Chrysler L Head Heat Riser - Repair or Scrap?

[likaleica]

Happy New Year from England to all - and happy Moparing in 2017!

I've seen conflicting views on here about the merits of the heat riser between the exhaust and inlet manifolds on the L head straight six - as on my 39 Plymouth Touring Sedan. As we know, this is the device that when closed, allows some of the exhaust gases up into the inlet manifold and when open (see photo), directs virtually all the exhaust gases out through the exhaust system. On the one hand,  the excellent forum technical article on the heat riser says:

Quote

The heat riser is an important part of the fuel system. A riser valve stuck in the open position causes problems such as slow warm up, carburetor icing, stalling and flat spots during acceleration. Another serious problem is the potential for crankcase dilution caused by excessive choke during starting and warm up. A valve frozen in the closed position is just as detrimental. Common problems include hard starts when hot, detonation (pinging under load), overheating, burned valves and a warped manifold. Regular maintenance of the heat riser should be on your monthly checklist.

End of argument then...............but  I've seen other members proposing to scrap the heat riser!  So who is right? My own experience of interlinked inlet/exhaust manifolds is from the famous Riley twin cam hemi head 1.5 litre. This is a crossflow engine with tubes running through the cylinder head and engine block to link the manifolds. Enthusiasts commonly blanked these tubes off and reckoned the engine ran better as a result.

I've been forced to replace the exhaust manifold on my 39 Plymouth and cant decide whether to repair the heat riser,  weld it in the open position - or remove it and blank off the exhaust from the inlet manifold with thick steel plate. I'd be very interested in any views on this!

 

[plyroadking]

It probably depends on the weather conditions that you are expecting to drive in.

 Ten years and around 75,000 miles ago I bent my very frozen in the closed position one back and forth until the welds on the flapper broke. I lived in California at the time and it never seemed to affect the engine, I routinely drove it it the winter when it was in the 30s. After I moved to Iowa and visit the car in wonter storage I notice that it takes a lot longer for it to come up to temperature when it's really cold, teens and lower. I'd imagine if I started the engine numerous times in the winter here I would over choke it and would need to replace the manifold with one that had an operational riser.

[1949 Wraith]

My heat riser was frozen closed when I got my 1949. I didn't realize that it was, until the exhaust manifold cracked at number 1 cylinder. I forced the heat riser into the full open position and have been driving it like that ever since.

I have been driving the car in below freezing temperatures when the roads are dry and I have not had any issues with freezing gas(might be because of all the ethanol used in gas these days). Spark plugs are a nice light brown so no issues with rich mixtures. I do have a new manifold and heat riser assembly and I will install both when I get around to doing the job. 

My 1937 Hudson has a heat riser that you would adjust and fix in place with a nut according to the season. Not much different than I have now with my Dodge.

Seems like lots of people loose the heat riser, without problems, when they do multiple carb set-ups or dual exhaust.

[likaleica]

Thanks plyroadking and 1949 Goat for those very useful remarks. Living in England we can encounter cold weather tho usually not extreme - and thanks to climate change now mostly avoid very harsh winters. Added to that I usually store the Plymouth over winter, just giving her the occasional start up and run along the lane on my land to keep everything free. So far I'm erring toward welding the riser plate in the open position............

[knuckleharley]

1 hour ago, likaleica said:

Thanks plyroadking and 1949 Goat for those very useful remarks. Living in England we can encounter cold weather tho usually not extreme - and thanks to climate change now mostly avoid very harsh winters. Added to that I usually store the Plymouth over winter, just giving her the occasional start up and run along the lane on my land to keep everything free. So far I'm erring toward welding the riser plate in the open position............

I'm confused. To me,the "open position" is when the exhaust gases are free to get into the intake manifold to heat it and the carb base. Another reason for this is so the automatic chokes will kick off once they reach a certain temperature. No big deal for those of us who have manual chokes,but I can see where it might be a problem for the guys running automatic chokes.

To ME,if  you want to shut this down you would weld or braze the flap shut so no exhaust gases would be able to reach the intake manifold.

 

Of course,once the engine warms up the intake manifold and carb will be warm enough an automatic choke SHOULD shut off anyhow. I just don't see how not having a open flap so exhaust heat gets to the intake manifold and carb would be any more than the tiniest of inconveniences in a properly operating engine with a thermostat. Once the engine is up to normal operating pressure the flap would shut anyhow to keep the intake and carb from getting too hot and boiling the gas away.

I guess it would make a difference if you lived in the great frozen north and only drove a mile or so at a time to work or shop and the engine never really warmed up,but that's it.

 

Am I wrong? Anyone?

 

[likaleica]

Quote

To me,the "open position" is when the exhaust gases are free to get into the intake manifold to heat it and the carb base. Another reason for this is so the automatic chokes will kick off once they reach a certain temperature.

I agree the term "open" is open (sorry!) to interpretation. I was following this description for the flap position when the gases go mainly through the exhaust system because that is the term used in the forum technical article on the heat riser.

[1949 Wraith]

3 hours ago, likaleica said:

Thanks plyroadking and 1949 Goat for those very useful remarks. Living in England we can encounter cold weather tho usually not extreme - and thanks to climate change now mostly avoid very harsh winters. Added to that I usually store the Plymouth over winter, just giving her the occasional start up and run along the lane on my land to keep everything free. So far I'm erring toward welding the riser plate in the open position............

likaleica, I don't think you will have a problem welding the plate so exhaust has free flow. Car ran great  today in the upper 20's, with riser frozen in the open position. Doesn't run any different than when it is in the upper 90's

[desoto1939]

The heat riser is used when owners live in cold climate areas and is to help prevent the formation of an iceing effect onthe bottom of the carb base. When the weather is cold the vaporized fuel then ices up and forma a white layer of ice on the base of the car and then this will cause you to have a very rough idle and also cause the car to stall out. 

The option is to pull out the throttle to keep the car running at a higher rpm and then when you are going again after a stop then you can lower the rpm via the throttle.

Rich Hartung

desoto1939@aol.com

[st63]

My own experience with this issue came when I purchased a 1950 P-20 sedan last summer.  The previous owner had issues with hard starting when hot and fuel boiling in the carb.  He installed a heat shield under the carb, and that seemed to "fix" the issue, at least temporarily.  He also had issues with the automatic choke not functioning, so he converted to a manual choke.  I discovered that the heat riser was frozen in the position where exhaust gasses were directed up at the base of the carb...ie: in permanent "warm up" position.  The manifold had also warped from the excess heat buildup. I wound up replacing the manifold with one with a working heat riser, and all of those issues went away. Personally, I figure the original engineers built it that way for a reason, and the best option is to return it to original.  Once you start monkeying around with the factory specs you'll open yourself up to a whole host of unanticipated issues and wind up having to use guesswork to figure it out. Just my 2 cents anyway.   

[Tom Skinner]

I wound up replacing the manifold with one with a working heat riser, and all of those issues went away. Personally, I figure the original engineers built it that way for a reason, and the best option is to return it to original.  Once you start monkeying around with the factory specs you'll open yourself up to a whole host of unanticipated issues and wind up having to use guesswork to figure it out. Just my 2 cents anyway. 

Bingo