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Author Topic: John Twist on Detoxing the MGB (1975-1980)  (Read 3329 times)

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  • AMGBA Club Tech Staff
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    • University Motors
John Twist on Detoxing the MGB (1975-1980)
« on: May 06, 2018, 06:30:15 PM »
One of the most frequently encountered questions once asked by owners of late model MGBs (1975-1980) was how to remove the smog equipment.  Today most of these vehicles have been “de-toxed.”  Detoxing involves removing the array of emissions equipment with which these cars were burdened by the engineers attempting to meet government regulations.  The equipment was added during a time when the this technology was in its primitive stages and could only do its job by impeding performance and drivability.
   Why detox? It often seems difficult, if not impossible, to adjust the various components of the emission control system to allow for good acceleration, good deceleration, yet avoid a red-hot converter or the popping common during deceleration. The belt-driven air pump draws a bit of horsepower from an already detuned engine. The major improvements gained by detoxing are better acceleration, faster deceleration, and a smooth idle. 
   The instructions are divided into four steps, the first two of which must be performed at the same time. The third step, removal of the  EGR valve, is helpful to smooth the idle. The fourth step, soldering the overrun valve, requires the removal of the carburetor.
   (1) Air pump – air manifold – gulp valve: Remove the two air hoses to the air pump, then remove the adjuster bracket bolt and fixing bolt and nut with a ½” wrench and socket. Twist the smog pump back and forth and remove it from the thermostat housing.
   Remove the ½” head bolt at the rear of the air manifold securing the pipe to the right rear head nut. Then unscrew each of the four injectors with a 7/16” wrench. It may be helpful to remove the spark plug wires for easier access. Then, plug the four injector holes with 7/16-20 x Ύ Allen set screws.  Bolts can be used, but do not provide the clean look afforded by the Allen set screws.
   Hold the tall thermostat nut with a ½” wrench and remove the two 7/16” headed bolts. Pull the small vacuum line off the unit. Either remove the hose from the 90 manifold fitting, or sometimes the 90   fitting lifts away from the manifold without any restriction. Use a 1” socket and remove the bolt from the manifold that secures the gulp valve bracket. Leave both copper washers on the bolt and retighten the bolt, having removed the bracket.
   If the 90 fitting did not easily lift away from the manifold, then grasp it with pliers and twist it back and forth until it lifts free. Then tap the manifold Ό” NPT. Use a good sharp tap and coat the flutes with grease to keep the shavings and swarf from dropping into the manifold. Screw the tap into the manifold about ⅔ of its length. Then, fit a Ό” NPT allen screw or brass plug into the manifold.
   Finally, remove the extra pulley and bracket associated with the air pump. On the 1977 to 1980 models this is an easy task as the two items are exposed. On the 1975 and 1976 models, however, it is easier to remove the radiator first to gain access. The reason the pulley is removed is simply for appearance.  The adjuster bracket should be removed so that it can never loosen and fall into the alternator fan!
    This completes the first step – removal of the air pump, air manifold and gulp valve.
   (2) Vacuum Advance: Reroute the vacuum advance tubing directly from the distributor to the intake manifold. On 1977-1980 models the vacuum advance line is run through the TCSA switch on the master cylinder box. This switch can be removed for a better appearance, and the wires taped and hidden.
   If the MGB does not have overdrive, then the gearbox power wire MUST be disconnected. If the MGB does have overdrive, it is advisable to install an in-line fuse (10 amp) to this unfused circuit. The single wire involved is white with brown and either runs to the gearbox by itself or is incorporated into the gearbox wiring loom. It is disconnected at the junction of the main, rear, and gearbox looms at the rear of the right inner fender. The main loom has about a hundred wires, the rear loom twelve, and the gearbox loom three.
   Check the ignition timing. With the vacuum disconnected, set the timing at 32° BTDC at full mechanical advance (about 3-4000 rpm).  This timing specification is true for all MGs (except Twin Cams) from 1946-1980.
   This completes the second step, rerouting the vacuum advance and setting the timing.
   (3) EGR Valve: The EGR valve is located on the top of the intake manifold, just to the rear of the carburetor (as seen from the left fender). Remove the two bolts (½” wrench or socket) securing it to the manifold and remove the valve. Make two gaskets with only the holes for the bolts. Sandwich the old asbestos gasket with these two new gaskets, using a liberal amount of Permatex “The Right Stuff” gasket compound to hold the sandwich together. Fill the cavities on the underside of the EGR valve with the gasket compound and replace the unit.
   This stops a fresh air leak that is always present and makes adjustment of the carb easier. If the EGR valve has failed, then the air leak can be quite massive. Reconnect the tube to the carb even though it serves no purpose – it looks better than stuffing a small screw in the end of the hose.
   (4) Carburetor: The last step involves the overrun valve and carb adjustment. Remove the carb from the manifold by (1) removing the air cleaner with a ½” wrench and disconnect the throttle return spring(s); (2) removing the heat mass and water lines out of the way with a piece of wire around the unit pulling it to the front valve cover stud; (3) removing the fuel line and overflow line by first twisting the hoses on their brass fittings – this loosens them and makes removal much easier; (4) removing the throttle cable using two 7/16” wrenches; (5) and finally removing the four nuts holding the carb to the manifold with a ½” wrench.
   Turn the carb upside down and allow all the gasoline to escape from the vent tube. Place the carb into a vise (carefully) and hold the throttle disc open with a screwdriver wedged into the external linkage or cam. The overrun valve should be positioned with the spring upwards.  Using a small propane torch to heat the underside of the throttle disc, flow some soldering flux onto the topside of the disc, and finally flow a generous amount of solder into the base of the valve (under the spring). Allow this to cool before removing from the vise. Ensure that there is no solder on the edge of the throttle disc – if some has crept there, scrape it away with the edge of a screwdriver.
   Close down the idle CO adjuster screw – the air bleed -- by unscrewing the brass screw, tightening the white plastic nut, and then retightening the brass screw.
   The carb can now be replaced.  Use new 5/16” fine nuts to make installation easier. If a new gasket is required, use Moss part 366-235. Use GREASE on the gasket to ensure there are no air leaks. Leave the throttle cable loose until the final adjustments have been made to avoid getting it positioned incorrectly.
   (5) Adjusting the carb: Start up the engine and allow it to reach operating temperature. Check the mixture by using the SU technique of lifting the piston. As the piston is lifted a very small amount (⅛” or so) judge the change in rpm. If the rpm climb as the piston is lifted then the mixture is too rich; if the rpm drops off immediately then the mixture is too lean. The optimum setting allows an increase of 50-100 rpm then a steadying or dropping off. Use a Stromberg adjusting tool to make the adjustments (clockwise richens, anticlockwise leans). Refit the throttle cable and air cleaner and take the car out for a test run.
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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