Derbi GPR 50 Belson project

[toratora]

Looks pretty good. Some of the mid throttle seems a bit rich, so if your needle could go lower (the clip go higher), it might help your throttle response. It's also good to show in the video the bike sitting at idle, and show it sitting and rev the throttle, and let it close. This can show a number of things.

[jkv357]

I agree on the low and mid-throttle sounding rich.

When I got my GPR it sounded similar, and would barely run until it was at WOT and cleaned out.

I leaned both the Needle Jet and Jet Needle significantly (24mm Mikuni Flatslide) in order to get it to run cleanly. Now it pulls clean from idle all the way.

The mixture at WOT (Main Jet) and full revs sounds pretty good to me.

[mace]

Looks pretty good. Some of the mid throttle seems a bit rich, so if your needle could go lower (the clip go higher), it might help your throttle response. It's also good to show in the video the bike sitting at idle, and show it sitting and rev the throttle, and let it close. This can show a number of things.

Cheers.

The needle was in N4, N5 didn't feel much different but I'll retry that tomorrow.

How much should I rev the throttle? I'll do another video tomorrow of that too.

[mace]

I agree on the low and mid-throttle sounding rich.

When I got my GPR it sounded similar, and would barely run until it was at WOT and cleaned out.

I leaned both the Needle Jet and Jet Needle significantly (24mm Mikuni Flatslide) in order to get it to run cleanly. Now it pulls clean from idle all the way.

The mixture at WOT (Main Jet) and full revs sounds pretty good to me.

From the Aprilia forum, current setup:

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

32 feels fairly small already, what do you think?

[toratora]

Maybe something like this:
https://youtu.be/r7VKLd084Ls

[jkv357]

From the Aprilia forum, current setup:

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

32 feels fairly small already, what do you think?

The Idle/Pilot Jet is probably fine, and is fine tuned with the idle mixture screw. Basic adjustment is turning the screw to get the highest RPMs at idle when warm. I like to go slightly richer (1/8 turn or so) to smooth out the transition as you open the throttle.

The issue is more with the Needle Jet and Jet Needle (they work together but are separate). That's what controls the mixture as you start to open the throttle and get into 1/4, 1/2, and the 3/4 open range - that's where it is currently way too rich. Adjusting the clip on the needle to drop it lower into the Main Jet (reducing fuel flow) will help, but sometimes (like with my situation) that wasn't enough and I needed different sized Jets.

[mace]

I rechecked the needle position, I'd already moved it back to N5 for that previous video, so it's as lean as it's going to get.

Didn't see your respective comments until a moment ago, and made the videos earlier, so apologies if they're unhelpful.

Revving: https://youtu.be/xjXHNsHUFmo Another test ride https://youtu.be/lnPtag0ZszI

[mace]

I did find the factory needle size somewhere, but I've lost it at the moment. It might have been A15, it's quite similar to the A25 needle that's in it just now, but with a wider tip.

[toratora]

A slightly wider tip would help with where you are at as it would lean it out some where the bike is too rich. The other thing you can do is change the pilot jet, but if you have another needle to try that sounds like a better procedure to try first.

[jkv357]

What is the idle speed set at? Seems like it is pretty high, but I can't tell for sure.

Have you checked for vacuum leaks, it also looks like the revs don't drop quick enough - which usually suggests a leak somewhere.

If you can get it to idle around 1500 when warm, turn the idle mixture screw until you get the highest RPM, then go 1/8 richer (not sure if the carb you have has an air-bleed or fuel-bleed adjustment for idle mixture). The reset the idle speed to 1500 (or whatever is the recommended speed).

What do you think, toratora?