Derbi GPR 50 Belson project

[toratora]

From looking at the photos I would really just go for a new carb. The CPs are affordable. As for size it really depends on what you plans are for the future. If you are going to just stay with the stock cylinder then yeah a 17.5 is probably the way to go. If you are going to replace the cylinder then it would depend on which one you plan to get, and that becomes a bigger conversation!

The carb comes with a pull choke, but you can get a cable adapter. You might be able to find the 17.5 with a cable style. I was able to get a 21, and a 23mm with the cable style choke, but I don't see the 17.5 on Treats with a cable style. You could ask Treats if they could get one for you.

Another thing I like to get for the CP carbs is the float bowl with a main jet access port. Makes it really easy to change main jets.

The CP and the PWK share jets so I usually get a ten pack of jets. The 17.5 comes with an #80 jet so depending on if it's rich or lean you could go with the 60-78 pack, or the 80 to 98 pack. You can get individual jets, but make sure they are high quality jets. I've gotten jets in the past that made the bike run crappy. When you figure your entire experience flows through this one little part it's worth getting a good one.

If the Treats prices seem a bit high you could wait for one of their sales.

[mace]

Cheers for the info.

I had another go at cleaning the carb yesterday, and discovered that there was more crap in behind the idle jet and the choke jet and passageway next to it were both, ironically, quite choked too. Gave them both a good clean out and reassembled. It's still not running perfectly, but it will now run without as much choke, well enough to try and get it through an MOT anyway, which will allow me to ride it on the road so I can clear it out with a bit of load on it, and do any tuning that's needed - regardless of carb.

How is the CP carb for economy compared to the original?

[toratora]

Apologies for being so late with this response—I would open this thread in a tab, and then get caught up with other stuff.

I don't really track fuel milage with my little strokers. My main concern is can the bike get to the next fuel stop, and as long as they can I don't really worry about it. That said bikes with 17.5mm carbs can go a lot further than ones with 21, or 24mm. On our rides the bikes with smaller carbs end up buying less fuel than those with the bigger carbs (usually). I noticed a huge improvement on the Mito when I went from a 34mm to a 28mm, but I also went from a Mikuni to a Keihin.

So for me performance, and tune-ability are the primary features I look for in a carb. The Dels are great carbs, and I got a lot of fun rides out of them over the years. I like the CP a little bit better, so the $100 or so spent on them feels like good money spent to me. The throttle response seems a little better with the CP, and tuning seems a little easier with the CP. Again this is totally subjective. I've not done proper testing at all between the two carbs, and my skill level has significantly increased from when I was tuning with Dels to now with the Polini carbs.

The first ride I went on when I switched to a 21 Del on my first GPR went up into the mountains around here. I had a really difficult time with the bike stalling out on me. This was a simple enough adjustment, but at the time enough of one that it took me a while to sort out! So it feels like to me that the Del was harder, but again I've gotten much better at this stuff than I use to be, so that could be the real reason, and not the changing of the carb—but I would still buy a CP over a PHBG at this moment in time.

[mace]

No worries mate, it's all good.

I'll paste my last engine post here from the Aprilia forums, see whether you've any thoughts. It's feeling like there might be light at the end of this particular tunnel now, I'm fairly happy the engine is good enough and sufficiently air tight, so perhaps finally it's just down to carb tuning, fingers crossed. I would like to retain the Dellorto carb if possible, as I'm trying to keep it original.

For context, the carb/manifold gaiter was slightly split, so I ordered up a new one and.....

So good call on the air leak from the carb gaiter, new one is on and after a bit of fettling, it runs again!

Tweaked the idle and mixture settings as per the manual, but it still sounded like it was four stroking a bit.

Test rode it again, and pretty rubbish, disappointing acceleration for the first 5 minutes, until by chance it managed to make it past 5k5 rpm, then it took off like a scolded cat. Keeping the revs high it felt like I imagined it would, but the lower rpm stuff clearly indicates something isn't set up right.

I've done a fairly decent amount of reading up on Dellorto carb tuning, but it doesn't seem to explain what I'm seeing here. Set off in first, get it moving and work up to maybe third. WOT it, and it slowly accelerates, until the revs hit this magic 5.5k, then it flies up to near the redline. What might cause that "low" rpm sluggishness?

[toratora]

Basically these bikes have no power down low—this is just the characteristic of the two stroke engine. That you are seeing something at 5.5K RPM is pretty amazing. The PMPR bike doesn't get on the pipe until 10K! I kid you not. Then boom all of a sudden there's a burst of 20+hp, and it's time to shift again!

I have some posts on leak testing, but I've yet to do a proper tutorial on doing a leak test.

[mace]

I suspected that might be the case, but from looking at a handful of youtube videos, I'm pretty sure I've got an issue - it's a real fight to get it up through this barrier. But I don't really know what's wrong with the fueling - if I'm WOT from, say 3k, and hold it there, it'll very, very, slowly gain speed, up to the 5.5k mark then suddenly pick up like the fueling has corrected itself. If I'm in the wrong gear, it'll never reach 5.5k and will struggle along at ~ 20mph at best.

I know I'm used to a 650 (4 stroke obviously), but it doesn't feel right - useless diagnosis I know! I have a 40 year old single cylinder 2 stroke engine in my boat too, so I'm kinda used to the initial low RPM sluggishness there, but even that isn't as bad as this.

[mace]

Have got some stuff on order to do a DIY leak test, if it comes to that. Whilst looking at that post, I noticed viewtopic.php?f=15&t=49&sid=83073216eaa ... t=60#p1345 where you talk about similar symptoms, and dropping the needle two.

I've currently got the needle set to N4, it was originally at N5 from the previous owner. There was chatter on the aprilia forum at one point about whether the needle on these carbs clears the diffuser or not. If not perhaps I should try N5 again?

I suspect that means that my main jet is too big?

[toratora]

There are a number of factors here that can change things. I don't think they are necessarily the issue, but are good to know. The needles come in different sizes and shapes to allow for different tunings. I don't think you should start here though because your carb is most likely compromised. There are many small passages in a carburetor, and if these are even slightly clogged up it can make the bike run poorly. This bike was in a very bad state when you got it so running poorly isn't very surprising. Unless your budget is totally strained, I would just replace the carb.

There are things your can do to clean the carb like get those buckets of super nasty carb cleaner, and let the metal bits sit in it. There are also sonic cleaners. And pipe cleaners (although these can leave things behind). And you can replace all the jets, emulsifiers, etc. to get the carb clean. But this can take a lot of time, and iteration, and again this bike started out in a very poor state.

It is also possible that there isn't a problem. These bikes are not four strokes, and they behave different. If that is your reference it could be distorting your reality. There's basically no power until you get to around 7Krpm. That's totally normal. I don't think I even let the clutch out on my bikes with the R's at 3K. If you are doing this it would explain why you are having problems. I would suggest finding someone in your area with a similar bike, and compare the two. You may find that at this point it is just learning the behavior of the bike.

[mace]

I know of someone with the facelift version of the GPR 50, might give him a shout and see if we can compare notes.

When I get a chance I'll gopro it and post the vid up, so you have something a bit more tangible to go on.

As mentioned I have changed the needle position, so it wouldn't hurt to revert that back to N5, as there's a reasonable chance the last time it ran, the carb was fairly close to being set up correctly.

Just need to find the time to get near it

[mace]

Needle moved back to N5. Took it for a quick spin.

https://youtu.be/gPUwztnEQrY