timers (maybe just a few years before me:)) used to make them out of car bumpers (when car bumpers were made of steel). You can fab one easily out of 1/8" or thinner plate.
Have a great time making memories!
 

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Discussion Starter #3
Thanks, I bunged up my knee several years ago (fooling around in the driveway, on a DT1) so I will be looking into knee braces. The YDS2 heads don't have 2 plug holes and I don't think there is room to drill more? I did see that they run steel shoe plates and I'm sure I could maybe copy one if I saw it up close. Anyways, I worked a lot more on this today cause parts for it came in. I found some old paint that was still good so after I had all the bodywork done, I painted it (candy red) . I also cleaned up the wheels and mounted new tires and tubes, I got these cause they were cheep and were "dual sport 20/80 off-road/ on-road and the rules don't require any special pattern is the motor is 100cc or less, I'll see if they work ? here's another photo from this
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afternoon.
 

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Very nice.
The tires should be fine. As you know, you want to be able to have the rear tire break loose in the turns. You will surely be experimenting with tire pressures, from track to track.
If you and your nephew are able to make or purchase steel shoes (really a must-have item), you will want to make sure that you have rubber on the left footpeg and the toe of the shifter. Automotive heater hose and safety wire should do the trick.
As you are probably aware, most factory dirt track bikes have the shifter on the right side, either by original design or by modification. You and your nephew will probably find that you will want the shifter to be installed so that the toe is a little higher than it would typically be for trail or street use.
I have cut fins from cylinder heads, to be able to install a compression release. However, if there's no room, I guess you can't do that to yours. Not sure how hard it would be to find another cylinder head to give it a try.
Have fun!
 

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Discussion Starter #5
I have a few "extra" heads but I don't see where there would be room for this, The YDS Yamahas are a 2 cyl. 250cc so the individual heads are = to a 125? kind of small, I guess it would need a release valve on both heads too. I will see if I'm even good enough to need that trick if I get it going and maybe try to do it. for the SL100, I've been working on the seat more, and made some studs to laminate to the bottom of the seat to hold it to the frame and 2 tabs to go under the front mount, they seem to hold it to the bike fine. I also built a "push bar" to reinforce the rear fender so it wont crack when we push start the bike. I used 1/2" mild steel tube to make the frame and it's attached in 3 places so it's pretty solid, I can pick up the bike with it and it wont flex. I fitted up an air box, mostly to use the battery mount and put a "UNI" filter in it. I'm still waiting for a condenser and rectifier to finish the ignition and electrical, I plan to run an on off switch and neutral light on the handle bars and maybe a digital tach? The last thing today was to mold a seat upholstery pan, I just laid some plastic over the seat and put 2 layers of class over it. When it cures tomorrow, I will trim it and bond some studs into it to hold it to the seat then cover the foam and glue the material to the under side of the pan Here's a few more photos;
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I have a few "extra" heads but I don't see where there would be room for this, The YDS Yamahas are a 2 cyl. 250cc so the individual heads are = to a 125? kind of small, I guess it would need a release valve on both heads too. I will see if I'm even good enough to need that trick if I get it going and maybe try to do it. for the SL100, I've been working on the seat more, and made some studs to laminate to the bottom of the seat to hold it to the frame and 2 tabs to go under the front mount, they seem to hold it to the bike fine. I also built a "push bar" to reinforce the rear fender so it wont crack when we push start the bike. I used 1/2" mild steel tube to make the frame and it's attached in 3 places so it's pretty solid, I can pick up the bike with it and it wont flex. I fitted up an air box, mostly to use the battery mount and put a "UNI" filter in it. I'm still waiting for a condenser and rectifier to finish the ignition and electrical, I plan to run an on off switch and neutral light on the handle bars and maybe a digital tach? The last thing today was to mold a seat upholstery pan, I just laid some plastic over the seat and put 2 layers of class over it. When it cures tomorrow, I will trim it and bond some studs into it to hold it to the seat then cover the foam and glue the material to the under side of the pan Here's a few more photos; View attachment 103541 View attachment 103542 View attachment 103543 View attachment 103544 View attachment 103545
 

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I have never installed a compression release on a twin. I would presume, however, that you would only need it on one of the cylinders. That should provide adequate engine braking. There should be enough room to do it. I have installed them on 125s.
Do you have access to a mill, so that you can drill the hole into the combustion chamber and mill the surface to allow the gasket at the base of the compression release to seal on it? As I'm sure you already know, you could use a hole saw to cut through the fins.
Should you choose to do so, you can re-phase the flywheel on the SL100 to eliminate the need for the battery, etc. There is probably an article or 2 on the Internet to explain how to do it. If it were me, I wouldn't install a tach. Just something else to break when he falls down.
Keep up the great work!
 

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Discussion Starter #8
I worked most of today and half of yesterday on making molds of the seat and upholstery pan. When I molded the upholstery pan on the first seat, it blistered the paint and ruined it. I decided to make a mold of the seat and pan so I could duplicate them if they get crashed, very possible, I've never done this on such a big parts before and it took some work to make it work out.
After I got my molds, I had to re- prep and still need to re-paint the seat but I have a set of molds to play with now so I hope it was worth it. I have a XL125 in the wings if nephew needs a bigger bike later so I'm sure it will be worth the work. I finally got the condenser and throttle for this bike so I should get it running next week? They have a race at Perris raceway next month on the 8th so I'm hopeing to get it ready for that? here's more photos;
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Discussion Starter #9
Both molds need more work to be ready to use, but they also need to cure for a while, that's probably why the paint blistered too, if it had cured for a few weeks, it might have been ok? I will order fiberglass form release material, and gel coat, before I try again. I used Carnauba wax on the seat first but it wasn't enough and it still ruined the paint and was really tough to get the mold off the seat. I will read more about doing this before I try again, I think it's important that you pay attention to the angles on your form so the mold ca come off easier?
 
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