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Derbi GPR 50 Belson project

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mace
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Re: Derbi GPR 50 Belson project

Post by mace » Sun Apr 21, 2019 2:28 pm

The 4/5k problem is basically only an issue when setting off in first gear, and needing to gain speed at a sensible rate. Not pulling wheelie acceleration, simply enough to, for instance, join an active roundabout without forcing all traffic on the roundabout to take avoidance. Once you're through the dead spot, which takes ~ 10 seconds or more on the flat, you've got the power to get out of the way.
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Re: Carbs

Post by mace » Sun Apr 21, 2019 2:31 pm

toratora wrote:
Sun Apr 21, 2019 1:39 pm
However, since your bike is stock you really shouldn’t have trouble getting it to run decently with the stock carb.
The bike isn't stock - has an arrow exhaust system on it, all restrictions appear to have been removed, and gearing has been adjusted for a top speed of 50mph approx. I changed the drive sprocket to increase the torque to the wheel (and reduce the top speed by ~ 8mph), but even with this, the dead spot is just as bad, so I may well put the gearing back to how I found it, as it's much more ridable like that without having to redline it constantly.
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Re: Derbi GPR 50 Belson project

Post by mace » Sun Apr 21, 2019 6:16 pm

Apparently I didn't actually post the carb details in this thread, which is a bit of an oversight.. Cut'n'paste from the Aprilia forum:

https://www.apriliaforum.com/forums/sho ... ost4487579

D50B0/1 engine.
Cylinder is assumed to be stock or thereabouts, 50cc.
Reeds were replaced as one was cracked, with voca racing carbon reeds, going by the website they are 0.35mm thick: https://www.pedparts.co.uk/product/3...n-reeds--derbi
Dellorto PHVA 17.5mm carb.
208HA atomiser
Needle: A25 N5/F1
Main jet: 95
Idle jet: 32
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toratora
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Video desu ka?

Post by toratora » Sun Apr 21, 2019 6:39 pm

Hey Mace, would it be possible for you to do a video of the bike doing the things about which it is having issues?

I’ve run the Arrow pipe on a number of my bikes without needing to change the jetting. Admittedly they were probably all the stock EBE075 bikes, but you can imagine that the 50s might be just as okay as well. Even with an upgraded pipe if the air box doesn't change you can usually stay with the same carb.

On my 05 when it was stock if I removed the 50 inlet the bike wouldn't run. Once I hopped it up with a kit ad a 24mm carb it wouldn't run with the 50 inlet. Ultimately I switched out to the 125 inlet, but only after switching to a 24mm carb.

The dyno runs will likely tell the story too, which is why I asked about them previously.

I only have one dyno run of the stock cylinder. This was with a 23mm CP carb, a Metrakit SP pipe, and an HPI ignition. The HPI was fucking up so you can see it was fairly pointless. Still lets focus on the initial beginning of the chart, because that’s fairly accurate.

As you can see below 8K there’s nothing. Nothing at all! No power. It doesn’t start to see any power until around 8.5K. So if you are not getting anything until around 6K well then you are doing great!

Post the charts when you can, and some video too, and maybe we can get this all figured out. :paw_prints:

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Which Carb is it?

Post by toratora » Sun Apr 21, 2019 7:39 pm

Are you sure you don’t have the 14? That’s what my bike came with initially. When looking through the carb chart from Derbi with the Euro2 engines (basically the same thing) that came with a 17.5 here’s the configuration they shipped:

Image
This is similar to what you have, but the emulsifier is a 212 instead of the 208. It’s also for a mediterranean climate (Spain).
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Re: Derbi GPR 50 Belson project

Post by mace » Sun Apr 21, 2019 7:40 pm

99.9% sure it's a PHVA 17.5: :)

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Re: Which Carb is it?

