Tech Stuff

Moss Motors MGB Tech Tips Your Carburetor Running Rich The most common complaint is that no matter what is done, the mixture is still far too rich. There are five major areas of concern: The air cleaner is dirty. Leaks in the manifold or exhaust will soot up an air cleaner. This causes a much higher vacuum in the venturi, and more gasoline is drawn into the air stream. 1. The ELC (Evaporative Loss Control) system is pressurized. This is very uncommon. Remove the vapor line, the vent line, and the carburetor overflow line. This travels across the valve cover to the charcoal adsorption canister. There should be NO CHANGE in the idling of the engine. If there is any change, clean the ELC system, and check for blockages. It is more common for this system to develop a vacuum which creates a lean running condition. 2. The automatic choke is the most common cause of rich running. These problems are:   a. The choke ass’y. has come loose from the carb. body. The vacuum from the carburetter then draws fuel from the float bowl around the valve which should be closed during warm running. Tightening the three slotted copper coloured screws eliminates this as a problem.   b. The choke lever and cam have stuck in a part-open position. A good tune-up requires removal of the choke assembly, thorough cleaning, tightening the pin nut, and lubrication.   c. The bi-metal spring in the heat mass is not correctly calibrated. Place the heat mass in a pan of boiling water; the bi-metal spring will rotate clockwise to its fullest extent. The notch on the bi-metal spring should be in line with the mark on the aluminum housing. If it does not line up, scribe a new mark. The line on the heat mass, on the black plastic insulator, and the choke body should all be in line.   d. The small passageway from the carb throat (past the butterfly) to the top of the auto choke ass’y. allows vacuum to pull off the spring-loaded enrichment plunger. When this passageway fills with soot , the plunger remains seated, holding the choke ON. By cleaning this passageway with a small wire, the vacuum can take its proper route.   e. The mating flange of the auto choke can become warped and allow fuel to bypass the choke valve. Resurface this mating flange and thoroughly clean after the surface is smooth.   f. Occasionally the brass choke valve is not fully inserted into the housing, so that the choke pin does not seat in (and therefore not closing off) the choke. This can be positively determined by removing the automatic choke and covering the screw holes and fuel passageways with a piece of masking tape, then checking the mixture again.   g. If the rubber diaphragm is torn or perforated, then the engine will run very rich, and maximum speed will be limited, spewing clouds of black, sooty, uncombusted exhaust. A torn diaphragm is easily found by removing the top of the suction chamber (four Phillips screws). While the diaphragm is being inspected, pay attention to the following: The tiny pin which retains the metering needle can break, allowing the needle to pop upwards. Adjusting cannot correct this. The needle’s shoulder should be seen on the bottom of the air piston. If not, and the needle has a lot of up and down movement, check the pin. Running Lean Leanness at road speed is often described as a hesitation, as if a wind was blowing against the car. Causes can be retarded timing or a too lean mixture. There are several possibilities:   1. The ELC System is plugged, placing a vacuum in the float bowl. Remove the vent line from the carb. and note any change. A plugged charcoal adsorption canister, or a plugged vent line from the bottom of the anti-run-on valve are the most common problems.   2. The float height is set far too low. The air cleaner is not fitted to the carburetter. The Stromberg carburetted MGB will not run with the air cleaner removed, nor with a filter which replaces the original assembly. (Continued on page 8)