|The MGA With An Attitude
SYNCHRONIZING Air Flow and ADJUSTING MIXTURE - CB-117
On Apr 17, 2010, Juan Gil Gutierrez in Barcelone, Spain, wrote:
"After resolving the leak I tried to tune the carbs but here we have another problem. I don't have a vacuometer and used a tube to "hear" the aspiration. But I can notice that the front carb aspirates much more than the rear one.
"In order to increase or decrease the air flow, we have three screws. One for the choke and one for each carb. Even if I unscrew them to the minimum, the rear carb aspirates less than the front one. If I screw the rear one a little, the revs go to 1200 easily. So to keep not 800 revs but 1000 (the minimum I can get without stalling and fall) I had to assume that the rear carb aspirates less.
"Another thing I noticed is I'm running lean. It's EXTREAMLY complicated to reach the nuts to increase the mixture. I guess I'm right when I say I should release the nut to increase it. Am I right?. Those nuts have a spring beside them and its almost impossible to reach there with your fingers, apart from the fact that it burns like hell. How can you make this Barney? After all this, I SHOW YOU MY RESPECT more than ever.
To synchronize air flow, do this. With engine off, loosen one clamping screw on one of the accordion clamps on the shaft between the carbs. This will allow the carb throttle plates to work independently. Put a finger on the throttle arm of one carb to feel the motion. Turn the idle adjusting screw with a screwdriver. Rotate screw anti-clockwise until the throttle valve is completely closed. Turn screw back and forth to find the point where the throttle arm just starts to move. Turn the screw exactly 1/2 turn clockwise to open the throttle plate some. This will likely be a little too much, but it is a good starting place. Do the same for the second carburetor. Then tighten the accordion clamp to lock the two throttle shafts together, and the throttles will be in perfect synch. Then start the engine. When adjusting idle speed, turn both idle screws the same amount in the same direction so they both stay in contact with the rest pads.
Adjusting the choke mechanism is an entirely different procedure. See page on my web site that covers that job.
The mixture adjusting nut has right hand threads. Unscrew the nut to lower the jet and enrich the fuel mixture. That will be clockwise looking at it from the top, or anti-clockwise looking up from the bottom. Opposite rotation makes it leaner. After each adjustment of the mixture nut, press upward on the jet to assure it will be seated against the nut, just in case it may want to stick in the lowered position.
In good clean condition you should be able to turn the mixture nut with your fingers, but no guarantee. You should acquire one SU wrench for that nut. It is less than $3-USD by itself, or it is included as part of the SU Tool Kit (for lots more money).
It is touchy to adjust mixture on the MGA without burning your arm. Best to start with a fairly cool engine, cool exhaust system. Start the engine, and allow it to warm up minimally until it will run without choke. Then adjust the mixture before the parts get too hot to touch. When you get close on the mixture adjustment, use the "Lift The Pin" technique detailed in the Workshop Manual for fine trimming.
The front carb mixture nut may be easy enough to reach if you stand in front of the car and reach under the carb with your right hand (palm up). To reach the rear carb mixture nut I stick my entire fore arm underneath the carburetors with my elbow almost touching the side of the radiator. This is where you can get burned if things are too hot. Short arms work best here. If you have long arms it may be a much tighter fit, tougher to get your elbow down level with your wrist.