Derbi GPR 50 Belson project

[toratora]

And I finally stumbled upon a photo.

There are others, and eventually I might find those too.

[mace]

That picture is the closest I've been to finding one so far. On one of the facebook groups, there was a guy in the US that said he might have one he could send me. Otherwise it's out of stock everywhere, and air boxes aren't coming up on ebay either. Actually that is not entirely true, there is an airbox on ebay, but the insert is missing

[mace]

Finally back.. with more questions

I fitted the 125 air intake restrictor, and rejetted the carb up to 104/105. A plug chop just before pulling in to the dyno place showed a decent burn.

We then ran it up on the dyno countless times that day, playing with the main jet size and needle position. However regardless of what we changed,
we couldn't sort the fueling out.

The problem seems to be around the 4/5k area, during which the fueling goes ultra rich. After it gets past that, we were able to get the fueling about right.
We couldn't find a sweet spot between needle position and smaller main jet to try and lean off the 4/5k area without making the top end dangerously lean too.

I'm about ready to chuck this carb in the bin, but would appreciate any thoughts on whether it's worth persevering with, or whether I should just give up and
go for the Polini CP or similar?

There's only two months before my daughter's birthday, at which point she could ride a 125 anyhow!

[toratora]

Hey Mace, what carb is on the bike now. I looked through the thread, but didn’t see it.

[mace]

PHVA 17.5, if memory serves me correctly. Stock for the bike anyhow.

[toratora]

Okay I get it now. If you are running the stock carb you wont be able to run the 125 insert. The stock carb needs more restriction on the air box. Go back to the stock insert, and you should be fine.

[mace]

I swapped the stock insert out because it was too rich with it in, but too lean with no insert. If I go back to the 50 insert I'll be back to where I was with being too rich across the board.

Would the CP carb alleviate any of my issues, or just create a new set?

[toratora]

I’ve found the CP carbs to be easier to tune. But they only have two choices of needles. Really though you should be able to get the PHVA to run properly on the bike.

I think maybe your expectations are the challenge. 4K-5K is pretty much no power on these bikes. Did you get any charts from the dyne action? It would be interesting to look at them.

[mace]

I'll need to go back to the dyno place and get the charts. I've not tried replacing the needle or slider, only changing the needle height.

Suspect changing the needle would solve my problem, but the problem is picking a needle with the right profile. They aren't cheap, and parts cannoning it would cost more than a CP carb!

[toratora]

Changing the needle is really only necessary for very fine tuning. This only happens once the rest of the bike is totally sorted. In the case of the CP currently it only has two needle choices. This would tell me that it’s not that big of a deal with the CP.

However, since your bike is stock you really shouldn’t have trouble getting it to run decently with the stock carb.

And again, 4K-5K on these bikes isn’t something for which you should be concerned. I can’t even remember the time I tried to do any thing at that RPM range.