03 650 v star classic: what is the factory

The carb mixture screw may add air or fuel, depending on the model. If the bike will idle it can be adjusted without knowing the factory setting. What you want is the highest idle that can be achieved using just the mixture screw, not the idle setting screw. Start the motor, then slowly turn the mixture screw in. If the idle speed increases, continue in that direction. If it slows and becomes rough, turn the screw back out until you reach the highest idle speed. That will the correct setting for the bike at your altitude.

Let's focus on adjusting the air/fuel mixture. This adjustment can help balance the fuel-to-air ratio for both cylinders. Here are the steps you can follow:

Idle Screw (Air/Fuel Mix):
The idle screw, also known as the pilot screw, adjusts the air/fuel mixture at idle. It affects the fuel flow from the pilot jets (idle jets) into the carburetors.
Locate the idle screw on your carburetor. It's usually a small black knob near the choke.
Turn the idle screw to achieve the desired idle speed. Listen to how the bike sounds while adjusting it.
Note: Adjusting the idle screw shouldn't significantly affect the mixture setting.

Pilot Screw (Idle Mixture):
The pilot screw (idle mixture screw) fine-tunes the air/fuel mixture from idle rpm up to about 1/8th throttle.
To access the pilot screws, you may need to remove the air box and carb side cover.
The factory default setting for the pilot screws is typically 2.5 turns out.
Carefully screw each one in until it lightly seats closed, and then count the number of turns.
You can adjust the pilot screws to make the mixture richer (more turns out) or leaner (less than 2.5 turns).

Dual Carburetor Sync:
Since your bike has dual carburetors, it's essential to ensure they are synchronized.
Use a vacuum gauge set to synchronize the carbs. Adjust the linkage screw until both throttle butterflies are in the same position for idle2.
Synchronization helps maintain consistent performance between the front and rear cylinders.

Check Rear Cylinder Idle:
If the rear cylinder isn't running correctly at idle, it could be due to a partially plugged idle jet.

Consider cleaning the carburetors if needed. While it can be a bit of work, it's essential for proper performance.
If the issue persists, consult a mechanic or consider opening up the carburetor for a thorough inspection.

Remember to take your time and make gradual adjustments. If you're unsure, consult your bike's service manual or seek professional assistance.

Adjusting the Mixture Screws - VT750DC.com

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If your bike is equipped with the original Keihen CV carb, you should not be able to adjust the idle mixture as the mixture adjustment screw was covered by an "anti-tamper" plug from the factory. If someone has drilled this out, there has probably been some other modifications made as well. The initial setting for this carb is about 3 1/4 turn off the bottom.

Good Luck
Steve

From the factory, when the engine and carb and gaskets were new, the mixture screws were set at 1 1/4 turns out. As fuel deposits form and things wear, it may be necessary to open up the screws a little. Lo side (closest to engine) effects the engine as it goes from idle up to speed. Hi side effects the engine only at full rpms.
Good luck.

This is not the original carburetor that came on the bike. The orignial was a Kehien "butterfly" carburetor. This carburetor is a later "CV" version with the large square looking top on it.

I'll tell you how to access the idle mixture screw on each model. The "butterfly" carb had it's idle mixture screw in a "tower" on top of the carb just forward of the carburetor mounting flange. The square topped "CV" carb has the idle mixture screw underneath the carb at rear of the float bowl.

They both came from the factory with an "anti-tamper" plug of about 3/16 inch diameter (5mm) pressed in over the adjuster screw so that the owner could not mess with the idle mixture.

To get access to the idle mixture adjustment screw, you must drill a small hole in the "anti-tamper" plug. Be very careful not to drill too deeply or you will damage the brass screw underneath the plug. Drill just enough to go through the plug with a small drill bit. Screw a sheet metal screw or start a tap in the hole. If you use the sheet metal screw, use a pair of pliers or something to pull the plug out. If you're using a tap, once you get the hole tapped screw a screw into the hole and use the pliers to pull the plug out.

If your bike will idle at all, start the bike up and allow the engine to warm up. If it won't idle, screw the idle mixture in until it just barely "bottoms" in the hole. Back it out 1 1/2 turns on the "butterfly" carb and about 3 turns on the "CV" carb and start the engine. Adjust the screw to obtain the highest smoothest idle. Then use the throttle stop screw to set the idle speed of the engine. Once you're finished, put a small dab of RTV sealant in the hole over the screw. If you don't do this, water will seep down by the fine threads permanently seizing the screw making it impossible to adjust the idle mixture in the future.

YOU ARE NOT BURNING ALL THE FUEL IN THE CYLINDER HTE FUEL THAT ISNT BURNT GOES IN THE EXHAUST AND WHEN YOU DEACCELERATE IT WILL BACK FIRE SO THERE IS TO MUCH FUEL GOING INTO THE CYLINDER ON CARB THERE ARE 2 SCREWS 1 FOR IDLE AND 1 FOR MIXTURE ADJUST THE MIXTURE ONE SCREW IT IN 1/4 OF TURN AND TRY IT