GPR Camp: Replica Racers

For sometime now I've been collecting parts that would ultimately end up on a bike to take to the track--kart track that is, not a full on moto race track. Just something fun to ride in an environment where you can push the bike a little harder, and not have to worry about things like cars, curbs, and other similarly dangerous obstacles.

Somewhat recent events caused me to change my plans a bit and accelerate this build. My Red bike has seen significant abuse due to bad drivers (it's been knocked over ten times since the build), and vandalism. So that bike is sadly coming apart in prep far a rebuild. In the mean time a lot of its dinged up parts are being repurposed to the track bike where scratches and such are just a fact of life. I've been given lemons as it were, and it's time to spend the afternoon in the sun!

The idea for this bike is to basically take parts I have laying around, and assemble a fun ridable bike for the track. It's something I plan to do in stages with regular revisions as parts become available. The frame is from another project that was being assemble for a friend that gave up due to life getting complicated. So he sold me a roller. Most of the bits from that bike I wont be using, but the frame and swing arm were decent. They were sent to the powder coater, and came back all shiny black. I did this at the same time as the Red bike to optimize the discount, knowing that eventually I'd use it for something.

The frame when it was at the powder coaters a couple of years ago. Originally this was one of the infamous GPR75's that made it into the States. Ozzie from Cycle Imports brought 200 of these over in a couple of containers. Somehow I've ended up with three of them. :lover:


The bike as it was once mis-envisioned.


Ed had suggested that for now I should take parts from the Red bike, and get the Track bike running--so that we could go ride at the Stockton track event. The Track bike and the Red bike on the way to Ed's where the swap will begin.


A track bike needs proper tires. These are Sava F001 (95/70-17 ), and R001 (115/70-17) track tires picked up from Treats to go on some 17 inch Derbi Grimeca rims (the stock rim size for the 2000 GPR is 16 inch). My friend RedSix did a late night tire swap to get ready for the track.


After some time at Ed's we headed to the track, where I would have to do a bunch more work.


Speedy brought out some cardboard on which to set up shop, and my Sacto moped buddy's provided some necessary shade.








Most everything went pretty well, and I had a bunch of help from my friends. I wasn't able to get the bike started, so it didn't get to go on the track. Current belief is that it's an ignition issue. I harvested an HPI mini rotor that I had installed on my moped. The stator didn't fit all that well, so I may have to get a proper mounting plate made up if I can't get this one customized to work.


Some cool things currently on the bike:
Metrakit ProRace pipe (for an RS50)
Brembo RCS16 master
Risse Racing Shock
Sava F001, R001 tires
Poor Man's ProRace Airsal 50mm top end
Metrakit 24mm PWK carb
Domino roller throttle
E clip sprocket mounting (for quick sprocket changing ability)
2005 GPR clutch lever assembly
Trail Tech kill switch
Handmade aluminum oil pump block off cover
Lots of custom black powder coating action

We went to the Sonoma kart track today, and I had a chance to work on the bike. I figured it was an ignition thing. The plug was wet, which meant fuel was at least getting to the bike. I pulled the plug and verified that it was sparking. Then I repeated the process with setting the ignition timing. But it turns out the HPI doc didn't provide the correct information. I then remember that my buddies at Motomatic had posted HPI information with regards to setting the timing. I then applied their research, and the bike started right up.

But I was out of time, again. But next week we should be going to Stockton again, and I should be able to do some tuning. :}

The Motomatic info is not all that different from the HPI docs, but the HPI docs have you set it to the other side of the center, and that's enough to make it not work. My friend Taylor suggested setting the timing to the third big tick from the left. This is where I currently have it set. I also had a bit of trouble centering the stator plate because the Puch plate doesn't quite fit the EBE. I had to modify the the stator plate. The mounting bolts are such that I wasn't able to make it so that I can easily rotate the plate, which means I have to pull the rotor, and reset it to make adjustments. Pain in the ass, but it works.

The thing that I'm currently employing as a seat is the fuel tank. The folks at Cagiva used an interesting method to get the weight were they wanted it, and also created a place to store one's helmet!

The racing that's currently going on out at Stockton seems to be gaining momentum. I went to the most recent race. I probably knew about half the racers from the rides I've organized, track days, and moped riding. It was super fun. There's another race this month, and hopefully I'll be able to attend, although I probably wont race. They host open practice from 9am until an hour before the races start. Hardly anyone runs a 50cc top end. There is a class for kids that does, but even the mopeds are running around 75-80cc, and there are a bunch of ugly sounding 4T bikes out there too. I suppose that's to be expected, but the mopeds were beating them. ;p

One of my buddies got so inspired by the email I sent out to my friends after the previous Stockton race that he started building a bike to take to the track that very day. I was out at his place a couple of times this week helping out. Here's a work in progress photo.


