GPR Camp: Replica Racers

. The Derbi Reactivation Project
My all time favorite bike is the 05 GPR that I sourced from Atlanta. A Saab buddy of mine picked it up, crated it, and delivered it to a dock where it was then trucked out to Oakland. I picked it up there with my Saab. The warehouse guys said “theres no way you are getting that in there.” I just proceeded to prove him wrong.

I put a lot of effort into this bike. Engine, brakes, suspension, tires—the only thing I didn’t really mess with was the looks, because I really love the styling of the 05 GPR.

Then almost exactly three years ago I was crashed on it by a young woman in a Honda Civic. This broke many of my bones, messed up my knees, and a bunch of rear teeth. Even three years later I’m nowhere close to the physical state before the crash. But I wasn't the only one hurt—my bike suffered greatly too.

I tried a few things to get it running again, but it really needed some serious effort. So I pulled it apart, brought it into the kitchen, and set it to non-op. CA speak for I’m not paying any reg fees while I can’t ride my bike.

I’ve slowly been gathering parts for it, and now I have enough to maybe get it running again. This will very much be a project where the bike comes together, and then I slowly rebuild it to where I want to take it.

I was going to wait until I had all the bits, but I so miss this bike that I have to do something about it. That means a temporary engine, temporary plastics, and well a few of the things that I wanted to do. But being a long term project it’s okay to do things in steps.

Some fun things I’ve already picked up for it

The Forks!
I was able to get the forks! Yeah that's all I’m going to say about them for now, but Scotty knows what I’m talking about—and if you want a set let me know. I’ll help you get them too.

Jolly Moto
I always wanted a Jolly Moto pipe for the GPR. I have one on the Monza, and it’s so awesome. One popped up on eBay a while back, and no one went for it. It got relisted, and still—so I bought it. It's really not the right match for my engine, but well it’s a Jolly Moto!

Xtreme
The previous build featured the 45mm stroked Airsal Xtreme kit. I still haven’t figured out how to make that happy, so for this temporary engine build I'm going to go with tho 40mm stroked Xtreme kit, also with the Airsal crank.

Sava
I’ve been running the “MotoGP” tires on this bike. I had sourced a rear wheel from the 2T 125 because it’s wider. But there were too many differences so I went back to the 50cc rear wheel. This meant that the tires didn’t properly fit the wheels.

Since then Sava has come out with the MC50 super soft. Oh yeah! Same compound that they use to make the track tires. They probably wont last all that long, but who the hell cares when you can ride street tires like that on a fifty! Benji only had the front in stock, so I’m going to have to wait for the rear.

Plastics
I finally found a place that still makes the GPR50 fiberglass farings. It takes a while, and they are pricey. But I fully intend to go for it—eventually. For now though I have a set from an 02 GPR, and I'm going to try to make those work.

Brembo
I’ve been running the 16x18 radial Brembo on the GPR, but it’s time to upgrade (the 16x18 went to the Red bike, and is now on the Track Bike).

Okay that's a good opener. I’ll probably be updating the OP from time to time.

Last week I made an appointment to visit Treats. Need to get some parts, you know?


To get this bike back on the road meant building a temporary engine. My friend Jason put an EBE075 into his 2003 GPR, and gave me the left overs of his stock engine. It's missing a bunch of bits, including a crank, and all the clutch parts, but it's a great start. Tonight it was a good opportunity to split the cases, and find out what's inside.

I use to have a Puch clutch puller, but that got lent out to those whom will not be mentioned. So I needed a replacement. Now the Puch puller isn't really the right part for the job so you have to modify it. It's the closest thing that Treats sells for this application, and with a bit of cutting with the hacksaw you are in business. I didn't make great cuts. The bolts needed to split the cases are M6 (this might be off I'll check it and update). You can actually use the engine mounting bolts (you'll have to source a second).


Once you get the clutch and stater off you can employ the puller. It's right at the edge of not being able to work, so you need to be careful.


If you are careful the cases split right open. You might need to use a rubber hammer to get the transmission end to open up.

The cases had some work done with port matching. Maybe to a Metrakit. I'm going to have to do some mods to the Airsal Xtreme to make it all line up. Since that requires noise that will have to wait until the morning. Thankfully it will be pretty straight forward work for the rotary tool.

