Re: Use of spark/gas rods & gas adj valve

Posted by Mike Flanagan on June 09, 1997 at 10:51:13

In Reply to: Use of spark/gas rods & gas adj valve
posted by Fred Neely on June 08, 1997 at 19:15:08

: I've read bits and pieces about use of the spark and gas rods, and the gas adjusting valve.I've read the Model "A" Instruction Book and Service Bulletins.What I want to know is what others have learned by experience.How does road, weather, or engine conditions govern your use of these driving aids?
The three items you refer to are not truly "driving aids" but rather engine controls. The functions controlled by the spark, hand throttle and GAV are controled for you in the modern car by vacuum and centrifical advance and later electronic ignition. The throttle setting is for maintaining a specific RPM setting over extended periods (cruise control). The GAV supplements the main jet in the carburetor when more or less gas is needed for the conditions extant.
When the timing is set correctly under the hood the spark control must be reset as engine RPM increases. This advances the spark. When driving I usually advance to near full as I leave a light and leave alone until I am stopped once more. When cruising on the open road I set spark lever at full advance and forget. When climbing a steep hill I usually retard the spark a little as engine speed decreases. Watch the heat and slow down and down shift if necessary. Care must be used in the use of full retard. If your timing is set several degrees retarded under the hood and you are idleing in full retard the spark can reach the plug before the piston has completed it's upward movement on compression stroke. Not good. I personally check my timing with a light after each change to make sure I am not retarded before top dead center at full retard. This is easily done with a little thought.
The GAV must be experimented with to find a methodology that works well with your car. This depends a lot on the condition of your carburetor, plugs, points, compression and all other things that contribute to the operation of the engine. All things considered I like to run the car as lean as possible without endangering the valves. I do this by running the GAV as near the closed position as possible at the time. Sometimes this is closed sometimes open 1/4 turn. My car never needs more than 1/4. I do this to minimize carbon deposits from a running rich situation. To tell if your to lean when driving if you hear a slight popping in the exhaust when shifting or when stopped rev slightly and let off quickly, popping in the exhaust means to lean. Richen by turning the GAV counter clock wise (CCW) until the popping goes away. As you climb in altitude you will have to adjust to account for run rich. Check the condition of the plugs periodically for excessive carbon (soot) buildup. This indicates run rich. Don't be surprised if the GAV settings change from day to day. The gas we are buying today is unseating a lot of crud out of the old tanks and unless yours is exceptionally clean you are probably experiencing "junk" in the carb. Clean the sediment bowl frequently and if you have a late 31 add a glass sediment bowl between the fuel shutoff and the carb.
The best way to understand the controls is to understand the system. Buy the books and read til u understand. Mr. Mollers book on carbs is excellent.
Hope this helps.
Mike Flanagan, The Model A Fool, AFlan1@Concentric.Net

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