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The text following is part of a series of articles written by John S. Henry on the restoration and maintenance of air-cooled Volkswagens. While his experience is exclusively with the Beetle, many of the techniques can be applied to other models.

This text is copyrighted and duplication, re-distribution or publication is prohibited without consent of the author.

Article: Cleaning a Carbon Clogged Heat Riser Tube
Last updated: 10/31/01

Cleaning a Carbon Clogged Heat Riser Tube

Symptom: The heat riser tube is clogged / you are having icing problems with your carb.

{Note: I decided to re-write this article after actually having to clear the heat riser on my own ‘57s 36hp engine.}

Heat Riser Science

First, lets understand exactly what a heat riser is. It is metal tube through which hot exhaust gas circulates and whose function is to warm the intake manifold (namely the upper, mid rail below the carb) and allow the creation of warmer, denser fuel/air mixture to the heads. Exhaust gas flows from the right side pipe, which is connected to the muffler exhaust connection flange on the #2 cylinder; to the left side where the thin pipe in the muffler goes into the expansion chamber, right behind the left side pea shooter tailpipe. The reason the gas flows this way is that the right side connection, right off the left head, is at a high pressure area, the thin pipe behind the left pea shooter is a lower pressure point. And also, since the gas looses its ability to carry carbon particulates as it cools, most of the carbon build up is usually found on the left side of the riser tube. Thus, the right side of the pipe will be the hotter end, where the paint burns off of first.

And note that the tube is not round all the way through. Where it is “jacketed” with an alloy casting to the intake manifold in the center, it is crushed to a concave “D” shape to maximize contact and heat transfer to the intake manifold (hacksaw and old one apart in the middle if you want to see). Keep this in mind when are trying to shove all kinds of things through the riser pipe to clear it.

So what causes a heat riser to get clogged and what happens when it does? Well, on a properly maintained, stock engine, the tube should go 100,000 miles of service or more and never clog. But in a poorly tuned engine with incomplete combustion (poor timing, carb mixture, valve settings), over time, the carbon deposits can build up very quickly.

Equally suspect are poor quality “aftermarket” muffler and exhaust systems which do not maintain the proper flow through the pipe. One common poor quality “replacement muffler” has the pipe joined to the muffler/head flanges on both sides. Think about what this does: as each cylinder fires, the gas is just shoved back and forth in the pipe, no heat is really transferred, there is no real linear flow, and deposits build up. The left side muffler tube must go into the back of the expansion chamber for this to work!! Also, some replacement mufflers have small, sheet metal plates welded over (or in) the heat riser openings (some newer and/or aftermarket intake systems don't use the heat riser system). Prior to installing these mufflers, the blockoffs should be drilled out.

When a heat riser becomes fully clogged, the net effect may never be noticed. Mileage and performance may suffer a bit, and in cooler, humid climates, carb icing may occur (ice will form on the intake throat below the carb, and often in the carb itself).

Always check your heat riser by blowing through it any time you have it off (then wash off your lips so you don't look stupid the rest of that day). Another quick check is to run your engine for a few minutes and then feel each end of the riser tube. Both sides should be about the same temp. If the right side is warm/hot, but the left side is cool, the tube is probably clogged.

Fixes- The "SRB" *
{* Solid Rocket Booster}

 I actually employed one of the techniques that I originally suggested, and learned a few new tricks. But first, let me present the alternate, “torch” method that this article originally contained.

Note: I have not personally attempted this method of clearing the riser tube. But I have heard from some folks who have and have been successful at it, including Rich Kimball who writes for Hot VWs magazine.  Be aware that the alloy "jacket" that holds the intake and riser tubes together has a much lower melting temperature than the steel tubes. You must not get the manifold "glowing" hot. It will also distort under intense heat easily. As it has been explained to me, the trick is to use a very small torch tip and heat the carbon, just inside the the riser flange, then hit the oxygen.  One day I'll get an old junk manifold and try this.........

The torch trick is this. Remove the manifold and carb. Now, using and oxy acetylene torch with a very small tip/flame and the manifold secured in a vise (ie. not holding it with your hands), heat the inside of one of the riser tube openings until the black carbon glows red. Not too hot as to get the metal glowing; you don't want to deform the tube. Then quickly cut the acetylene on the torch and boost the oxygen when the flame goes out. The pure oxygen will fuel the carbon burn all the way into the tube. You may have to do it several times, alternating end to end until the tube is clear. USE GLOVES AND FULL FACE PROTECTION, this carbon burn can proceed like a small solid rocket booster when oxygen is applied!!!!