Had to wait for some day light to make noise with the rotary tool. First I slid the barrel onto the studs to see where adjustments would be required. Mostly I just want to remove any shelf that might disturb the flow of the fuel into the transfer ports. I feel that Airsal already knows more about port design than I do, and I just need to match things up.

Marked up with the Sharpe


I tend to take it slow, checking the work as I go. It's easy to remove material, and a bit harder to add material. Folks that do add tend to use epoxies like JB wield, or some such.


Almost there


I'm probably going to swap out the drive seal, and the shifter seal as well. The main drive seal is a 17x35x7, and the shifter is a 12x20x5. Lots of mopeds use the same size as the main seal for cranks. The shift seal can be found on some Hondas (part #91205-KF0-003), and also on some KTMs (part #0760122050). Even O'Reily's has these online.

Inspired by one of Matt's posts I decided to get the items necessary to build a leak down tester for this bike. O'Reily's has a large variatiy of rubber freeze plugs which are perfect for the job. That with a brass fitting, and that's basically all I needed. I already have a MityVac.

With the Airsal Xtreme the exhaust port worked with a 1"-1 /8th" plug. The intake on this bike will be one of the Malossi units for a PWK carb. For this a 1 3/8th"-1 1/2" worked well.


Basically all I had to do was remove the bolt and washer from the larger one, and insert the brass fitting.


This fit nicely into the Malossi intake.


For the exhaust port it was just a simple insert.


And there you have it, a simple leak down test kit. I'll be testing this out once I get the engine built.

I've been searching for an EGT solution so yesterday I called up the Trail Tech cats. They said that none of their meters were comparable with EGT sensors, because their temps didn't go high enough. From their website the TTO temp gage that we all use is ranged at: 30-500°F, 0-270°C. Which isn't nearly high enough for EGT.

The Trail Tech cat on the phone said that they would have to employ different technology to enable their gages to read EGT, and because they hardly ever get any requests for EGT it is unlikely that they will ever offer the ability.

The Stage6 EGT setup sells for around $100 at some places. $150 at others. It's basically a rebranded Koso, and Koso does sell a couple of different solutions, but the snowmobile folks don't speak too highly of these units.

The Sensor Connection has some real industrial equipment. The sensor would probably work for about $65 and shipping once you got all the bits you needed. And they have a handheld unit at ~$70 is in the ball park of many other inexpensive solutions.


Another thing I'm looking into is an actual lambda sensor for strokers. But these are even bigger money.


Once you start thinking about this stuff all of a sudden something like the Aim Mycron4 shows up on the radar. This would be north of $400 so it's sort of pie in the sky at the moment. I think Matt has one of these, so I expect some education may be following soon.

The 4 2T keeps looking better the more I check it out. But I'm really not ready to take it to that level yet. But maybe by the time I get the bike running I'll be more open to the idea. If I had found the Stage6 EGT meter in town I would have gone that way this week.

I thought I'd be ready to drop the main bearings into the cases by now, but just too many other things have to happen first. I've got them prepping in the freezer.


I went by Treats to get the drive shaft seal. They didn't have a shift shaft seal. I'll have to source that from another vender.

The Barn shipment showed up this morning.

I thought I was making great progress today. Finished up with a bunch of the cleaning last night and had planed to do the assembly today. Everything was going well, and got it the main cases buttoned up. The cranks spins nice and smooth, but the transmission doesn't want to spin well in fifth, and wont go into sixth. I hadn't taken the trany bits apart so I didn't expect this to be an issue. But really since there were signs that this engine had been opened previously should have given me pause to do a proper inspection.

The Heat! This is pretty much the only kind of gun I have--I say pretty much because there's a glue gun around here somewhere too.


The bearings dropped in.


Checking the new gasket.


The crank sitting in the new bearing before full insertion.


Almost time to put it together.


The sexy new Malossi drive shaft seal.


The manual states to make sure the bolts are all clean. This is the before shot, and they aren't too bad, but I did some cleaning on them anyway. The torque setting on these bolts is 6NM, or about 53 inch pounds.


All together, and waiting to come apart.

Yeah I took it apart, and cleaned it all out. Then made sure by the docs it was all together correctly. It's working now.

Sadly due to my current circumstances I don't have access to the usual tool action. Notice I'm working on the kitchen table.