Normally when the counterweight is forward the exhaust gases are recirculating around the inside of the manifold. This is the 'cold' position. This is done to heat up the manifold and the incoming fuel mixture as fast as possible to get more complete combustion. The reason,I think, the counterweight is in the recirculating mode when it's forward is because the forward movement of the car would 'fight' this position if it were the 'normal' position. Just my guess. The counterweight slowly moves backwards to it's rear rest position as the bi-metalic strip heats up and expands. Unwinding and 'rotating' to position the deflection plate inside the manifold in the non-recirculating postion. Where it remains for the remainder of the time the engine is running. You can move the counterweight or it's shaft rapidly back and forth and 'feel' and hear the delfection plate slapping back and forth. For the most part it's not needed in the warm weather we operate our cars in but it shouldbe there for authenticity. I repaired mine a couple of times when the bi-metalic strip broke at one end. I just rebent it over to lock it in the shaft and it worked again. I suppose the calibration is off now but I have watched it and it swings over in the first 3 or 4 minutes and doesn't appear to have any effect on the engine. But it must remain in the non-recirculating position to keep away from pre-ignition which can damage your engine on a hot day.? ? ?

The best solution is to take the manifolds apart and make a separator plate between them.  Dump that heat riser nonsense.  If you want to look stock, put a spring on it and leave it.  It does nothing but add heat to an already overheated intake manifold. 

In addition to the other posts, For the spring, check with Bernbaum, Atlas Motor Parts, or Len Dawson in Wash state. (see the bulletin ads). Also, The original counterweights are scarce and should be bolted securely to the shaft and checked regularly as they can loosen. If the spacer/stop is missing from the other shaft, most good  hardware stores like OSH/ ACE  carry a sleeve with a set screw which will work as a substitute. After separating the intake from the exhaust manifold, you can see the internal baffle and check the gasket condition. The spring tension should be wound only enough to keep the counterweight in the approx.2 o'clock position or against the stop when the engine/ spring is cold.
LR

Quote from: Phred on May 02, 2009, 07:40:12 AM
I have some questions about the heat riser mechanism on my P4, and would like some suggestions from others more experienced with the Plymouths than me.

The heat riser shaft that goes through the "pot" under the carb in the exhaust manifold is very sloppy. On the outboard side of the manifold has a visible and audible exhaust leak at this area. I am assuming the shaft goes all the way through the manifold and there is probably a similar condition on the inboard side of the manifold.

Is there/was there some sort of bushing for the heat riser shaft, or was it originally just a hole in the cast iron of the manifold?

Do any repair kits include some sort of bushing? If not, what type of material would make a good bushing in this hot environment?

When the thermostatic spring for the riser is broken and the counterweight on the shaft is down, is the deflector inside the manifold in the free flowing orientation, or is deflecting exhaust to the intake manifold?

Thanks in advance
Phred

               I can tell you from experience on a bit later vintage Plymouths, that the heat riser shaft does consist of a small sleeve or bushing, that is pressed into the exhaust manifold around the riser valve.
               When the counter weight is down, it usually means the exhaust is going straight down to the exhaust pipe.
You would not want the valve to be closed when it's broken.  A heat riser valve stuck closed would cause there to be a cracked intake manifold as well as poor performance.
               My suggestion is remove the manifold and take it to a machine shop to have the valve serviced, someone here may have a new thermostatic coil spring.

I have some questions about the heat riser mechanism on my P4, and would like some suggestions from others more experienced with the Plymouths than me.

The heat riser shaft that goes through the "pot" under the carb in the exhaust manifold is very sloppy. On the outboard side of the manifold has a visible and audible exhaust leak at this area. I am assuming the shaft goes all the way through the manifold and there is probably a similar condition on the inboard side of the manifold.

Is there/was there some sort of bushing for the heat riser shaft, or was it originally just a hole in the cast iron of the manifold?

Do any repair kits include some sort of bushing? If not, what type of material would make a good bushing in this hot environment?

When the thermostatic spring for the riser is broken and the counterweight on the shaft is down, is the deflector inside the manifold in the free flowing orientation, or is deflecting exhaust to the intake manifold?

Thanks in advance
Phred