2009 Derbi GPR50

[Reefyboy-15]

Place holder post for Reefyboy's GPR.

[Reefyboy-15]

2009 Derbi GPR50 de restricted Ginnelli Exhaust. What would be the best road tyre wet and dry (it rains a lot in the UK) for this bike with a inexperienced young rider. Cost is not an option. Building it for my Grandson.

[Reefyboy-15]

Hi has anybody removed the spring off of a standard GPR50 Olle shock. If so how did you do it ? Any advice please.

[jkv357]

Typically you need to compress the spring in some way and slide out or unscrew a retaining collar.

[Reefyboy-15]

Thanks, My son and I have tried that to no avail. I contacted Olle and they say there is a C clip so have another go. Thanks for your help.

[Reefyboy-15]

Hi Toratora
I am Paul, Reefyboys Grandad based in Christchurch UK.
Sorry that I haven't picked up your thread earlier but thanks for creating a Project page for me i was not sure how to do this.
I started rebuilding a Derbi GPR50 from a crashed bike that had been dismantled for my Grandson Reef 16th Birthday in April 20 To give you and idea i carried out the following. Cleaned parts, assembled, tested, stripped bike back to parts, stripped and rebuilt engine with new barrel, pistons bearings and seals and Giannelli exhaust. rebuilt front forks with new seals, wheels new bearings, new brakes new levers etc etc then sprayed and decals, looks fabulous. A lot of the answers to my questions i found on this brilliant forum.

However i am having awful trouble with the carburettor since i fitted the new exhaust. Stock Dellorto PHVA 17.5. main jet 80 Idle jet 34 power jet 50 Atomiser 211A
Slide 40, Needle A7 clip on 2nd groove. Spark plugs NGK BR9EIX Iridium / B9ES. 2T oil Fully Synthetic fed in to carb at standard settings. Stock reeds ?The Giannelli exhaust came with a restriction washer welded across the entry point from the exhaust port which i removed. power band kicks in at 6000 rpm. the problem is that it is running to rich from about 3000 till 6000 rpm and has a problem clearing. With the throttle wide open its then pumps to much oil in and is hard to start once stopped which it has not been doing. It idles at 1500 well with the air screw out at 5 +- turns which is saying its too rich. Dropped the needle down and the problem moves up the rev range. Float is fixed height but within tolerance. I don't want to go to lean on the Main jet which i have a full range of from 60 to 150.
I have had lots of 2 strokes in my time including road, racing, Motocross, enduro, but this little baby has got me a bit stumped !! any help you could give me would be much appreciated.
Kind regards Paul

[Reefyboy-15]

Also How do up load photos please??
Paul

[Jaxx]

Here's nice tutorial how to upload photos: https://www.gprcamp.com/foro/viewtopic.php?f=7&t=8

[Jaxx]

About that barrel, is it original size (50cc) or big bore? I assume it's 50cc... From my experiance sharp needles like A2/A7/A8 and PHVA 17.5 don't work well with aftermarket exhausts. Racing exhaust will always have flat zone (richness) before powerband but this can be corrected to some degree with proper needle selection. I've found the best needle to be A20 with main jet size somewhere between 96 and 100. Note that main jet size depends on exhaust shape AND needle shape AND atomiser size, some will say that with this exhaust you should have main jet that size, that's BS if you don't know which needle and atomiser you have... For example, I have found that with A7 needle (1.0mm tip) you are in MJ range around 84 to 90, for A25 (1.4mm tip) 90 to 96, for A20 (1.6mm tip) 96 to 102 and for A13 needle (1.8mm tip) 102 to 108...

[Reefyboy-15]

Hi Jaxx
Thanks for the reply, I have looked at the post re photos and that makes a lot of sence. I am sorting the photos out into order and will upload.
Re carb. 50cc standard bore and piston. 17.5mm Carb with 65 main jet as standared. When I installed the exhaust I upped the main jet to a 100, this was far to large and I have progressivly reduced the main jet to a 72 now. The engine is now running cleaner in lower revs 1500 plus and drops in to the power band at 6k. However I can't test to see if the engine revs cleanly through the rev range as I need to take it on a open road which I can't do yet as it has not got a MOT, Tax or Insurance. Once thats completed this Thursday I can give it a test and see how it well it runs.
Thanks for your help.