Topic: 67 390 GT carb heat

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67 GT 390 has two tubes that come from the exhaust manifold to the carb, the first comes from the top of the manifold to the carb choke housing and attaches with a compression type fitting, the second comes from the bottom of the manifold and ends   about the top of the passenger side valve cover, from there I thought it went by means of a hose to the choke clean air pick up tube on the passenger side of the  carb its the small tube coming out on an angle. It seems that set up like this the choke housing has vacuum pulling from two directions is that correct ? Thanks Paul.

[WT8095]

67 GT 390 has two tubes that come from the exhaust manifold to the carb, the first comes from the top of the manifold to the carb choke housing and attaches with a compression type fitting, the second comes from the bottom of the manifold and ends   about the top of the passenger side valve cover, from there I thought it went by means of a hose to the choke clean air pick up tube on the passenger side of the  carb its the small tube coming out on an angle. It seems that set up like this the choke housing has vacuum pulling from two directions is that correct ? Thanks Paul.

I believe it works like this:

Both ends are lower than ambient air pressure, however the choke housing is receiving a stronger vacuum signal than the other end. Air enters the tube at the top of the carb. This is just a convenient location to draw filtered air from, as opposed to using an unfiltered source outside the air cleaner. The air is then drawn down through the tube, through the passage in the exhaust manifold (a tiny heat exchanger, effectively). It then continues upward and into the choke housing, where it heats the bimetal choke thermostat spring. The air is then drawn into the carb and into the intake manifold and beyond...

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That makes sense its the fact that the port opening on the filtered side tube is much bigger than the choke housing side which has a pretty small restrictor in it.  also I found that the choke spring has a number stamped on it, 132 but I haven't  found any info to identify the correct spring, I'm having choke issues, thanks Paul.

[Bob Gaines]

67 GT 390 has two tubes that come from the exhaust manifold to the carb, the first comes from the top of the manifold to the carb choke housing and attaches with a compression type fitting, the second comes from the bottom of the manifold and ends   about the top of the passenger side valve cover, from there I thought it went by means of a hose to the choke clean air pick up tube on the passenger side of the  carb its the small tube coming out on an angle. It seems that set up like this the choke housing has vacuum pulling from two directions is that correct ? Thanks Paul.

The air is heated in the choke furnace (exhaust manifold ) and rises through metal tube with the insulated sheath to the choke bimetal heat actuated spring. Heat rises. The lower port on the choke furnace is for fresh cooler air which is drawn in displacing the rising heated air.  The cooler fresh air comes from the carburetor by way of the uninsulated metal tube (not necessary to be insulated) and connected to the carb by a rubber hose (because it doesn't get hot like the heated side). I hope this helps understand the loop.

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Bob, that's how I thought I remembered it working but it seems there is always something with parts this old. My carb C7OF-D seems to have a restrictor of .052 in the carb where the choke housing mounts to it is this correct ?

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It seems that with this restriction the spring is winning the tug of war with the vacuum and so Im having problems with setting the choke and getting it to stop, thanks Paul.

[Bob Gaines]

Bob, that's how I thought I remembered it working but it seems there is always something with parts this old. My carb C7OF-D seems to have a restrictor of .052 in the carb where the choke housing mounts to it is this correct ?

Sorry I don't remember seeing a restriction on any of the cars I have worked on. That is not to say some were not supposed to have them. It sounds like a solution to fine tuning a choke. Others may have more info on this point.

[WT8095]

The air is heated in the choke furnace (exhaust manifold ) and rises through metal tube with the insulated sheath to the choke bimetal heat actuated spring. Heat rises. The lower port on the choke furnace is for fresh cooler air which is drawn in displacing the rising heated air.  The cooler fresh air comes from the carburetor by way of the uninsulated metal tube (not necessary to be insulated) and connected to the carb by a rubber hose (because it doesn't get hot like the heated side). I hope this helps understand the loop.

Yes, heated air does rise relative to cooler air. However, with the size of tubing involved in this application, convection alone would not produce sufficient flow. The amount of force from convection would not be sufficient to overcome the amount of vacuum present on the inlet end of the tube (the connection on the air horn portion of the carb), which would result in flow AWAY from the thermostat housing. Thus, a stronger vacuum source is required at the choke housing to pull air toward the choke housing. And that vacuum is provided as shown in the illustration attached below.

These images come from the 1968 shop manual. I believe the choke is identical to the 1967 choke. If not physically identical, operation would be the same. Vacuum from the intake manifold is routed through the carb to the choke housing. It is this vacuum source which is drawing air through the choke heat tube, starting at the air horn inlet, making its way downward through the manifold connection and upwards until it reaches the choke housing. Some chokes such as this have an assist piston, but others that rely entirely on the thermostatic spring have vacuum routed the same way. I've included the description of operation as explained in the manual; I'm sure it will make more sense than my brief explanation.     The only thing it doesn't explain is the inlet connection to the air horn - why connect that end to what appears to be a vacuum source?

