Modern Buddy

Can someone please confirm which way to turn the air/fuel mix screw to make a more fuel rich mixture on a stock 2009 Stella 2T?

After rebuilding the Carb and and replacing gaskets under the carb and carb box, everything is running great but looking at My spark plug, it appears I'm running on the lean side. For my own piece of mind I would like to adjust the mix screw so It is OBVIOUS I'm running rich and then adjust from there.

I have done a bunch of searching and reading but it seems like there are a number of opinions or possibly different years had different carbs that worked differently. Most people say to start at 1.5 turns out and adjust from there. However, I think I have the screw with finer threads so the 1.5 turns is not accurate.

When I got the scooter used, the screw was turned out three full turns. This is where I had it this weekend, but after driving about 40 miles with a new plug it seemed a bit on the lean side. Maybe I just needed to ride more miles before I checked?

If I knew for sure which way to turn the screw to make a more fuel Rich mixture, then I would feel more confident in my ability to safely Adjust and recheck. Thanks for your time and any advice!

JHunt,

You are correct on the turn count versus thread pitch. On the coarse threads, you start tuning from 1.5 turns out of fully home. On the finer threaded ones, you can start 3 to 3.5 turns out of fully home.

Before spending too much time fiddling with the mixture screw, can you speak to the configuration of the bike?

Stock air filter, de-blued stock air filter, drilled and de-blued stock air filter or P200 air filter?

Jetting configuration: What are the numbers for the installed jets?

- Main jet? Stock is marked 94.
- Mixer (tube with the holes in the side)? Stock is marked E3.
- Air Corrector? Stock is marked 140.
- Low speed jet? Stock is marked 40/130.

Exhaust? Is it the stock pipe with a catalyzer, or an aftermarket such as a Sito Plus or SIP Road? A more aggressive expansion pipe ?

It seems like a lot of information to ask for, but these are all factors in tuning the bike for proper mixture. Even knowing you're near Atlanta is relevant, as altitude / air density are factors for tuning. The closer you are to sea level, the richer your mixture needs to be.

Additionally, how are you doing your spark plug readings? Plug readings should be taken via "plug chops". This involves warming up the engine, then accelerating under load. I've used third-gear roll-ons to load the motor. Once the motor is loaded under throttle, hit the kill switch and pull in the clutch. The engine must not be allowed to return to idle, as this will change the plug color and not accurately reflect the mixture condition of the loaded engine. Coast to the side of the road and pull your plug for examination.

az_slynch wrote:JHunt,

You are correct on the turn count versus thread pitch. On the coarse threads, you start tuning from 1.5 turns out of fully home. On the finer threaded ones, you can start 3 to 3.5 turns out of fully home.

Before spending too much time fiddling with the mixture screw, can you speak to the configuration of the bike?

Stock air filter, de-blued stock air filter, drilled and de-blued stock air filter or P200 air filter?

Jetting configuration: What are the numbers for the installed jets?

- Main jet? Stock is marked 94.
- Mixer (tube with the holes in the side)? Stock is marked E3.
- Air Corrector? Stock is marked 140.
- Low speed jet? Stock is marked 40/130.

Exhaust? Is it the stock pipe with a catalyzer, or an aftermarket such as a Sito Plus or SIP Road? A more aggressive expansion pipe ?

It seems like a lot of information to ask for, but these are all factors in tuning the bike for proper mixture. Even knowing you're near Atlanta is relevant, as altitude / air density are factors for tuning. The closer you are to sea level, the richer your mixture needs to be.

Additionally, how are you doing your spark plug readings? Plug readings should be taken via "plug chops". This involves warming up the engine, then accelerating under load. I've used third-gear roll-ons to load the motor. Once the motor is loaded under throttle, hit the kill switch and pull in the clutch. The engine must not be allowed to return to idle, as this will change the plug color and not accurately reflect the mixture condition of the loaded engine. Coast to the side of the road and pull your plug for examination.

Thanks so much for your detailed response. I will check the jets tonight to see what they are marked, but to my knowledge the bike is totally stock (air filter, jets, exhaust).

My spark plug readings are far from scientific. I am simply pulling the spark plug when the bike has cooled down after using it to run some errands over the weekend (20-40 miles of riding).

**So based on what you are saying, am I correct in stating that turning the mixture screw out (counter clockwise) will richen the mixture with fuel making it less lean? In other words it is better to error on the side of more turns out to avoid running it too lean/hot?

Thanks!

JHunt012 wrote:Thanks so much for your detailed response. I will check the jets tonight to see what they are marked, but to my knowledge the bike is totally stock (air filter, jets, exhaust).

My spark plug readings are far from scientific. I am simply pulling the spark plug when the bike has cooled down after using it to run some errands over the weekend (20-40 miles of riding).

**So based on what you are saying, am I correct in stating that turning the mixture screw out (counter clockwise) will richen the mixture with fuel making it less lean? In other words it is better to error on the side of more turns out to avoid running it too lean/hot?

Thanks!

Yes, turning it out will richen it. A word of caution, though: You shouldn't ever make significant changes in the mixture screw position. When you adjust it, never turn it more that one-quarter turn at a time in either direction when adjusting the mix ( I would even recommend changes of no more than one-eighth of a turn). If everything else is functioning properly, it will have a significant effect on fuel added to the mix across almost all of the engine's rev range. The Dell'Orto/Spaco SI-series carburetors are a bit involved in how they mix the incoming charge than most other small motorcycle carburetors; The settings of air corrector, mixer, throttle slide (particularly the cutout in the bottom of the slide) low-speed jet and main jet all are as relevant to the fuel mixture at a given engine speed as the mixture screw is. When you do adjust it, be sure to follow it plug chops at mid-throttle and "loaded" throttle so you can get a better idea of how the change has affected the mixture across the rev range.