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The first thing to check on the A series is probably the vacuum system. With age these vacuum pipes deteriorate and crack, split and even drop off. If your vacuum system isn’t top notch your car will never run right. Vacuum pipe bits and pieces can be bought from most good motor stores or you may even be able to get the original pipes from MG-Rover still.
The vacuum system consists of 3 or 4 pipes (depending on the age of your car)
The first pipe comes from the inlet manifold (1) into the airbox (2). The second runs from the air box (3) to the air flap (4).
The air flap when the engine is cold moves to direct warm air (collected from around the exhaust) into the engine to help it run more smoothly. When the engine is warm the flap reverts to supplying cold air from behind the grill area. To test the flap disconnect the hose (3) from the airbox and suck on the end. You should see/feel/hear the flap in the air box move. You shouldn’t be able to suck air easily through the pipe, if you can then there is a fault with the air flap (uncommon). In my experience even if the flap is seized it doesn’t affect the running of the car at all when warm and only marginally when cold.
The next pipe comes from a stub directly in front of the carburettor (5) this pipe then leads to the thermostat housing (on later catalytic converter equipped models (6)(7)) or on most models straight to the distributor (8) This pipe has the largest effect on the engines running so make sure it is in good condition. To test the distributor diaphragm remove the end of the pipe not connected to the distributor and gently suck on it. If you can suck air through easily then either you have a split in the pipe or the diaphragm is broken and you’ll need a new one. If it is working correctly you’ll only be able to suck a small amount of air through and if you listen carefully you may hear the diaphragm click, as you apply suction indicating that it is working correctly.
The catalytic converter models also have a vacuum switch in the thermostat housing which prevents the ignition timing advancing with vacuum until the engine is warm in order to make the catalytic converter work more efficiently. This switch in the thermostat basically blocks any vacuum from reaching the distributor until the engine coolant is warm.
Now that you’ve carefully checked all of the vacuum hoses for leaks you can move on to the next step of tuning your carburettor.
At this stage if you have an auto choke mechanism I would suggest that unless you have already done so that you renew the sealing O rings on the stepper motor. The original ones degrade with the more modern petrol we are using today and cause many running problems. The club sell upgraded O-rings which don’t suffer this problem. If you need some then let me know (as I currently hold a stock of them for the club which are available very reasonably priced) also do a forum search for “Viton” and you can read all about changing them and the benefits in detail.
Right now that you have your vacuum system working well and I’ll assume the ignition timing is approximately right then we can start tuning the car. Many people say that you need lots of expensive gas analysers etc to do the job but really you don’t. I tune mine by ear and it runs great and passes the MOT every year, (well on emissions at least!). All you need are a couple of screw drivers, one thin and long with a flat head and the other with a wider flat head.
You need to have a warm engine to tune it correctly so after you’ve come back from a drive is the best time to do it. (Most of this info is taken from the Haynes manual so if you need further advice I suggest you look there as there is lots more advice in there but I’ll just give you the basics that seem to work for me.).
Firstly make sure that the oil reservoir for the carburettor damper is filled with oil. Normal engine oil is usually fine for this. Be careful when screwing the plastic nut/damper assembly back in as it is very easy to cross thread it. Then locate the idle speed screw. If you are lucky you can reach it between the side of the airbox and the carburettor. If not the airbox can be removed using a 10p piece or similar to remove the large flat screws (x3). Start the car and turn the screw to give you a fast idle speed, slightly above the usual idle speed. Then with the airbox reattached locate the mixture screw (this has 2 slots cut in it but you can use a large flat screwdriver to adjust it. Turn the screw clockwise to richen the mixture and anti-clockwise to weaken it.
(If you have looked at the spark plugs after a good hot run they can tell you a lot about the mixture of the engine. If they are dark or sooty then the mixture is too rich and if the plugs are very white then it is too lean. You want a nice beige/tan sort of colour ideally.)
During the tuning process you should occasionally blip the accelerator pedal (or you can manually operate the throttle lever on the carburettor) to rev the engine occasionally. This is needed to stop fuel collecting in the inlet manifold and giving you incorrect tuning. I usually rev the engine once every 30 seconds or so and then again to check that everything is fine when I’ve finished tuning.
Ideally you want the weakest mixture you can get away with as it’ll give you the best fuel economy. Turn the screw half a turn at a time each way for several turns until you obtain the fastest even idle speed you can. Then I usually weaken the mixture by ¼ of a turn, (but that is my personal preference). Then reset the idle speed screw back to a sensible idle speed where it idles evenly but slowly, you’ll soon realise when you’ve gone too slow!. Then floor the accelerator a couple of times to clear out any remaining fuel and check the idle speed is correct. Then take it for a test drive.
