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Tarboy56 01-13-2013 04:33 PM

Heat riser issue
Who out there knows anything regarding the heat riser located on the passenger side at the back of the exaust manifoild? Is this supposed to be closed when the engine is cold and open when warm?

RWENUTS 01-13-2013 08:45 PM

THat's the way it works.

Tarboy56 01-14-2013 08:44 AM

Exhaust riser
I thought so. Mine was bound up I took it out cleaned and soaked and freed it up found out whoever installed it had it in upside down and thus when the engine got hot it actually closed the butterfly it works now HOWEVER my intake manifold barely gets warm even when the engine is up to temp. My car simply falls flat on it's face when you try to take off from a stop it acts exactly like the accelerator pump is bad BUT I have had three NEW carbs. on and there was NO change. Starts fine runs fine at idle but if you don't feather it the engine just dies. Going down the road at 50 mph runs good no issues. Any ideas?

RWENUTS 01-14-2013 09:04 AM

Couple of things possible.
Someone installed new intake gaskets and blocked off the passage under the intake or the passage is plugged with carbon. I had one that was plugged off.
Your symptoms are typical cold intake problem.
THe fuel vapour reverts back to a liquid when unheated. No fuel vapour to the pistons. Lean bog.
Do you have the stock heated air cleaner that has a heat tube running to the exhaust manifold?

I see your other post. The cold intake will answer your question over there.

Tarboy56 01-14-2013 09:31 AM

56 enging issues
Thank you funny you brought this up checking the intake was NEXT on the list there is a 1/4 inch tube runs from the left side of the engine manifold going to the choke but nothing to the air cleaner and it is the stock air cleaner

RWENUTS 01-14-2013 09:48 AM

Probably a stock oil bath then. No heat to it.
On my old truck I just do the heat soak method when I want to drive it. I run it on high idle for 5 minutes. Shut it off. Give it 5 more minutes then fire it back up. The heat from the motor radiates up and warms up the intake. But even then it sometimes wants to fart around when I drive it. I just keep the rpm's up a bit till it's fully warmed up. I'm running an open element air cleaner so no heated air.

Tarboy56 01-14-2013 09:52 AM

56 engine issues
Well I plan to pull the intake off just to eliminate that as the problem as I mentioned earlier there is a heat riser located on the passenger side of the engine which directs exhaust to the manifold when cold then under operating conditions it opens and directs exhaust to the exhaust pipe.

MouseFink 01-14-2013 11:39 AM


The heat riser butterfly valve on the passenger side manifold is equipped with a bi-metal spring. The bi-metal spring keeps the butterfly closed and the LH exhaust is diverted through the RH cylinder head, through the intake manifold, under the carburetor, through the LH cylinder head into the LH exhaust system in order to heat the carburetor and atomize the fuel. As the bi-metal spring heats up it relaxes and allows the exhaust pressure to open the heat riser butterfly valve. As the intake manifold it heated, a bi-metal spring in the carburetor choke control is heated and opens the choke butterfly and drops the engine idle speed. Some systems use a bi-metal spring in the intake manifold with a rod to the choke control or use a 1/4" tube from the manifold to a bi-metal spring in the choke control to open the carburetor choke butterfly. When the exhaust pipe butterfly valve is fully opened, exhaust gasses flow in the usual manner, out the exhaust system on both sides. After the engine reaches operating temperature, the carburetor is heated by exhaust gasses from the cylinder heads. The carburetor choke butterfly is fully open and the idle is stepped down to normal speed. All this equipment must be operational for the automatic choke system to work properly. The automatic choke and heat riser system replaced the manual choke and pull cable. Modern cars with a carburetor have a electric choke system and it is not effected by corrosion.

On most cars after five or ten years, the exhaust heat riser butterfly valve is corroded shut or open and the bi-metal spring is missing or non-functional. The engine is more sluggish due to the exhaust restriction. The later cars with electric choke did not have a exhaust system butter fly valve. If tubular headers are installed, a manual choke with a pull cable or a electric choke is required to operate the carburetor choke in cold weather.

We always cut the exhaust butterfly valve out with a acetylene torch. You can buy a electric choke kit for most carburators. I had one on the center carburetor of my 1963 Pontiac tri-power. I had to install a electric choke system because I was stupid and blocked the heat risers in the heads with aluminum. The Pontiac would not start without a choke when the temperature was below 40 degrees and never would run correctly because the fuel was not fully atomized.

Tarboy56 01-14-2013 02:20 PM

Heat riser
Mouse Fink,
Thank you for your comments interesting. IF you cut out the butterfly valve how does the intake get enough heat to atomize the fuel properly ? Is it by using the choke till the engine comes to full operating temp?

MouseFink 01-14-2013 04:42 PM

If the RH exhaust pipe heat riser butterfly valve is removed, the carburetor base still gets indirect heat from the the center cylinders via the center cylinders exhaust valve heat ports and the heat passage under the carburetor in the intake manifold. . With the RH exhaust pipe heat riser valve is removed, it takes the carburetor base slightly longer to warm up and the choke slightly longer to fully open. The choke has a rich (winter) or lean (summer) adjustment which can adjust for the delay. The rich or lean adjustment raises or lowers the pre-load on the bi-metal spring in the carburetor choke mechanism.

After I filled the heat passages in the heads with molten aluminum, the bi-metal spring in the carburetor choke mechanism received no heat at all. The main reason I filled the passages with aluminum was to prevent the exhaust heat from burning the paint off the intake manifold. That is dumb mistake for a car that is sometimes driven on the street in winter. Blocking the heat passages in the heads is used on race cars at the strip in order to cool the carburetor base and the fuel for a denser mixture but it is ineffective for a car that is driven on the street after the engine reaches operating temperature. A cool air carburetor intake is a better idea and is sometimes worth 2 tenths in the quarter mile.

Tarboy56 01-15-2013 08:19 AM

56 engine issues
Mouse Fink,
Interesting comments I'm anxious to pull the intake manifold off and see what is going on my intake manifold does not get very hot even once the engine is totally up to temp. I may be blocked due to carbon build up or possibly wrong caskets.

oldbogie 01-15-2013 10:18 AM


Originally Posted by Tarboy56 (Post 1635069)
Mouse Fink,
Interesting comments I'm anxious to pull the intake manifold off and see what is going on my intake manifold does not get very hot even once the engine is totally up to temp. I may be blocked due to carbon build up or possibly wrong caskets.

I'll X2 Mouse Fink and add that the crossover of the intake usually gets clogged with carbon as the engine ages and becomes less to not functional.

As far as the heat riser valve is concerned, unless you live someplace that gets really cold there is little need to keep it functional. You'll find that when the crossover passages are clear; the intake gets plenty hot, very fast even without this troublesome device.

The small tube you mention is the Choke Stove. It supplies air heated by the hot exhaust to the bi-metallic spring in the plastic choke housing of the carb. It may, or not, source air from the air cleaner assembly old models tended to draw unfiltered air newer ones starting from the 1960's often took filter air. Some of these choke-stoves passed into the actual exhaust stream, when they finally burn out, they drew exhaust gasses into the choke spring housing which pretty much destroys all the parts from corrosion.


Tarboy56 01-15-2013 10:33 AM

Heat riser

Thanks for the reply most if not all my parts are new and seem to be working I'm hoping when I take the intake off I find something that solves my problem I will be changing the car over from powerglide to three speed on the tree and in the process plu the engine and freshen everything up

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