A few decades ago, everyone knew how to use the choke, because most cars with petrol engines had one. But modern engines don’t require this manual input from the driver, so if you learned to drive in the last 20 years or so, you might have never learned how it’s done. It’s easy though.
This isn’t an absolute rule, just a very basic guide. All cars need slightly different treatment, and other factors such as the ambient temperature will also have an effect. But if you’re new and have no idea how to use the choke, try this before honing your technique.
How to use the choke – THE BASICS
You adjust a manual choke by pulling the knob in and out. On some cars the knob is turned clockwise to lock the position; anti-clockwise releases it so you can adjust it.
- If you’re starting the engine from cold, you need to pull the choke all the way out before starting the ignition. When you fire up, the engine will rev quite high, and the exhaust gas will smell rich. This is normal. Leave the choke out while you drive away.
- As the engine warms up, it gradually needs less and less choke. Don’t worry about being too precise. As a very general rule: push the choke half-way in after about a mile, and all the way in after a couple of miles.
- A more accurate method is to look at the temperature gauge. Once the needle just begins to lift, try pushing the choke half in. By the time the needle is about a third of the way up, push it all the way in.
- On a cold day, the engine takes longer to warm up, so you need to leave the choke out for longer.
- If the engine starts to splutter (especially when you accelerate), it needs a bit more choke, so pull it back out a little until the engine is warmer.
- When the engine reaches normal temperature, with the needle in the middle of the gauge, make sure the choke is pushed all the way in. Now you can forget all about it.
That’s all there is to it. As you bond with the car over time, you’ll hopefully learn how to use the choke in more subtle increments and anticipate how much your engine requires.
ONCE YOU’VE GOT THE HANG OF IT…
The choke is infinitely variable, so you can make very small adjustments if you need to.
If you’re starting the engine but it isn’t completely cold – let’s say you’ve parked up for 30 minutes, so it’s still got a little warmth under the bonnet – use just a little choke. But you’ll probably be able to push the choke in quite soon after you set off.
You may find the engines run better with a bit more choke if you are at higher altitude. However, if you live high up and the car is rarely at low altitude, you shouldn’t use excessive choke all the time, as your fuel mixture settings should be calibrated to according to the atmosphere. If you don’t know how to do this, find an old mechanic with a oily beard to have a look at your carburettor. He’ll know what to do.
SO WHY IS IT CALLED THE CHOKE?
A petrol engine requires a mixture of air and petrol going into its cylinders. When the engine is cold, it needs a richer mixture (a higher proportion of fuel) for the combustion to be efficient.
Pulling out the choke literally chokes the air supply to the engine. By decreasing the flow of air, a higher proportion of petrol is sucked into the cylinders.
The more you pull the choke out, the more the air flow restricted. When it’s pushed all the way in, the air flow is no longer restricted.
WHAT HAPPENS IF YOU GET IT WRONG?
If you use too much choke you’ll burn more petrol and cause higher emissions. Longer term, you will decrease the life of your spark plugs, and potentially cause your engine to ‘coke up’ over time, meaning it’ll become less efficient and less powerful.
Learning how to use the choke properly is part of the fun of driving old cars. They can be temperamental mechanisms, but they help keep you in tune with the condition of your engine.