Packard Forum Index » Pre-War (1899-1942)

Hi All,
Three questions:- Firstly what is the actual function of this valve? Secondly:- When looking at the manifold directly under the carb I have two springs. On the left hand side an expansion coil spring which connects to a bracket and the valve shaft. On the right hand side a spring which looks like a bi-metal strip connected into the main shaft and has a hooked end which is just floating about in the breeze. There is no other bracket in the area for it to hook onto. Should there be one and what does it look like?
Thirdly what effect on the running of the car with this being inoperative?
All help much appreciated
Regards
Terry

The function of the valve is to aid vaporization of fuel during cold startup by diverting exhaust gases against the base of the intake manifold, sometimes called the heatbox. If the valve is inoperative but stuck in the open position, the only negative will be slightly poorer performance for a few minutes while the engine warms up. If the valve is stuck in the closed position, you'll basically be constantly heating the fuel mixture and indirectly the carburetor which could lead to vapor lock, gasoline boiling within the fuel bowl, etc.

The spiral spring is bimetallic which is to say it winds or unwinds with the application of heat. In a cold engine at rest, it is holding the flapper valve in the closed position, as the engine warms, it relaxes and the counterweight on the other side insures that the valve begins to open. The little coil spring is probably just an anti-rattle spring.

If the shaft is free to rotate, you might want to just buy a new heat riser spring and repair it. If the shaft is frozen and you don't know which position it's in, you'll face removing the manifolds to free it up unless you get fortunate with some PB Blaster and a bit of physical persuasion.

Hi Dave,
Thanks Dave for the reply. Everything is nice and free but the bi-metal spring is only connected to the shaft and there is nowhere to connect the hooked end to. Should there be a bracket or cover or something for it to latch onto/
Terry

There should be a "pin" that is connected to the manifold parallel with the shaft which the other end of the bimetal coil attaches to. See attached picture:

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Glad to hear it's not frozen, by all means make a proper repair. On the V8 models it's a separate piece between the maniflold and the exhaust system and decided to leave it off when I fitted a new exhaust - I've regretted it ever since, even here in a moderate climate. If I ever need to redo the exhaust, it's going back on.

Hi Kev,
I don't have anything as simple as a pin such as the one shown in your photo. There is no sign of a sheared stud or any holes for it to go into.
This is what I have got. I hope it has come out and shows up on my screen opposite the remove panel below
Thanks
Terry

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I do not know about pre war Packards specifically. So i can only guess.

Somekind of control cable or other mechanism may have been attached to the heat riser to allow controlling it from inside of the car. Look for any extra holes in the dash or evidence of a bowden cable set up.

29-32 L29 Cord used a seperate "carb heat" control cable thusly labeled on the the dash.

Somekind of control cable or other mechanism may have been attached to the heat riser to allow controlling it from inside of the car. Look for any extra holes in the dash or evidence of a bowden cable set up.

No cable or control mechanism. Do you see the two protruding threaded studs? A circular stamped steel cover with vent holes for the heat flow mounted over the circular spring and retained the end, and nuts and washers on the two threaded studs retained the cover. When the nuts and washers were loose, the cover could be rotated to adjust the tension on the bimetallic spring.

I believe this was essentially the same on the 320 and 385 engines from when they went to downdraft carburetion in 1933 thru 1939 perhaps. Here's a sketch I made of my 34 heat riser many moons ago, the cover/spring retainer detail is in the lower corner.

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This same system was used on the 356 engine at least until 1947.

And on through 1950.