MG Car Repair: 1973 mg midget, fluid resistance, plastic cap

Question
Howard
I just found out that one of my carburetors does not work.  It is the one on the left side if you are looking at both carbs.   the silver cylinder inside the carb does not open and close like the other one does.
Is there a way to troubleshoot before I buy another carb?
What makes that silver cylinder move up and down ?
Thanks
David


Answer
Hi David,
That is the piston and it should slide up and down. First unscrew the plastic cap in the top of the pot (cylinder) With the engine off lift the piston with your finger and let it fall. It should fall so freely that it makes a "Clunk" noise when it hits bottom. Then open the throttle by pulling on the cable housing while you look closely at the linkage to see that both throttle shafts are turning the same.
Lift the plastic cap up and push it back down, it should lift freely and feel fluid resistance as you push it down. Now screw it back down and lift the piston again. You should feel that fluid resistance again and when you release it the piston should freely fall to the bottom.

These are the first tests to see if the problem is the carb or your engine.

Next remove the screws holding the top pot on the the carb. They are usually tight. Carefully lift the top pot, holding the inside piston in the top pot. Do NOT drop either one as you will destroy them.
Hold them up right and slid the top pot off of the piston. You will note a long soft spring in the piston. Lift the spring out and set it aside. Pour the oil that is in the piston inner tube into a container or cup. Be carefull not to bend the long needle sticking out of the bottom of the piston. Note that there are two holes in the bottom of the piston and they exit toward the engine. There is a tooth in the carb that keeps the piston aligned so that the two holes always point toward the engine. Look at the tooth, the groove in the piston and the two holes to see that they are all correct. (This is what makes the piston raise when you rev the engine.)
Air going across the bridge in the carb speeds up causing a low pressure area at the jet and the two holes. This low pressure area is transmitted up into the pot and that lifts the piston. (if it is free to move)

If all I have covered is OK than you have an engine problem and you must do a compression test.

Let me know,
Howard