Thursday 1 March 2012

Choke? Isn't That What Athletes Do?

Something I hear from youngsters – that’s anyone younger than me by 10 years – all too often is that life was easier in the old days. I’m not too sure that it applies to owning or fixing cars. Compare the following oft-repeated scenario to today’s fill - drive - change oil (maybe) - sell car lifestyle.

Q.Last year I had a tune-up done on my 1972 Chevrolet Caprice (454 engine). Since then, whenever I start it up it stalls. I have to repeat the starting procedure five or six times before it stays running. Is there any way to cure this problem? Will I have to have the carburetor overhauled or replaced?

A.First off, I don't think that the carburetor necessarily has to be rebuilt or replaced. Since the condition didn't start until after the tune-up, it must have been caused by the mechanic, not the car. Somehow, the automatic choke system or the fast idle mechanism was disturbed. Or something that was hiding the problem was fixed so that the stalling now appears to be the worst concern.

The automatic choke system reduces the amount of air entering the engine while at the same time feeding more gasoline to the cold engine as it turns over. To stop the engine from stalling as it starts, a vacuum operated diaphragm opens the choke plate an extra little bit just at the moment that the engine starts. Then, as the engine warms up, the choke plate moves open slowly as the need for additional gasoline is reduced.

At the same time, the fast idle mechanism holds the throttle open to speed up the engine during warm-up. This fast-idle speed is a critical adjustment on a tune-up, but it is often overlooked or adjusted incorrectly.

The trick to getting a car to start -- and to stay running -- is in having all of the adjustments dead right. One error and frustration sets in. On the other hand, all of the adjustments could be correct and the problem could lie solely with the driver. Many cars require a certain starting technique. If this ritual isn't followed to the letter, the car won't start.

To solve the problem, get the carburetor set right first. The air cleaner should be removed. With the cold engine shut off, step on the gas pedal and watch the choke plate. It should move freely and close the top of the carburetor. If the plate sticks or is all gummed up and dirty, free it up by spraying it with a solvent such as WD-40 or an aerosol carburetor cleaner. If the choke plate is free but will not drop closed, adjust it so that it does. It's usually a simple matter of loosening three small screws which hold a round, choke thermostat in place. Open the throttle by hand a touch and turn the choke thermostat in the direction which closes the choke plate.

Tighten everything back up. Leave the air cleaner off and use some golf tees to plug any unplugged vacuum hoses. Pump the pedal ONCE, take your foot off the gas and start the car. It ought to start right away. The choke plate should be open about half way and the engine should be running at fast idle.

The fast idle speed of most engines ranges anywhere from 1700 rpm to 2600 rpm. This roaring speed sometimes seems excessively noisy so mechanics sometimes slow it down. Other times, owners complain of the roar and request that it be slowed down. Either way, the engine will usually stall on start-up when the fast idle speed is set too slow.

Speeding up the fast idle is simple: turn the fast idle adjusting screw clockwise. This is the screw which contacts an odd-shaped, notched piece of metal called the 'fast idle cam'. Remember how much you turn the screw in case of an error. Engine speed change should be immediate. If it isn't, you're turning the wrong screw. Since there are only two screws to choose from, you can't get into too much trouble. Turn the wrong screw back to where it was and turn the other one.

Once the fast idle speed is right and the choke adjustment is right, the car should start and run each and every time. Pump the pedal once, foot off the gas, crank the engine over and the car will run. The automatic choke -- when it is working properly -- makes engine starting much simpler than ever before. There is no more need to pull on choke cables or push them in at the right moment. It will work at any temperature and under all conditions.

Nothing to  it, right? 


  1. Those were the days. I think there was more skill involved back then. Carb overhauls, distributors - points and condensers, timing etc. Now it's more parts replacement

  2. I remember those days very well, It reminds me of the old tv ads where they showed the mechanic just tweaking the carb with his tune up screw driver. Life was simple then!