VW Bus - Heat Riser Clearing

Most stuff is sorted at this point in the Bus-saga. There are a few things to do, but overall the bus is, as expected, in pretty good shape.

One issue is that when the car is running and warmed up, the base of the carb is sweaty with condensation and cold to the touch. This is a function of the swirling fuel charge on its way into the combustion chambers, and the increase in density of the charge as it makes its way to the plugs to be fired to run the engine. Usually though, that cold dampness is mitigated by a design feature of VW intake manifolds called the heat riser.

Here in preparation for the process to clear the heat risers, the rear valance panel and the tins have been removed from around the engine, but the muffler is still on. This is to diagram the process by which a heat riser works in warming the intake manifold and counteracting the 'frosty carb syndrome' described above.

And here is the highly specialized diagram cooked up by our crack team of highly specialized technicians here at the science and engineering suite of One Utahdog Center, World Congress of the Americas. In this shot, you can see the counterclockwise loop the heat riser makes which is symbolic of the flow of heated exhaust gases from the muffler and looping around underneath the carb and back to the muffler. Now of course, the system isn't specifically designed to flow as a loop I don't think, but there is a discernible warm side and a cool side which indicates to me that counterclockwise flow is present. Anyway, the heated air travels somewhat in the loop (purple) where it joins in a figure 8 profiled section of the manifold (red) that acts as a radiator of sorts on one side to warm the intake charge on the other side, and then the warm air returns to the exhaust where it is expelled from the back of the car. Clear as mud? Good!

Now the problem comes from when the heat riser, which is carrying carbon soot exhaust gases, gets clogged with sooty nastiness. The carbon soot cakes and coats everything in the heat riser over a period of years of operating the vehicle, and at some point it flows no air at all, and then 'frosty carb' ensues. The engine will run rough as it tries to ingest moist cold intake charge.

The way to clear the heat riser is to take a length of braided steel wire, in my case I went to the auto parts store and got a transmission selector cable for a full size truck and used the inner braided cable and threw the rest in the trash. The cable I used had a hexagonal bit formed on the end which I attached to the chuck of my cordless drill. Then, with the muffler removed leaving the ends of the heat risers open, and using the drill and a pair of heavy gloves, I guided the cable until I had punched through the carbon scale and had the cable hanging out of each end of the risers. Then I worked the cable back and forth and spun it with the drill to clear the scale from the walls of the risers.

The cable hanging out of each end of the risers is the goal here. Seeing this means free and clear risers and no more frosty carb!

Left side cable hanging out of the riser, complete with the natural occurring burr on the end from being worked up in the riser. This burr works like a chimney sweep to clear the riser.

And the right side riser, showing the hexagonal end i fitted into the drill chuck, and also the knotted mess that spinning the cable into a tight hole produces when the cable binds and wraps up on itself under power from the drill.

And finally, using the beheaded Silca bicycle pump, I pushed the rubber air hose as far as I could into each side of the riser and gave a few forceful toots on the pump to clear any loose debris that may still be sitting in the riser.

Job done! No more frosty carb!