If the car seems to hesitate when you apply the throttle under load then the mixture is probably too weak and you can the mixture screw clockwise half a turn and try again. Using this method you should find that you get the perfect mixture quite easily.
The vacuum system consists of 3 or 4 pipes (depending on the age of your car)
The first pipe comes from the inlet manifold (1) into the airbox (2). The second runs from the air box (3) to the air flap (4).
The air flap when the engine is cold moves to direct warm air (collected from around the exhaust) into the engine to help it run more smoothly. When the engine is warm the flap reverts to supplying cold air from behind the grill area. To test the flap disconnect the hose (3) from the airbox and suck on the end. You should see/feel/hear the flap in the air box move. You shouldn’t be able to suck air easily through the pipe, if you can then there is a fault with the air flap (uncommon). In my experience even if the flap is seized it doesn’t affect the running of the car at all when warm and only marginally when cold.
The next pipe comes from a stub directly in front of the carburettor (5) this pipe then leads to the thermostat housing (on later catalytic converter equipped models (6)(7)) or on most models straight to the distributor (8) This pipe has the largest effect on the engines running so make sure it is in good condition. To test the distributor diaphragm remove the end of the pipe not connected to the distributor and gently suck on it. If you can suck air through easily then either you have a split in the pipe or the diaphragm is broken and you’ll need a new one. If it is working correctly you’ll only be able to suck a small amount of air through and if you listen carefully you may hear the diaphragm click, as you apply suction indicating that it is working correctly.
The catalytic converter models also have a vacuum switch in the thermostat housing which prevents the ignition timing advancing with vacuum until the engine is warm in order to make the catalytic converter work more efficiently. This switch in the thermostat basically blocks any vacuum from reaching the distributor until the engine coolant is warm.
Now that you’ve carefully checked all of the vacuum hoses for leaks you can move on to the next step of tuning your carburettor.
At this stage if you have an auto choke mechanism I would suggest that unless you have already done so that you renew the sealing O rings on the stepper motor. The original ones degrade with the more modern petrol we are using today and cause many running problems. The club sell upgraded O-rings which don’t suffer this problem. If you need some then let me know (as I currently hold a stock of them for the club which are available very reasonably priced) also do a forum search for “Viton” and you can read all about changing them and the benefits in detail.
Right now that you have your vacuum system working well and I’ll assume the ignition timing is approximately right then we can start tuning the car. Many people say that you need lots of expensive gas analysers etc to do the job but really you don’t. I tune mine by ear and it runs great and passes the MOT every year, (well on emissions at least!). All you need are a couple of screw drivers, one thin and long with a flat head and the other with a wider flat head.
You need to have a warm engine to tune it correctly so after you’ve come back from a drive is the best time to do it. (Most of this info is taken from the Haynes manual so if you need further advice I suggest you look there as there is lots more advice in there but I’ll just give you the basics that seem to work for me.).
Firstly make sure that the oil reservoir for the carburettor damper is filled with oil. Normal engine oil is usually fine for this. Be careful when screwing the plastic nut/damper assembly back in as it is very easy to cross thread it. Then locate the idle speed screw. If you are lucky you can reach it between the side of the airbox and the carburettor. If not the airbox can be removed using a 10p piece or similar to remove the large flat screws (x3). Start the car and turn the screw to give you a fast idle speed, slightly above the usual idle speed. Then with the airbox reattached locate the mixture screw (this has 2 slots cut in it but you can use a large flat screwdriver to adjust it. Turn the screw clockwise to richen the mixture and anti-clockwise to weaken it.
(If you have looked at the spark plugs after a good hot run they can tell you a lot about the mixture of the engine. If they are dark or sooty then the mixture is too rich and if the plugs are very white then it is too lean. You want a nice beige/tan sort of colour ideally.)
During the tuning process you should occasionally blip the accelerator pedal (or you can manually operate the throttle lever on the carburettor) to rev the engine occasionally. This is needed to stop fuel collecting in the inlet manifold and giving you incorrect tuning. I usually rev the engine once every 30 seconds or so and then again to check that everything is fine when I’ve finished tuning.
Ideally you want the weakest mixture you can get away with as it’ll give you the best fuel economy. Turn the screw half a turn at a time each way for several turns until you obtain the fastest even idle speed you can. Then I usually weaken the mixture by ¼ of a turn, (but that is my personal preference). Then reset the idle speed screw back to a sensible idle speed where it idles evenly but slowly, you’ll soon realise when you’ve gone too slow!. Then floor the accelerator a couple of times to clear out any remaining fuel and check the idle speed is correct. Then take it for a test drive.
If the car seems to hesitate when you apply the throttle under load then the mixture is probably too weak and you can the mixture screw clockwise half a turn and try again. Using this method you should find that you get the perfect mixture quite easily.
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