1967 Jeep CJ5 V6 Heat Riser

[JohnnyCO]

Hi All.  I am doing a complete strip down and rebuild of a 67 CJ5 v6.  I am never done this before so I am learning as I go.  I am finally taking it in (as a roller) to get a new exhaust system.  I am going custom because I have not been able to find after market.  I have gotten different opinions on what to do with the heat riser.  I have the original coil but I have not idea if it is good.  I could not find a replacement coil anywhere.  Many tell me to cut out the flap in the heat riser and just use it as a spacer.  I read an old post by Moses where I believe he said it is important to leave it in for the function it plays during engine warm up.  So now I am not sure what to do.  Any advice or direction would be appreciated.

Thanks

John

[Moses Ludel]

JohnnyCO...Yep, you do recall my earlier comments.  Restoring heat risers has been my approach since growing up at Carson Valley, Nevada, cold starting engines in -20 degree F weather (before the climate shift).  If you're intending to cold start in the winter and have a reasonable warm-up period for the intake manifold's crossover passage, I do recommend the use of a heat riser.

On that note, there are more modern heat riser approaches that the muffler shop could incorporate.  Of course, you can find a generic heat riser with a coil spring and weighted valve arm.  A little searching, maybe a type the muffler shop knows already, could easily be part of the exhaust work. 

A quality muffler shop can certainly do a better job with the exhaust system than Kaiser did originally.  The early V6 CJs had the strange topside cross pipe and other quirks.  If you go with dual exhausts (not mandatory if the single exhaust pipe size is large enough), be certain to run the pipe(s) (single or dual exhaust) over the rear axle and out the rear end of the Jeep.  I never run side exit pipes.  Rear exit exhaust pipe(s) will help prevent fumes from asphyxiating you or passengers when crawling on a trail.

If you want to simply go with a spring loaded valve and weighted valve arm, here's one example.  There are likely sizes to accommodate what the muffler shop has in mind for exhaust pipe sizing and routing.  Make sure the weight is in the correct orientation to open the valve:

https://www.classicindustries.com/product/all-years/chevrolet/k5blazer/parts/3887023.html

Here are some additional choices, including Crown Automotive's 2-inch heat riser valve:

https://www.summitracing.com/parts/cwa-j3238998

There are also vacuum actuated heat risers that use a vacuum ported thermal sensor (coolant type) that serves as a thermal vacuum switch.  These vacuum switches have a pipe thread and screw into a cooling port on the engine.  They are preset to a specific temperature opening point, usually around 140 degrees F.  Many manufacturers switched to these vacuum actuated heat risers in the seventies and eighties to meet emission standards.  The vacuum (typically manifold vacuum sourced) applies when the thermal sensor switch opens the vacuum port to the heat riser.

All of this is about heat beneath the carburetor for complete combustion.  The intake manifold has a heat transfer passage that warms the carburetor base quickly.  The heat riser blocks exhaust at one bank of the V-engine and forces it across the intake manifold heat passage.  Make certain the heat passage in the intake manifold is unrestricted.  (This passageway is at the middle of the intake manifold, passing exhaust from one cylinder head to the other by way of the intake manifold.)  The passage can clog with carbon over the years. 

If the passage is not free, the heat riser valve would be useless and substantially restrict exhaust on one bank when cold.  If the exhaust pressure becomes high enough to overcome the heat riser's spring pressure, it will force the heat riser valve open. 

Another approach is a coolant spacer between the carburetor and manifold.  (Clifford does this for its manifolds.)  The only drawback here is response time.  The coolant will heat up gradually and not offer a great amount of fuel vaporization improvement until the engine is nearly warmed up.  By then the intake manifold has warmed.

Moses