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Thread: Derbi GPR Track Bike

  1. #76
    apriliaforum expert mattology's Avatar
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    ah. i thought you had the modified jolly moto or welded modified whatever on it. must have been a different bike.

    florida is at sea level with about 95% humidity ...

    Get your confidence back with Aprilia.

    caution: the use of motorcycles may be habit forming. side effects include purchasing multiple examples of the same bike, in different states of tune, different years, or different models altogether. some motorcycles may be not running when purchased. some parts are no longer available. there are rare instances where aprilias are bought over the internet and upon arrival, they are not as were originally disclosed. there is a small group of people who cannot handle two stroke engines and motorcycle maintenance. if this is the case, you may not be a candidate for Aprilia RS250's.

    Aprilia. Ask your doctor if the RS250 is right for you.

  2. #77
    apriliaforum expert ToraTora's Avatar
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    Wink Dyno print outs

    Ed sent me photos of his dyno print outs.

    The KTM

    Metrakit PR GPR
    Last edited by ToraTora; 12-07-2015 at 01:44 AM.

  3. #78
    apriliaforum expert ToraTora's Avatar
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    Wink Smoking on the Pipe

    I do have a Jolly Moto on the Monza, and I recently picked one up (years ago already!) that will go on the Black bike once I get it sorted--but it's one of those projects that takes years and years.

    Here it is in mock up mode.

  4. #79
    apriliaforum expert ToraTora's Avatar
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    Wink More Charts

    Here are the scans of the charts from my bike this year.




  5. #80
    apriliaforum expert ToraTora's Avatar
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    Wink More Dyno Action

    Here's the dyno of the 45 stroke Airsal Xtreme with the same pipe and carb that are currently on my track bike. I also found that the dyno run from three-ish years ago was also done with the 24mm PWK carb, so that's no longer a factor. Now the interesting thing is this dyno run was with the same carb and pipe as the dyno run on the engine currently in the track bike. They even both have HPI ignitions, so those are effectively the same. The air box is different, and maybe I need to do something about the air box on the track bike, but that engine was using the air box used on this dyno run back when it was in the Black bike (I know this is all rather confusing!). But what's interesting is basically with the only difference being the top end/crank configuration the peak power is in very different locations. So is the pipe really controlling the peak? I use to think so, but the data is messing with my head.


  6. #81
    apriliaforum expert ToraTora's Avatar
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    Wink Mr. Sticky comes to town.

    Sticky Sava track tires for Mike's bike were the project for today. We employed Greg's bead breaker again--I was too busy getting a work out to take photos of that action, but here are a handful of nice ones, and one of the Brembo RCS14 Master.

    We also did a carb swap to see if the bike would run better. It ran a little bit better, but still suffered major bogging with the throttle open.

    To start things off we used Mike's tire warmers. We got the new tires nice, and warm, but as it turned out we needed to heat up the old tires to get them off. They were really stuck on the rim--probably from having been warmed up previously. It was so much easier having help to mount the tires on the wheels. Didn't take hardly any time at all, and I'm not nearly as sore as I've been from previous tire outings.


    Front tire on the bike. I should have taken a photo of the rear one too. Have to get that later I suppose. :P




    And the pretty Brembo action. This is becoming a very nice bike.


  7. #82
    apriliaforum expert ToraTora's Avatar
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    Wink Forking Around

    I finally had opportunity to put the All Balls KTM SX65 seals into the Derbi forks today. Ed granted me access to his bench so that we could use the pneumatic action to loosen up the bolt on the hydraulic tube. I've not yet figured out a way to loosen this bolt with hand tools.

    Now these SX65 seals aren't an exact match to the stock seals. So far the forks looks to be well sealed, but I wont know for sure until I mount these on the track bike, and do some testing. I really hope these work, because then we have a very nice solution for these bikes. I also intend to play around with the fork oil weights, and quantities. Those of you with a sharp eye will notice that I'm using 5 wt, rather than 7.5 wt. Yup, trying out a little experiment.

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    The Forks: These are the 2003-2004 GPR Sebac forks. The require slightly different seals than the older ones. The 2002 forks do share the same seals, but those are anodized gold rather than silver.