The Forks

I also started working on the forks. Had the wrong seals though, so the fork repair will have to wait until I can get the proper seals. For those interested the stock oil weight for these forks is 7.5, at 175cc. Once I get the proper seal dimensions I'll post that info too. Because these forks do not feature adjustable damping, the easiest way that you can change the reaction times is by changing the oil weight. A lighter weight will make the response times quicker, and a heavier weight will make them slower. Adding more oil will change the progressiveness because there will be less air to compress. For the track, since it is smoother than most roads a faster response time would likely be preferred. So I'm going to try 5 wt this time.

It would also be possible to switch out the springs, or adjust the sag by inserting a spacer with the spring. I wont be playing around with either of those at this time.

There is a trick to working on these forks. You can't actually do it with hand tools because the bolt that must be removed will just spin. So you have to employ a power tool to remove the top socket head bolt. There's also a trick to putting the screw back in. There is a hole near the end of the shaft. You slide the spring over the shaft, and then slip a small auger into the hole. Spin the spring to get the nylon piece over the shaft, and then spin the bolt in. It's a bit tricky, and the manual doesn't explain any of this stuff.

The fork seal says these numbers for the ID OD Height:

34.74 x 47 x 9

I found some different numbers listed for these forks, but that's not what's in them, and they are different than the Showas that are also used by the Derbi. I've successfully used the All Balls seals for the Showa forks with part number 35-1011, seal size 35 x 48 x 11, but the Sebac forks for whatever reason employ a different size. And interestingly enough some websites list the wrong size for the Sebacs. Which is how I ended up with the wrong seals. Additionally, the parts manuals for these bikes don't list the seal size either.

A couple of UK sites list seals is this size, but so far I've not found a dust seal. There's an older Guzzi that has the same size, but the dust seal is the old style wrap around. The Gilara Runner seems to have the same dust seal which is a 35 x 47.5 x 13. More research to be doing.

Found some images of the seals on ped parts. These are the actual parts from Piaggio.

Dust Seal


Fork Seal

Dcrac3r wrote:looking good! I love that you are putting part numbers out there for people!

Yeah I do a lot of that sort of thing. Partially because I can't remember them, and then also because I'm sure others can benefit from my effort.

When I first started getting into the whole Derbi thing I could't find anywhere that listed part numbers. I found braggadocios that would claim they knew how to do things, but didn't share any info, or worse made fun of people that couldn't figure it out. Very annoying people. I didn't want to be like that—so when I found stuff I posted it.

A bit more news on the seals front. The KTM SX 65 from around 2003 has a very similar size seal setup. It's not the same, but it might work.

KTM SX 65
All Balls Part #55-128
Oil seal 35x47x10

All Balls Part #57-139
Dust seal 35x47.5x11.5

All Balls makes kit that includes the oil and dust seals for about $30. I've gone ahead and ordered that kit. I'll let you know how it goes. I'm mostly concerned with the oil seal being 1mm taller. It's possible that the lock ring that keeps the oil seal in place may not be employable with the slightly taller seal.
All Balls KTM SX 65 Kit Part number 56-143

Compared to the Sebac fork:
Oil seal 34.74 x 47 x 9
dust seal 35 x 47.5 x 4.8/13

Athena also makes a seal that is the proper size for these forks P40FORK455136

Adrenaline sells a Texnix oil seal that is the proper size.

Racing Planet sells the proper sized oil seal from Naraku NK158.20.

Interestingly a Tomos A35 main shaft seal is pretty close too: 35 x 47 x 7
The nice thing about the Tomos seal is that the height wouldn't be an issue.

If the All Balls oil seal doesn't work I'll source either the Athena or Texnix. That with the All Balls KTM SX65 dust seal should produce a viable affordable seal kit for the Sebac forks found on the 2002-2003 GPRs.

On this Moto-Guzzi site they list all of these bikes employing the 34.74 x 47 x 9 oil seal.

Fits Moto Guzzi Le Mans 1, Le Mans 2, T3, Le Mans 3, Mille GT (35mm Forks), 1000 SP, Convert, G5, T4, V65, Lario, V7 Sport, V35 Florida, Imola 2, Monza 2

I was able to source some All Balls for the weekend. Stephanie over at KC Engineering found a set, and got them to SF this morning. I also picked up some 5wt fork oil. If it doesn't work out I can always put the 7.5wt in the forks. ;P The thing is since these forks are not adjustable the damping can only be effected with changing the fork oil. This will provide slightly less damping, where if I were to go to 10wt it would produce slightly more damping.