Seems like a flow restriction more than anything, but is there really any restriction if it is working properly?

Probably minimal, but there's the issue of heating up the incoming air/fuel misture too.

Cons are the valve getting stuck in the closed position. Mine did that on my 318 back in 1981. I cut it out. However, at that time I went to a junk yard and saw a hill of exhaust manifolds and found the exact same one but without a heat riser valve and used it and never looked back.

My 2 cents....
please take this for FWIW...
just an opinion and nothing else.
My experiences are drawn from a 70 318....
big block experiences may be different...

Here goes...

Background
==========
The heat riser, the choke, the heated air intake including the heat stove and air cleaner, are all suppose to work together to help the car start quickly when cold, warm the engine up quickly
when cold, reduce emmissions at initial startup and fast idle, and allow the car to be driven cold.

In addition, 2 very important settings on the carb are also quite critical, the 2nd step on the fast idle cam (choke postioned nearly closed, and the fast idle speed (RPM) for that setting which is set after the car is fully warmed up...

When all of these are functioning properly, and
set correctly, the car fires first time everytime
(COLD), goes immediately into fast idle, steps down twice (tap on the gas pedal) to normal idle speed within 1 minute and is driveable cold.

Except on very, very cold days ...below freezing etc. The fast idle to normal idle speed drop down time is longer, maybe up to two minutes.

Take any of those pieces out of the equation for any reason, deliberate or otherwise and the whole system is compromised... but what gets comprised is cold weather related stuff..

Meaning its either hard to start, slow to warm up, poor driveability cold etc...

If the heat riser is stuck open (or removed)the car will spend quite a bit of time in fast idle..assuming the choke closed corrrectly and the fast idle cam was set right... .it will be very slow to warm up...and probably have very hard to drive cold. You can't put it in gear until the fast idle steps down to normal idle...

This is classic scenario for an old mopar you
have to start and let warm up for 10 minutes before you can go driving it.

At least that has been my experience.

If the heat riser is stuck closed, the car may
run warm and even overheat in the summer time especially if you run factory A/C, but you will have none of the cold related issues...

Also, the carb will get so hot it will
eventually warp (Most of the old Carter 2bbl carbs suffer from this although you'll be hard pressed to have anyone admit it)

You'll lose some performance because of the back pressure increase...etc..

On a car with a heat riser stuck closed.. after the car warms up you can take a long screwdriver
and rotate the counter weight to open it. You'll
actually hear the engine idle slightly faster
when the restriction to the exhaust is removed (by opening the valve!)...

Bascially the car was designed to have the heat riser heat the base of the carb to get the fuel to atomize quickly when cold...

That's important if you want to start the car
and get of the driveway quickly in the winter time. Daily drivers of old needed this...

Today most mopar owners are'nt concerned about that. Most of us start 'em up and let them run for a few minutes... and then off we go...

SO what happens if yours doesnt work..
Well... your not alone I've NEVER felt like
they worked well at all even when rebuilt,
They are either slow to open never really seem to open all the way etc...

Given that original ones are 35+ years old.
Its hard to expect them to still work..
Replacements frequently just don't work very well..even NOS replacement rebuild kits (I've done those!) are'nt a fool proof solution.

Your best alternative...
If you remove the heat riser...
Make sure your heated air system works
and switch to an electric choke!

Electric choke conversion kits for the old carter 2 bbls are out there and they work very well...
I've done this..with almost no impact on cold weather starting, idle, or driveability...


Hope this helps

Good luck!

It can cause running problems when the car is warming up if it doesnt work. Ive never seen one stuck shut usually the stops or other parts get rusty and fall apart.

My GTX had it removed and it took it a good 20 minutes to warm up in the middle of the winter. Didnt bother me much though. But man would it bog if you tried to take right off! Still spun the tires though haha.

Also it makes the paint wear off your intake manfold on the crossover.