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Discussion Starter #1
Picked this up last week -



Needs a bit of clean-up, but overall it's in pretty good shape.

It's a 2000 Derbi GPR 50. Water-cooled, reed valve, 6-speed, 50cc 2-stroke. It has an Arrow chamber and a Mikuni Flatslide carb on it. Probably 200 pounds wet.

Pulls pretty strong, once you get past about 7500, all the way to 11,000.

Planning to powder coat the wheels to match the blue on the graphics, remove a few things, and get a new set of tires ASAP.
 

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Discussion Starter #3
Fun! watched vids of them after seeing this.

17" wheels?
This is a 2000 model, so it has 16s. But the older 50 engines are better than the newer ones. Piaggio made some changes in 2007 and messed it up. I think they did go to 17s in something like 2003. I would have preferred 17s.

This is only the 2nd one I've ever seen anywhere (and I've gone to a lot of cycle shows and meets over the years), so I snapped it up. It was only about 40 miles from here, which was a plus.

It sounds and pulls like a stock 125 for the most part once it's rolling, but it's not fast (of course). It is super light, which makes it a lot of fun!
 

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Discussion Starter #4
Cleaning it up -


Plan to pull the carb and check the jetting. Probably replace the reeds, change the trans fluid, brake fluid, and coolant while it's apart.
 

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Discussion Starter #6
Made a couple mods to eliminate passenger accommodations (like it could ever carry a passenger...) -

Milled the mount for the rear footpeg off of the bracket holding the silencer -



and cut off the passenger grab bars -

 

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Had the 1999 model when i was 16. Great little bike. Spend a few quid & bang in the 80cc kit, new reeds & carb. You won't be disappointed.

However im not familiar with the new engines. I used to work & rebuild AM6 engines on a weekly basis. Loads of " secret " things you can do to that engine :)

One thing i would recommend Is to bin the oil pump & go full premix. These things had a problem with failing or the adjuster getting loose over time. Good luck with the build. Can't wait to see more pics :)
 

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Discussion Starter #8
Had the 1999 model when i was 16. Great little bike. Spend a few quid & bang in the 80cc kit, new reeds & carb. You won't be disappointed.

However im not familiar with the new engines. I used to work & rebuild AM6 engines on a weekly basis. Loads of " secret " things you can do to that engine :)

One thing i would recommend Is to bin the oil pump & go full premix. These things had a problem with failing or the adjuster getting loose over time. Good luck with the build. Can't wait to see more pics :)
The big bore kits are few and far between at this point from what I'm hearing.

Is the engine designation EBE050 on the 2000 model?

It has been running premix since almost new - so that's good.

The tiny 24mm Mikuni Flatslide is legit -


the offset intake manifold, not so much -



I plan to check with an engine builder I know that does a lot of porting to get his ideas on how it should be opened up. The round opening is only 19mm and the backside contour looks like a mess.
 

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Discussion Starter #10 (Edited)
Replaced the chain with a new gold RK -



The chain is super long, but it does have top and bottom rollers. It surprises me sometimes the small details that they put into a cycle this size. I wouldn't expect it to have roller guides. They don't even make a chain that long in gold, so I had to buy 2 and pin them together to make one long one.

If you look closely at the photo you will notice one of the sprocket bolts isn't fully tightened down. Actually that the second one that's not fully seated. I will most likely replace the bolts with buttonheads and locktite them, but I have no idea why they wouldn't be tightened correctly before bending over the locking tab...
 

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Discussion Starter #13
Changed-out the standard bolts for some button heads with internal star washers instead of the crimp plates for a cleaner look -



Also found out that it's a factory 75 and not a 50. That means it has a longer stroke. I was pretty sure from riding it (and how it sounds) that it wasn't a 50.

In the process of modifying the intake manifold to get a smoother transition from the carb to the manifold and into the reed cage.
 

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Discussion Starter #14 (Edited)
This is the "before" shot -


Got it all back together after making a few changes -





I was planning to take it for a spin, but it started raining before I could. I've changed the manifold quite a bit, and richened the jetting some, so I'm anxious to see how it runs now. Still needs new tires, so I won't be hittin' the corners very hard.

Still need to finish the aluminum cover to replace the rear seat (removed all passenger stuff) and plan to powder coat the wheels blue to match the graphics later this season.
 

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Looking GREAT mate! Man i miss the smell of two stroke in the morning. :)
 

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Discussion Starter #16 (Edited)
Thanks.

I was running Yamalube when I got it, and it didn't smell that great. Changed to Redline ester race oil now and it smells much better! It was set-up as premix when I got it, which is a good idea.

Put a few miles on it yesterday, and it ran OK, but the jetting isn't quite right yet. I need to order a 1 size smaller main and make an adjustment to the needle.

I think the mods we made to the intake manifold helped peak power, as it feels like it's pulling stronger on top now.

EDIT: Got a new float needle and seat, main jet, and a needle jet with a fatter taper (the other was full-lean and still too rich). Also will drop the float level 2mm as suggested by the Mikuni tech. The carb sits at a 30 degree angle into the reed block, which is really too much. 12 to 15 degrees is the recommended max. Not much I can do about it.
 

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Discussion Starter #17 (Edited)
Though U.S. riders may not be familiar with the Derbi name, they might recognize one of its factory riders -



I believe this photo is from Derbi's 100th GP victory in 2010 (Marc Marquez aboard).
 

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Discussion Starter #18 (Edited)
Put about 75 mi on the Derbi this weekend. Finally running good. I had to purchase a new jet needle, needle jet, and main to get it right. Still plan to go leaner on the pilot, as the adjustment is almost max lean. Overall the carb was very rich, and now after leaning it out it will pull clean from idle to redline. Much easier to ride, and not really that slow if you keep it in the 8000 and over range - which isn't that difficult.

Here are a few photos from this weekend's cycle get together in Pine Bluff, WI -



My son rode the Derbi, I rode my buddy's '65 Bultaco Metralla, and he rode his new KTM RC390. I rode the RC390 on the way back.





Almost everything and anything shows-up.

Slimey Crud Run | Pine Bluff to Leland and back!Slimey Crud Run | Pine Bluff to Leland and back!
 

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Discussion Starter #20
Put another 70 mi or so on the Derbi this weekend on the backroads of WI.



It's running strong and clean. Looking into changing the gearing, as it's spinning about 9200 rpms @ 60mph. Redline is 11,000. Top speed is rpm limited for the most part, but it could easily cruise at a lower rpm. With the mods it has it is making enough power that the super-low gearing isn't necessary.

Trying to figure out the range. It holds 2 gal, and because it's running premix stopping for gas is a problem. I need to top it off with premix and figure out what the range is so we can relax a bit when out on the road.

Talked to a guy at Road America the other day that was riding a stock Aprilia RS50, and he says he is pegged at 55 mph.

My son has been riding it most of the time. He's 19 and about 130#, which helps. Everything makes a difference, and just tucking down behind the windscreen gives a noticeable rise in RPMs when cruising.
 

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