    I think I already showed how to disassemble these forks earlier in this thread so this is just the assembly section. Lube up the seal, and place it into the fork tube. It wont go in all the way, but it will slide down to the seat. With the new seal in the fork tube used the top cap to seat the seal. It doesn't take much force, but you want to apply it evenly. You will know it's in place because you'll be able to see the ring for the retaining clip.






    Once the inner seal is in place insert the retaining clip.




    Then insert the dust seal. You should be able to put this one in place with just your hands. It doesn't take much force, just a little bit of finesse.


    Then slide the stationary tube onto the sliding tube. Insert the spring. Reach in and pull out the hydraulic tube. Add the nylon spring cap. Now it's time for the secret tip on how to do this. It's nearly impossible to hold the hydraulic tube to insert the bolt. So take a small screw driver, and insert it into the small hole in the top of the hydraulic tube. Rotate the spring so that the tube reaches high enough that you can thread in the bolt. Then torque the bolt.




    These forks take 120ml of 10 wt fork oil. You can change the wt, and amount to change the dynamics of the fork damping. Lower weights will decrease the damping. More fluid will stiffen the forks. Grab your graduated cylinder (you can get these off eBay for less than $3), mark with a Sharpe where you want to measure to, and pour in the desired amount. Pour this into the fork. Insert and torque the top cap.

    Last edited by ToraTora; 01-06-2016 at 02:24 AM. Reason: fixed an error in the numbers

  8. #83
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    Wink La Reservoir

    We needed a nice affordable solution to the brake master reservoir for the Brembo on Mike's bike. I found some Chinese brand that was State side, and ordered up a few. They look pretty decent for what they are--now I just have to fashion up a bracket.






  9. #84
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    Wink Stockton

    Ed and Alex went to Stockton. Alex took second on his KTM. Ed ended up racing a bunch of motards. I think he took second--he's number 14 in the photo (the white GPR). I'll have to find out who took first. :P

    Click image for larger version. 

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  10. #85
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    Wink Correction!

    It turns out that the Sabac forks found on the 2002-2004 GPRs use 120mm of 10 weight! The inner top bolts should be torqued to 17Nm.


    I'll go back and update the previous post...

  11. #86
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    Wink Gp0

    Ed just picked this up for his son. It's pretty cool--but it is a 4T.

    180cc, and it puts out about as much power as that 72cc Derbi GPR behind it!


  12. #87
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    Wink PWK Activate!

    I've been posting PWK info in some of the tech threads, but I thought it would be nice to have them around in the Track Bike thread since I run PWKs on most of my GPRs.

    Here's a link to Duncan Racing's PWK tuning guide.

    Here's an exploded diagram and parts list for the PWK, and a needle chart.






    Here is a nice graph illustrating the effect of the various circuits, and their interaction on the PWK.


  13. #88
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    Wink Inside the GP0

    I went over to Ed's today to check out the GP0 that he just picked up for his son. Admittedly the 4T is a big turn off, but outside of that it's a really cool bike. Check out these photos. Note, that Ed replaced the stock 4T engine because it was too much for the class that he wants to enter his son. So he bought a Lifan 125 engine--which mostly just slipped right in.














  14. #89
    apriliaforum expert mattology's Avatar
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    that shock looks familiar

    Get your confidence back with Aprilia.

    caution: the use of motorcycles may be habit forming. side effects include purchasing multiple examples of the same bike, in different states of tune, different years, or different models altogether. some motorcycles may be not running when purchased. some parts are no longer available. there are rare instances where aprilias are bought over the internet and upon arrival, they are not as were originally disclosed. there is a small group of people who cannot handle two stroke engines and motorcycle maintenance. if this is the case, you may not be a candidate for Aprilia RS250's.

    Aprilia. Ask your doctor if the RS250 is right for you.

  15. #90
    apriliaforum expert ToraTora's Avatar
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    They snuck out to the track yesterday. If they had told me I would have gone too. :P I haven't heard the results yet. The bike was set up for a fat guy, but it's suppose to be for Ed's son, so they have to go through the suspension for a 60 pounder. :P

    I didn't recognize that shock though, but it looks pretty nice. Really all the stuff on the bike looks really nice (expect for the 4T action).

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