Post by mace » Sun Apr 21, 2019 7:41 pm

toratora wrote:
Sun Apr 21, 2019 7:39 pm
Are you sure you don’t have the 14? That’s what my bike came with initially. When looking through the carb chart from Derbi with the Euro2 engines (basically the same thing) that came with a 17.5 here’s the configuration they shipped:

Image
This is similar to what you have, but the emulsifier is a 212 instead of the 208. It’s also for a mediterranean climate (Spain).
Throttle valve.. Those are running 40's not the 30 that's in mine. If 40 is a bigger cutaway, that would lean out the throttle at around 1/4 slightly, which might be roughly in the right area.
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Re: Derbi GPR 50 Belson project

Post by jkv357 » Sun Apr 21, 2019 7:48 pm

Based on what you described, I would say the mid-range (needle) is too rich.

For carb tuning, I would recommend thinking in terms of throttle position instead of RPMs.

My Derbi, with a 24mm Flatslide and an Arrow pipe, fell on it's face and almost wouldn't run once it hit about 1/2 throttle. It would idle fine, leave the line fine, and run fine at WOT, but in between it stumbled and stuttered terribly. I ended up leaning the mixture significantly with a leaner Needle Jet and leaner Jet Needle. Now it pulls smoothly the entire way. So much nicer to ride!

Chances are it's not as extreme for you, but I would try leaning (lowering the needle/raising the clip) the needle setting 2 notches (if it's adjustable) and see if it's better or worse.

I would put some marks on the throttle housing with tape to tell you when you are at 1/4, 1/2, and 3/4-throttle position.

Have you adjusted the idle mixture to the optimum setting when fully hot? If not, I would get that set first.

Your gearing change shouldn't have any effect, so if you like it, don't change it.
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Re: Derbi GPR 50 Belson project

Post by mace » Tue Apr 23, 2019 3:52 am

jkv357 wrote:
Sun Apr 21, 2019 7:48 pm
Based on what you described, I would say the mid-range (needle) is too rich.

For carb tuning, I would recommend thinking in terms of throttle position instead of RPMs.

My Derbi, with a 24mm Flatslide and an Arrow pipe, fell on it's face and almost wouldn't run once it hit about 1/2 throttle. It would idle fine, leave the line fine, and run fine at WOT, but in between it stumbled and stuttered terribly. I ended up leaning the mixture significantly with a leaner Needle Jet and leaner Jet Needle. Now it pulls smoothly the entire way. So much nicer to ride!

Chances are it's not as extreme for you, but I would try leaning (lowering the needle/raising the clip) the needle setting 2 notches (if it's adjustable) and see if it's better or worse.

I would put some marks on the throttle housing with tape to tell you when you are at 1/4, 1/2, and 3/4-throttle position.

Have you adjusted the idle mixture to the optimum setting when fully hot? If not, I would get that set first.

Your gearing change shouldn't have any effect, so if you like it, don't change it.
Hi,

Does sound similar to what I'm dealing with.

We tried leaning/enriching by moving the needle position and it clearly had an effect on the dyno graphs (the mixture curve at 6k+ dropped down beautifully as the needle was dropped) but didn't solve the over-rich mixture in the dead spot. If we'd leaned it off further with a smaller main jet, it may well have helped, but the dyno chap was getting nervous about making the top end excessively lean,

The A25 needle is about as wide as they come. I wonder if a smaller main and a narrower needle would be the way to go.

Idle mixture has been tweaked and tweaked again. :)

Tape idea is a good one. I'll probably put it on the throttle so I can see what it's doing whilst moving. It's easier to see the limits of the dead spot when under load, as it's gets stuck for longer.

Agreed on the gearing, it was more to see whether a little extra torque at the wheel (thus less engine load) would be enough to work around the problem. It wasn't. Only solution (which is really bad) was to rev it up and dump the clutch.
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Re: Derbi GPR 50 Belson project

Post by jkv357 » Tue Apr 23, 2019 6:40 am

It sounds like you are on the right track, and the taper of the needle may be the issue.

On the Main Jet - if it is running well at WOT the Main is correct. I wouldn't lean it to try to get the mixture right in the mid-range - that's the Needle Jet/Jet Needle's job. Are there leaner Needle Jets (part the "Needle" drops into) available? I had to change (lean) both to get the mid-range cleaned-up.

Typically they say to get the Main Jet set correctly and work down from there.

The Main is really the most important - you want it right.
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