Post by Scoutpilot on Sept 9, 2016 6:04:56 GMT -5
A high idle which can only be maintained by closing the choke even partially and can’t be controlled with the Brass Idle Mixture Screw needs to be investigated. You’ll have to perform a vacuum leak test. Set up your vacuum gauge and Dwell/Tachometer. While the possibility of fire is remote using this method, keep a Fire Extinguisher handy. With the motor warmed up and idling as low as you can maintain it hands-off, use either an unlit propane torch or starting fluid (ether). You are looking for changes in RPM and motor sounds. Watching the gauges and listening carefully, play the propane, or spray the ether in short bursts, all along the manifold gasket, top and bottom. You’re looking for breaks in the gasket and possible cracks in the intake manifold. Next move to the base of the carburetor, apply the gas or ether all around it, paying particular attention to the throttle shaft and where it enters and exits the body, as this is the most likely location for a leak. Keep moving up and around the carb until you reach the base of the choke housing. Any spot you find changes in RPM or sound is a possible vacuum leak. If a leak is found around screws, try tightening them, but don’t strip the threads in the carb body. A leak found at the base gasket, which should be a correct diffuser gasket, necessitates further tightening of the mounting nuts up to 30-35 Lb Ft of torque. Leaks at the throttle shaft require either bushings or a new throttle shaft, and most likely both. The shaft and bores wear over time into an ‘egg’ shape. As the shaft rotates a gap opens and allows too much air in, necessitating an increase in fuel flow via the Idle Mixture Screw. Eventually Choke will be needed to maintain the idle. Un-corrected over time, the carb will eventually cease to function correctly up to, and including, failure of the motor to start.
Here is a quick reference chart.
Here is a quick reference chart.