Tuning Dellorto 17.5mm?

Hello,

I am now somewhat familiar with mopeds and how they work, and I'm thinking about upgrading to 70cc, an aftermarket exhaust and an unlimited CDI. For that I naturally would have to adjust the carburetor, but I have a lot of misinformated friends about the carburetor tuning and I can't find any answers to my questions. Everyone of them tells a different story and sadly no one answers my question directly.
I own a Derbi Senda 2009 Anniversary Edition, and it comes with the Dellorto 17.5mm carburetor. As I get it, from all the open source information, from online the carburetor gets controlled by:

• Closed-1/4 throttle: idle jet
• 1/4-3/4 throttle: needle pin (Don't know how to pronounce it correctly)
• 3/4-4/4 throttle: main jet

Until now I thought by lowering the needle pin, you would increase the main and idle jet's fuel amount overall, but seems I have been wrong.
As I understand now, after upgrading to 70cc and aftermarket exhaust, I will have to buy a new idle jet (if does not idle with the current one
), lower the needle pin by 1 position and install a new main jet?

I am very cautious about not running the bike lean, since I am very new to this, but I would also like to learn from your experiences. Thank you.

 

I've got the opposite problem. My bikes running too rich!

 

This is the best guide I have found: http://www.shiny-red.net/guides/how-to-jet-your-carb/

You're very unlikely to have to change the idle jet, should be able to tune the idle mix just fine using the mix screw

 

Ok, here we go.
Tuning the carb is called jetting or re-jetting. There is a comprehensive guide on here in the tuning section.
But I'll try to condense it for you.
It's all about getting a perfect balance between fuel and air delivery for the power produced by the engine.
For example, changing the exhaust alone will require a carb re-jet. The gases escape quicker through the exhaust so the engine needs more fuel input to compensate and maintain the back pressure. Follow so far?

Fit the 70 kit and new exhaust and cdi, without jetting the carb it will bog and not run right.
Remove the float bowl and note the number on the main jet. You will need to buy one or two jet packs, sets of 10 jets, to give you enough selection to get it just right.
For example, if you have a 56 size jet as standard, you should buy one jet pack of sizes 50 to 75 and one jet pack of 75 to 100.
Now it gets tedious. You are going to have the float bowl off multiple times during the next process.
Start with the biggest jet size in the packs and start to work down in size until the engine picks up nice and crisp with good power delivery through the entire rev range.
When you think you've found the right jet size, go down one or two more to prove it, then go one size higher than the best one to keep the engine on the slightly rich side.
Adjust the idle speed to your liking, take it for a spin.
After 3 or 4 miles, check the colour of the spark plug. Too white looking and it's running lean, too dark or black and its running rich.
The ideal colour is a nice dark ginger colour.

 

Hmm, okay, guess I'll find out. Huge thanks.

 

Beautiful! Couldn't find a better explanation even if I wanted to! I do get the jetting part now, but does that mean I should leave the needle as it is? Won't it run too lean?

 

The slide needle is only part of the process.
Jet it first, then if you find that the idle screw is all the way in to keep it running at tickover speed, raise the slide needle by moving the circlip down to a lower slot. This raises the needle up from the hole in the main jet allowing a little more fuel to be sucked through at idle speed.
Regular plug checks for colour is essential after jetting to make sure that it is running slightly on the rich side.

 

Thank you! Finally now understood. Now just got to wait for the parts.