SOOTY PLUGS

Around every three months my 1987 Metro begins to run rough when idling. When this happens, I remove and clean the spark plugs, which have taken on a black and sooty appearance.

Once the spark plugs have been cleaned the engine will start and run okay by keeping it revving slightly higher. After a short while it will then run as normal again at all engine speeds, including tick- over.

The air filter is clean and the oil and coolant levels are correct and don’t need changing. When this problem first began to occur, I changed the HT leads and distributor cap, followed by the fuel filter. I’ve also checked all the compressions, which are even and good.

I’ve also checked for any leaking vacuum or breather hoses and am now wondering what else I could do. Paul Williams Black and sooty plugs are a good indication that the mixture is too rich and as you’ve checked and confirmed that the air filter is in good condition, then the cause must be due to over fuelling.

From the details I have on your Metro, I believe it should have a manual choke and I would check the return spring and the operation of the choke mechanism, as this is the most likely source of your troubles.

Another problem within the carburettor may be that the needle float valve is sticking and allowing fuel to flood through into the engine. If you don’t have a choke mechanism but have the later electronically operated carburettor unit, then I would check the coolant sensor, which may be reading cold. This would trigger the carburettor to allow a greater amount of fuel in to the engine than is required.

I would suspect that when you do rectify this problem you will also have the advantage of a reduced fuel consumption from that which you are now experiencing.