Derbi GPR 50 Belson project

[mace]

I'm starting from scratch - I don't have any spare needles, jets or emulsifiers. Will need to buy a few online.

[mace]

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?

Idle speed is set to roughly 1800, as per the Aprilia factory manual. Did wonder if that was a bit high, but it was as good a place as any to set it in the absence of better information.

I've sprayed the whole carb and manifold area with carb cleaner, and there's no change in RPMs during or after, so I'm fairly happy there's not much leaking air?

PHVA has air adjustment for mixture. I've tweaked it previously to the point it's at, but will have another go as you suggest, ta.

[toratora]

Setting fuel mixture

HFFS

I've also added a thread on leak testing.

[mace]

Cheers toratora. That's pretty much the procedure I followed. Will redo it though to see if I can get it any closer.

[mace]

I took the bike to my usual bike shop who have a dyno (yes, I know), and we ran it up on there.

We quickly realised that there was another restriction in the air intake, which was causing the dead spot in the middle of the rev range.

The fueling was all over the place, both lean and rich either side of the dead spot, but actually bang on in the dead spot.

Any thoughts on what that means?

We then removed the restriction, and the bike pulled much better through that spot, but is now obviously running lean across the board.

They had a 122 main jet lying around, so we tried that but it was immediately obvious that it was too big, bike was clearly over-rich with that.

Should I stick with the restriction, and try to tune the carb for that? Or leave it out and tune it with a free flow of air?

If the latter, what should I be looking at changing? I'm guessing I'm going to need a pile of bits to throw at this thing, to try and dial it in.

[jkv357]

What was the air restriction you removed?

Depending on what it is, I would most likely tune without the restriction as long as you could get the engine to run well in the low to mid range. If it was a mess down low, but pulled harder on top, you would need to decide if it was worth it. That makes it a real pain to ride in traffic.

Generally you want to get the Main Jet set correctly and work you way down from there - as it changes almost everything when you change the Main.

[mace]

The parts diagrams describe it as a diaphram. it's a plastic plug that goes in the side of the air box, with two holes in it. Must restrict the air intake down to a third or so of the unblocked hole.

I'd kinda prefer to have it in there, as it's quite a bit noisier without, but I can't really argue with the fact it runs better when it's removed.

[mace]

I've ordered two kits of jets, 95 up to 104, and 105 up to 114.

[jkv357]

The parts diagrams describe it as a diaphram. it's a plastic plug that goes in the side of the air box, with two holes in it. Must restrict the air intake down to a third or so of the unblocked hole.

I'd kinda prefer to have it in there, as it's quite a bit noisier without, but I can't really argue with the fact it runs better when it's removed.

Because you are using the airbox it will still be a lot quieter unrestricted than if you were running a pod filter.

Open 'er up! (my opinion...)

[mace]

Yeah, I'm going to go opened up for now

Unrelated, I have fuel leaking from the petcock area, suspect it is the black plastic fitting in the tank rather than the petcock which is new - can you get replacement O-rings or the whole fitting? Parts diagrams don't show the fitting separately from the tank