CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

How does the carburetor choke heat riser tube work?

Early 70’s Ford Truck, Motocraft 2150 Carb (with auto choke)

The auto-choke has a small tube going to it. It seems to originate in the exhaust manifold area.

Does this tube pipe hot exhaust gas to the choke? Or does it simply pipe hot air using convection?

The reason I’m asking is the tube has rusted off and become disconnected at the exhaust manifold, and I’m trying to decide what to do about it. Is it now leaking exhaust fumes?

You are correct, the tube is providing heat to the choke, so the choke will open up as the motor warms up. Best option is to replace the tube so it is connected and no longer leaks. If you want to plug the hole in the manifold that will stop the escape of exhaust gases. You might not have a effective choke anymore, but you could replace the current choke with a choke that is warmed electrically.

My choke seems to have a dual warming system, both the heat tube and an electrical connection. So I can probably get by with just plugging the hole, at least for now. Since it is leaking exhaust gasses, I’ll do it straight away. Thanks.

I think kits may still be available to convert the automatic choke to a manual choke. J.C. Whitney may still sell them–you can go to their web site. Ford was the last manufacturer to go to the automatic choke. Real Fords had manual chokes–make your old pickup a real Ford.
I think the electrical connection may be for a choke pulloff. My 1978 Oldsmobile had this arrangement and it was operated by vacuum, but your system could be electric. I would solve the whole problem by installing a manual choke and plugging the hole where the choke heat tube comes out of the manifold.

Your choke is closed longer than it sould be without that tube. That bore in the exhaust manifold should not be in the exhaust stream, there should be no exhaust leak.

I had some old Fords and I replaced that choke heater tube with common 1/4 " steel tubing. Note that there is a second tube connected to the top of the carburetor or air cleaner for clean air. If your tube has been missing for any length of time you can expect the vacuum orfice inside the choke housing to be plugged with dirt. There is a vacuum supply that pulls heat though the tube.

The Dorman HELP line has a repair kit

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_x_5012120-P_x_x?cm_mmc=ACQ--Google--enhancedRM-_-5012120&ci_src=17588969&ci_sku=5012120&ci_gpa=pla&ci_kw={keyword}#utm_source=acq&utm_medium=google&utm_campaign=enhancedrm&utm_content=5012120

And venturi vacuum draws the hot air though the tube.

You don’t want exhaust gases in the choke cover. The gas will corrode the metal in the pipe and the choke cover.

The plumbing path is filtered air down to the manifold, air heated by the sealed tube passing through the manifold, hot air back up to the choke cover, and exiting by the vacuum pulloff piston into the intake manifold.

The “choke stove” on the manifold should never leak or provide exhaust gasses to the choke…Trying to restore 42 year old automatic chokes is loves labor lost, they never worked that well when they were new and wasted a lot of gas. They are easily converted to a manual choke with an inexpensive manual choke kit…

You have an old lawn mower or gas weed wacker laying around? The throttle cable is a cheap way to convert your automatic choke to a manual. Takes a little MacGyvering, but if I can do it you can too. Make certain that you disable the auto choke, they waste gas and become inefficient as they age. Rocketman

Sometimes MacGyver comes back to haunt you. With the somewhat mild to warm climate here I have often just opened the choke totally for men who seemed willing and patient enough to pump the pedal a few times to get the engine started and hold a fast idle long enough for the engine to warm up and idle without stalling. But for the more demanding the automatic choke can be kept functioning as designed if willing to find the correct pieces needed and setting it properly. Often on the Ford 2bbl an electric choke stat will function well with the stove blocked off. The manual choke kits can be successfully installed but for me they are more time consuming and troublesome than repairing the OE automatic choke.