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2009 Derbi GPR50

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Jaxx
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Re: 2009 Derbi GPR50

Post by Jaxx » Tue Aug 24, 2021 4:08 am

I still recommend you to to use A20 needle. With A7 you are running much too rich between 1/4 and 3/4 of throttle unless you compensate this with very small main jet, but this should be dangerously lean at 100% throttle... Did you test 72 MJ at wide open throttle? I bet it would be too lean...
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jkv357
Posts: 219
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Location: WI
Replica: '99 GPR EBE-075
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Re: 2009 Derbi GPR50

Post by jkv357 » Wed Aug 25, 2021 6:22 am

Jaxx wrote:
Tue Aug 24, 2021 4:08 am
I still recommend you to to use A20 needle. With A7 you are running much too rich between 1/4 and 3/4 of throttle unless you compensate this with very small main jet, but this should be dangerously lean at 100% throttle... Did you test 72 MJ at wide open throttle? I bet it would be too lean...
Better to be lean in the midrange than at WOT for sure.

My GPR75, with a 24mm Mikuni flatslide (too big IMO, but that's what was on it), was so rich in the midrange it wouldn't even run. It would just stumble and stutter without accelerating. I had to go leaner on the Needle Jet and the Jet Needle to get it to run right.

If it's too rich in the midrange it will burble and stumble, then clear out as it gets onto the Main Jet.

If it feels and sounds flat at WOT, it's most likely lean.
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Reefyboy-15
Posts: 13
Joined: Sun May 31, 2020 1:50 pm
Location: Christchurch Dorset United Kingdom
Replica: Derbi GPR 50 2008 model
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Re: 2009 Derbi GPR50

Post by Reefyboy-15 » Wed Jan 26, 2022 4:18 am

Place Holder Post ??
Trying to up load photos from Imgur.
https://imgur.com/boYUQDe
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jkv357
Posts: 219
Joined: Fri Dec 01, 2017 6:47 am
Location: WI
Replica: '99 GPR EBE-075
x 417
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Re: 2009 Derbi GPR50

Post by jkv357 » Wed Jan 26, 2022 6:59 pm

Reefyboy-15 wrote:
Wed Jan 26, 2022 4:18 am
Place Holder Post ??
Trying to up load photos from Imgur.
https://imgur.com/boYUQDe
Not sure what that means, but the bike looks great!

To embed a photo from Imgur -

Click on the image (pop-up opens)

Go to "BBC Code (message boards and forums)" on right toolbar and click "Copy" in the blue box.

Right click and select "paste".

The link will show in the body of the message you are composing, and the image will become visible when you click "Preview".
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Reefyboy-15
Posts: 13
Joined: Sun May 31, 2020 1:50 pm
Location: Christchurch Dorset United Kingdom
Replica: Derbi GPR 50 2008 model
x 8

Re: 2009 Derbi GPR50

Post by Reefyboy-15 » Sun Jan 30, 2022 8:52 am

ImageImage
ImageImage
Hope fully some images of the Derbi GPR 50 which i have bben working on and off for my Grandson.
Thank you for your help with the posts on your great forum.
Paul
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Reefyboy-15
Posts: 13
Joined: Sun May 31, 2020 1:50 pm
Location: Christchurch Dorset United Kingdom
Replica: Derbi GPR 50 2008 model
x 8

Re: 2009 Derbi GPR50

Post by Reefyboy-15 » Sun Jan 30, 2022 9:24 am

ImageImageImageImageImageImage
Got the hang of this now Haha
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scottydog
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Re: 2009 Derbi GPR50

Post by scottydog » Sun Feb 06, 2022 8:17 am

Looks good that!
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toratora
Posts: 3161
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Location: San Francisco
Replica: Derbi GPR
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Naisu!!!

Post by toratora » Fri Feb 25, 2022 10:39 pm

Yeah Paul it’s looking great! :paw_prints:
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al_pritchard
Posts: 28
Joined: Sun Jan 09, 2022 2:15 pm
Location: Atlantic Highlands, NJ
Replica: 2003 Derbi GPR 50cc
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Re: 2009 Derbi GPR50

Post by al_pritchard » Sat Feb 26, 2022 11:05 am

That looks very nice! :starry:
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Reefyboy-15
Posts: 13
Joined: Sun May 31, 2020 1:50 pm
Location: Christchurch Dorset United Kingdom
Replica: Derbi GPR 50 2008 model
x 8

Re: 2009 Derbi GPR50

Post by Reefyboy-15 » Tue Apr 19, 2022 12:10 pm

URGENT HELP Please!!
While I am very pleased with the outcome of my efforts to make a smart looking Derbi GPR50 the engine is very temperamental and is nightmare to start. I have spent on and off over a year trying to get it to start and run well and I am now at my wits end to fathom out a simple 50cc 2 stroke engines? When I first built the bike to see if it would run I started with an engine and boxes of parts to make the bike from. It was simple to construct and I had it built and running in a couple of days. I had to empty the engine; exhaust and carb of water, but it started well and ran ok in its restricted state.
I then stripped the bike, cleaning and making a list of parts to replace. I then set about the engine as I was concerned that it had water in it. Again a reasonable easy job, I decided to replace all the bearings and seals throughout and as the cylinder and piston were badly scored I ordered new basic pair to replace them. However after all this hard work it would not start with the standard 17.5mm PHVA or new 19mm PHBG. I had put a new Giannelli un restricted exhaust on so thought that this was the cause but it would not start and run with the standard exhaust. I then set about re jetting the 17.5 PHVA carb to 98 main 34 idle 20 needle. I did have some success with this but the engine would soon oil up and spew black oil all over the back of the bike. I thought with this amount of oil it must be coming from the clutch side of the engine through the crankcase seal. So I stripped the engine again checked it and put new crankcase seals in. I then rebuilt and pressure tested it. I found leaks at the base of the barrel and the reed block which I corrected and I carried out a compression test and noted that the readings were 90psi which I thought was on the low side. I had a spare Naraku piston which I put in; this increased the compression to 95psi but still not great. I put the Engine back into frame, it was slightly better, enough to start it after a good wind on the starter. Once hot I reset the carb for air and idle speed, I then gave it a run. All the power came in at 6.500-7000rpm and it took off like a frightened cat. However once stopped it would not start again. The plug was oiled? So after changing jets, needles, carbs, restricting the exhaust, new starter and many spark plugs I give up with a bike that I can’t start sitting in the garage! The only common denominator is the barrel and piston. The carb is the same the reed block, the timing and a restricted exhaust. Lack of compression seems to be the only other thing. What do you think??
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