TheSamba.com :: View topic

Sun Feb 28, 2010 2:59 pm

I have been here

http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm

and been able to turn my carb rather effectively. But I want to know about the mixture screw. I can't find any background on it. Does it control air flow or fuel flow? I would guess that turning it in restricts whatever it changes and out does the opposite, is that correct?

Sun Feb 28, 2010 5:28 pm

The mixture screw affects the mixture at idle and just above idle. It effectively regulates the amount of fuel entering the carburetor when the throttle valve is shut or nearly so. Turning it in leans out the idle mixture and turning it out richens it.

This is a simplified explanation, you need to study a book on carburetors if you need a more complete explanation.

Sun Feb 28, 2010 6:11 pm

Wildthings wrote: The mixture screw affects the mixture at idle and just above idle. It effectively regulates the amount of fuel entering the carburetor when the throttle valve is shut or nearly so. Turning it in leans out the idle mixture and turning it out richens it.

This is a simplified explanation, you need to study a book on carburetors if you need a more complete explanation.

This is what I wanted to know, thank you.

Sun Feb 28, 2010 6:11 pm

bottom the screw, then turn it out 1 and a quarter turn. They usually run best here. Set it and forget it.

Sun Feb 28, 2010 6:38 pm

Fuel. When the butterfly is closed properly air rushes past the edge. That air pulls fuel from a little hole. The ratio of air to fuel is set by how far that tapered needle screw is in the hole. The general procedure is with the engine warmed up, turn the screw in until it bottoms gently. Then open it 1 1/2 to 2 1/2 turns. Set the idle speed to around 700 prm and turn the screw in until the engine starts to die, screw it back out until the engine runs the smoothest. Now set the idle speed again to the factory specs. Timing must be correct too. Turn the screw back in until the RPM drops 50 RPM. That will be very close. Remove the air cleaner. The RPM should not change. Use your hands to cup over the top of the carb. If the engine speeds up as you come closer then turn the screw out a little. If the RPM drops, turn the screw in a little. The RPM should not change until your hands are almost on the carb. Put the air cleaner back on. Drive it around the block. Park it but leave it running. Check the RPM and set it to spec. Goose the throttle - Vrrrooom Vrooom. It should be very reponsive and not die when it returns to idle. If there is a little lag or hesitation try turning the screw out about 1/4 to 1/2 turn and try again.

Mon Mar 01, 2010 8:44 pm

SGKent wrote: Fuel. When the butterfly is closed properly air rushes past the edge. That air pulls fuel from a little hole. The ratio of air to fuel is set by how far that tapered needle screw is in the hole. The general procedure is with the engine warmed up, turn the screw in until it bottoms gently. Then open it 1 1/2 to 2 1/2 turns. Set the idle speed to around 700 prm and turn the screw in until the engine starts to die, screw it back out until the engine runs the smoothest. Now set the idle speed again to the factory specs. Timing must be correct too. Turn the screw back in until the RPM drops 50 RPM. That will be very close. Remove the air cleaner. The RPM should not change. Use your hands to cup over the top of the carb. If the engine speeds up as you come closer then turn the screw out a little. If the RPM drops, turn the screw in a little. The RPM should not change until your hands are almost on the carb. Put the air cleaner back on. Drive it around the block. Park it but leave it running. Check the RPM and set it to spec. Goose the throttle - Vrrrooom Vrooom. It should be very reponsive and not die when it returns to idle. If there is a little lag or hesitation try turning the screw out about 1/4 to 1/2 turn and try again.

I followed your instructions and I got to say my bus runs quite well. I recently moved from the mountains of NC to Sunrise FL so I think I need to rejet for it all to be perfect but I got it to run better. Thank for such detailed directions.