Derbi GPR 50 Belson project

[toratora]

You've got a bit of work to do.

The carb is a 17.5 PHVA. This is generally a very good thing, because it's the larger of the stock carbs for these bikes. But if you want the bike restricted this isn't going to help. Polini makes a very nice carb, that's easy to tune that you can get in a 14, the CP. That would make your life much easier with the restriction process. But I have to wonder, no one goes out and puts this carb on their bikes. This bike probably came with the 17.5, so that would lead me to believe the powers at be are okay with this bike having this much power. I don't remember seeing the CDI, but it's possible that's where the restriction exists on this bike.

The front master could be repaired with a rebuild kit, at least for the missing part, but I would expect it to have corrosion in there to the point that you might be better off sourcing a replacement.

Do you have access to someone that can re-chrome the sliding tubes on the forks? You can get seals for these forks, but replacing them without dealing with the oxidation will just tear the seals again. You can sand down the oxidation, and keep an eye on it, but eventually it will come back, and tear the seal.

The front sprocket is a 14, so dropping that to a 12 will help you with your target speed. These bikes use the same front as a bunch of Hondas so you should be able to get one for about $5.

[mace]

I've heard of this CDI before, what is it? If this is the stock carb, I'm inclined to keep it, as you say probably came with it.

[mace]

I suspect the sliding tubes are beyond what the UK MOT would consider allowable, so they've got to get either replaced or rechromed. I've seen adverts for two places in the UK, quite pricey, around £200 for the pair, and that's with me pre-stripping them down/rebuilding them. Seals etc on top of that of course.

Did you say you replaced your front master with a Brembo? Was that just the master, or the whole lot? I had a quick look on Brembos website but wasn't able to find any products that matched the GPR; if you have any part numbers still that would be great, ta.

[toratora]

Yes I'm using the Brembo RCS14, which I bought from Kurvey Girl. They are truly awesome, but expensive. There are some other options though. Usually when doing this upgrade I also upgrade the caliper, and rotor too. I'm not that far along with the Red Rocket thread, but I should be getting there soon with lots of photos. I'm running the RCS15 on the 05, but eventually I'm going to switch that out to a 14 also.

[mace]

Have been busy trying to get hold of parts and such forth, and am now beginning reassembly.

I got some forks from a GPR 125, which are in much better condition. Unfortunately they are also longer, so for the time being I've mounted them high in the triple trees so I can build up a rolling chassis.

Wasn't able to find a second hand rear caliper, at least not from anyone willing to have a conversation (long story) so ended up buying a new one.

My current problem is actually embarrassingly trivial, putting new brake pads in the front caliper. The old pads had metal backing plates which were clipped on with arrows for wheel rotation, so I swapped those to the new pads, so far no problem.

If I put the pads in with the backing plates attached, the locking pin fits fine and all is good. But if I also insert the bit of metal that sits behind the pads, I cannot get the holes to line up with the locking pin for love nor money. Does anyone have pictures of how the caliper is supposed to look built up, and have any suggestions as to why it won't fit back together?

Thanks!

[toratora]

I think I know what you are talking about, but could you post some photos to make sure?

Did you get the forks from the 125 2T, or the 125 4T? If you don't know, a photo would tell the story.

[mace]

Forks:

[mace]

Bit of metal from the front caliper:

[toratora]

Those look like they are from the 2T GPR, so they should be the same length as your stock forks.

The metal from the brake is a spring clip. What it does in keep the pads where they belong. It applies pressure on the tops of the pads, and the other springs that go through the pads. It will through off the alignment of the pin springs, but if you push on the clip when pushing in the pin springs it will go together. Just takes some practice.

[mace]

These are definitely over an inch longer. That's all I know

Re spring clip, I put a g-clamp on the assembly and still couldn't compress the pads in enough with that clip in situ to get the holes to line up. That seems like unreasonable force to me, so I'm guessing something isn't right. It's literally the thickness of that spring clip (way less than 1mm) that's stopping the pin lining up properly. Do you have any photos of a correctly assembled caliper (or better yet a disassembled one that was subsequently assembled fine)?