Engine Problems

Missing

Carburettor Fluttering
Missing at high revs can be caused by a lack of oil in the carburettor dashpot. Take a look at the top of your carburettor (it's an aluminium casing that looks like an upturned wine glass with the base broken off, or an upside down letter 'Y', and is generally covered in grimy, petrol-soaked dirt). There is a black plastic cap on the top of it that unscrews. Attached inside is a 3 inch long shaft with a valve mechanism on the end. Lift the shaft gently up and down inside the dashpot. If there is no resistence then you need to add oil. Add a small amount of light machine oil to the dashpot. The grade of oil should be around 10-15. Do not use engine oil as this is too thick and will cause the engine to be sluggish. Sewing machine oil or '3-In-One' oil is best. I've even used automatic transmission fluid with good results. Screw the shaft back in (finger tight only) and go for a test drive.

To understand what's happening you need to understand how the SU carburettor basically works. As air is sucked into the engine a vacuum is created above the carburettor plunger that sucks it upwards. On the bottom end of the plunger is a tapered needle that fits into a small hole and controls the flow of fuel out of the hole. The more suction there is, the higher the needle rises and the more fuel that is allowed into the engine. And of course, the more fuel and air there is, the faster you go. Simple really. (related to this, the mixture adjustment on the bottom of the carburettor simply lowers the hole that the needle blocks, and therefore lets more fuel in, simple isn't it?)

The dashpot is actually a reservoir of oil in the top of the carburettor plunger. Because there is a valve mechanism on the shaft in the reservoir it restricts how quickly the plunger can rise or fall. If there is little or no oil in the reservoir then the plunger is free to jiggle about causing an ever changing amount of fuel to be allowed into the engine, which of course causes rapid power surges. The same occurs if the oil in the dashpot is too light. If the oil is too heavy then the plunger cannot react quickly and the engine will be sluggish to provide power when needed.
 
 

Engine Running Rough

Spark Plugs
Mostly taken for granted, the spark plugs are central to good running of your car. I mean, no spark, weak spark or even miss-timed spark and your car wont go very well, if at all. So, the first thing to do is check that your plugs are OK. If they've been in the car for more than 20,000 miles, throw them away and get new ones. If the plugs have never been changed in your Minor, which is possible, or they've done a lot of miles, you'll probably notice that the sparking points are rounded off. This is because the spark process is actually destructive, Tiny portions of the plug points are eroded away with every spark. Not only does this eventually increase the gap between the points, making it harder for your distributor to supply enough power to make the spark jump the gap, but it means the spark is likely to wander around the points, never sparking in the same place twice and therefore making it difficult to set the gap correctly.

The recommended spark plug for the 948cc and 1098cc Minor is the Champion N9YC, however, I have had much better results when using the NGK BP6ES. The recommended spark plug points gap is 0.030" or very close to 1mm. I rarely bother checking the gap of new plugs these days as they always seem to be spot on, but it doesn't hurt to check, especially if you want things to be 'just right' or if you want to eliminate a possible source of bad running.

High Tension / Spark Plug Leads
The new silicone core leads are great, for a while. Sudden loss of power and intermittent rough running are a sure sign that the leads are faulty, as we experienced on our trip across France in our Traveller. Silicon core leads are cheaper than the standard wire core leads but you pay for what you get. Unfortunately, if you have to replace your coil for any reason (bursting, for instance) you'll probably find that the recommended Lucas coil now has a modern push fit for the lead instead of the old style screw in variety. This means that you either have to modify your existing lead or ditch it in favour of a silicone one. Catch 22.