The reason is, the engineers wanted a source of clean, filtered air to prevent dust from clogging the tubing and the choke mechanism (and the carb passages too). Instead of having an inlet right at the manifold and adding a small filter, they used another filter that was already present - the one cleaning the air entering the engine. So they tapped into an area in which air has passed through the filter. This area is at a lower pressure than the air in the engine compartment, but that's not a problem, since the choke housing is ported to an even stronger source of vacuum, thus ensuring flow goes the correct direction.

Ultimately, while this system works, it is a bit complicated and is subject to leaks, kinked tubing, and air filters that don't quite keep everything out that they're supposed to. Thus the eventual switch to electrically heated chokes.

[Bob Gaines]

Yes, heated air does rise relative to cooler air. However, with the size of tubing involved in this application, convection alone would not produce sufficient flow. The amount of force from convection would not be sufficient to overcome the amount of vacuum present on the inlet end of the tube (the connection on the air horn portion of the carb), which would result in flow AWAY from the thermostat housing. Thus, a stronger vacuum source is required at the choke housing to pull air toward the choke housing. And that vacuum is provided as shown in the illustration attached below.

These images come from the 1968 shop manual. I believe the choke is identical to the 1967 choke. If not physically identical, operation would be the same. Vacuum from the intake manifold is routed through the carb to the choke housing. It is this vacuum source which is drawing air through the choke heat tube, starting at the air horn inlet, making its way downward through the manifold connection and upwards until it reaches the choke housing. Some chokes such as this have an assist piston, but others that rely entirely on the thermostatic spring have vacuum routed the same way. I've included the description of operation as explained in the manual; I'm sure it will make more sense than my brief explanation.     The only thing it doesn't explain is the inlet connection to the air horn - why connect that end to what appears to be a vacuum source?

The reason is, the engineers wanted a source of clean, filtered air to prevent dust from clogging the tubing and the choke mechanism (and the carb passages too). Instead of having an inlet right at the manifold and adding a small filter, they used another filter that was already present - the one cleaning the air entering the engine. So they tapped into an area in which air has passed through the filter. This area is at a lower pressure than the air in the engine compartment, but that's not a problem, since the choke housing is ported to an even stronger source of vacuum, thus ensuring flow goes the correct direction.

Ultimately, while this system works, it is a bit complicated and is subject to leaks, kinked tubing, and air filters that don't quite keep everything out that they're supposed to. Thus the eventual switch to electrically heated chokes.

FYI there is also a metal fiber filter typically made into the exhaust manifold. If it is clogged up which is typical after 40 or 50 years then no matter how good the air draw the choke system will not work properly. NPD sells the rebuild parts. They are not fun to change. 

[WT8095]

It seems that with this restriction the spring is winning the tug of war with the vacuum and so Im having problems with setting the choke and getting it to stop, thanks Paul.

Can you describe in more detail what is happening, for example, is the choke opening too soon, too late, not at all? What is the air temperature where you're working? Is the engine hot or cold?

[WT8095]

FYI there is also a metal fiber filter typically made into the exhaust manifold. If it is clogged up which is typical after 40 or 50 years then no matter how good the air draw the choke system will not work properly. NPD sells the rebuild parts. They are not fun to change.

Oh yeah, I forgot about that. Thanks for reminding me, I have to remember to check that when I put my motor together! From a design standpoint, I believe it was not intended as a filter; it was meant to increase the amount of heating area for the air passing through the tube. Like radiator fins. It would be a lousy place to put a filter, especially since they went to all the trouble to route the inlet line up to the carb to take advantage of the filtered air!

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I have read the info you posted and it is the same in a 67 shop manual thanks. One thing it doesn't mention is the restrictor in the carb housing where the choke housing mounts to the carb base, its right at where the cork seal goes, in this carb it measures .052. I did make use  of a NPD choke stove kit as the  new tubes are stainless steel and are holding up very well. Also it comes with new material for inside the exhaust manifold, its a VERY course steel wool looking material. It seems I cant quite get the choke to set properly as the fast idle cam wants to stay on even warm and adjusting doesn't seem to help, Thanks Paul.

[67gtasanjose]

Is the port going into the carb open? Verified open? Is it possible to remove the restrictor you speak about? Otherwise if linkages are assembled correctly about the only thing else to try is possibly another bi-metal choke cover

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The spring that's in the choke is marked with 132 is it possible to get different springs ?

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the port is open and it needs the restrictor because of the helper piston and its surface area, Im guessing here.