Clearing a Blocked Heat Riser

The heat riser is the narrow pipe underneath the horizontal induction manifold. It is attached to fittings on the muffler/header pipes for the two rear cylinders, and carries exhaust heat up to the inlet manifold under the carburettor.

The heat riser forms two important functions. It prevents ice forming under the carburettor (caused by the fuel evapourating and cooling the inlet air below freezing) - icing will eventually block the manifold and stop the engine. It also ensures that the remaining fuel droplets stay suspended in the air stream and start evapourating, and do not stick to the insides of the manifold (a problem with all long manifold designs), which results in fuel pooling and uneven mixtures. Hopefully you are aware that LIQUID fuel does not burn - only fuel VAPOUR mixed with air burns, and a little heat under the carburettor certainly helps the fuel droplets turn to a vapour.

Since the heat riser uses exhaust gases, it can be blocked over time by a build-up of baked-on carbon inside the pipe. A worn, oil-burning, poorly tuned engine will block the heat riser faster than a fresh well tuned engine (there’s a message in there!)

You can do the first part of this procedure with the inlet manifold still in the car, but you get a better result if it's removed.

1. With the muffler removed (or the inlet manifold off the car) you have access to the ends of the heat riser.
2. Cut a length of old clutch cable (or similar stiff but flexible cable) about 18 inches long - just over half the length of the heat riser itself.
3. Fray a few of the wires at one end - bending them outwards to form a tiny wire brush on the end of the cable. Only a few mm of bent wire is needed since it has to travel through the squashed D section in the middle of the heat riser – where there isn’t much room. It doesn't have to be especially neat either. The steel in these cables are TOUGH - it will take some work to bend the wires. Another option is to cut the end of the cable at a 30-45 degree angle with bolt cutters - this will make a sharp pointed "chisel" end. Either system will work.
4. Put the other end of the cable into a battery drill and spin it BACKWARDS so the cable doesn't unravel. It's important to use a low speed like the battery drills have so that the cable does not whip about uncontrollably. A power drill with low speed and reversing functions might work OK too.
5. Feed it into one end of the heat riser. It's hard to get around the corner and also hard to get into the D section in the middle, but persevere until you know it's more than 1/2 way through, then try again from the other side. You will probably have to hold the length of cable steady as you feed it in - I found a leather glove useful. It may jump and jerk as it cuts into the baked-on carbon. If you try to use a full length piece of cable it will flex and whip about too much - hence the ½ length recommended. It may go in like a hot knife through butter, or it may be quite resistant, depending on how hard the carbon is baked inside. If it's really tough, you might need to stop and empty out any cabon crud before proceeding again.
6. If you have the manifold off the car, you now upend it and place on a bench or in a vice so you can fill the heat riser with carburettor cleaner and leave it to soak overnight. Next morning, drain the carburettor cleaner and ream the heat riser with the cable again - this will scrape out all the extra bits loosened by the carburettor cleaner. Flush with water and leave to dry before putting it back on the car.

Note: If you don't have any carburettor cleaner but have some paint stripper in the shed, you can make yourself some carburettor cleaner since many carb cleaners use the same active ingredient - Methylene (or Methyl) Chloride. Dissolve about 10% paint stripper in 90% kerosine. Always wear heavy rubber gloves (not thin latex) and eye protection when using carburettor cleaner - it's rough on skin and eyes. You can re-use carburettor cleaner over and over - just keep it in a large metal tin with a tight fitting lid (Methylene Chloride evapourates) - a large coffee can or similar.

7. If you live in a warm climate, try to get one "small hole" gasket for the heat riser. The seal kits seem to always have two large-hole gaskets, but in warm countries VW originally used a small hole gasket in the downhill side (the one with the curved pipe leading into the muffler) to reduce the total heat in the riser and the manifold, and some VW shops may still have the small hole gasket available. You could also make yourself a suitable small hole gasket out of copper sheet (copper is semi-crushable for a good seal). Try a hole of 1/4" (6mm), and if the manifold above the heat riser does not get very warm, enlarge the hole a little. You can of course just use the two large-hole gaskets in the seal kit, but the manifold above the heat riser may get hotter than it needs to - nice and warm is just fine. A too-hot inlet mixture reduces engine power slightly, and increases engine temperatures too. In cooler climates the two larger hole gaskets will work best.