Fixes- The "TBM" *
{* Tunnel Boring Machine}

The second technique is the old Roto-Rooter trick. Routing out the riser tube with flexible "bit". Carbon buildup is pretty stubborn stuff. But persistence can triumph over it.

A popular technique (and this one is very useful in clearing a clogged fuel line too) is to sacrifice an old clutch cable (or even a brand new one, they are only $3 for crying out loud) and use it. The threaded end will fit into a drill chuck nicely, and the "business end" can be fitted with a kinds of neat weapons. But getting the clog "broken through" initially is key.

First of all, when cutting the cable, cut it to only the length you actually need. Too long, and it can get pretty unmanageable when you get it spinning. For a heat riser, you want it to be just a few inches longer than halfway through the tube, as you will go at it from both ends. If your tube is only constricted, and not completely clogged, you might want to make a square end cut in the cable. But if you want to fashion a bit of a "bit" out of the cable end, cut it on a fairly steep angle. Slice the cable with a carbide disc for a super clean cut. I use either a Dremel or a 3" disk on an air powered die grinder (see my Doing a "#4" Restoration, Page 1 article for pics of these tools) And if you realize that you cut it too short, and/or want another tool, you can used the "eyelet" end too. Just cut the eyelet off and the remaining shank will fit in a 3/8" drill chuck.

If the slant-cut cable end doesn't do it, you can actually attach other "bits" on the cable end by soldering on a small barrel and the bit. I used a female "molex" type connector (just a single insert), and soldered it on to the end of the cable using a small butane powered torch (those things are great for stuff like this, better than a soldering iron, or a full size propane torch; dirt cheap too). Clean the cable end well, use your best shiny metal soldering techniques. You can solder Dremel bits to the end using the barrel, but you might need to cut the shank down a bit to make the whole rigid portion of the end pretty short. If not, it might not be able to make the curve at the riser tube ends. Another thing that came to my mind as a suitable "barrel" would be a hefty ("yellow size") butt splice barrel, bare with no sleeve. But the bit that scored it for me was a glass/tile drill bit. On a 1/8" shank, you can get these in sets. They have little bullet shaped, pointed, flat tips. From the right side end, I couldn't get past the blockage. And after trying a bit, I couldn't even blow through it any more. But from the left side, with some repeated shoving, my bit poked through.

And speaking of soldering, there are other advantages to soldering the cable braid. One is that it will allow you to use a raw length of cable as an auger. Simply clean to shiny, one end of the cable, about 2". Heat it slowly with your torch and apply some solder. The cable should "wick up" the solder and this end will now not unravel. You can place such a soldered end right in a drill chuck and use the other end as you wish. Thus, you can actually get 3 augers out of a single clutch cable; the threaded end, the eyelet end (eyelet cut off), and the center section if cut to length correctly. Another advantage to soldering the cable braid is to prevent unwinding of the cable behind a bit or "chimney sweep" end (see below). This is especially true if you reverse the drill.

Once you have a small passage established, you can auger the thing out with a raw cable. If the passage is very narrow, start with an angle cut end. Run the drill with the spiraling of the cable, not against. If you spin the drill forwards (against the cable spiraling) the whole thing unwinds pretty quickly. Don't do this unless you create a solder "collar" just behind the business end. Even then, the cable can unwind between the solder and the drill chuck. If it unwinds a lot, it is hard to get it to go back nice. Work the drill back and forth, from each end. You should be able to feel the blockage freeing up. Once you get the bare cable able to slip through from both ends freely, it is time to build your "chimney sweep" end.

With needle nose pliers or wire cutters, bend just a couple strands on the end over at right angles, only about 1/16" from the end, then auger again. Use compressed air to blow the black dust out, and keep going until the cable slips through freely. Then, bend over some more strands, auger again, blow out. More and and more, bending more wire each time (about 1/8" at right angles). You are creating a "Chimney Sweep" type end on the cable, about 1/4 - 3/8"" in diameter overall. Then, heat the cable just about 1" behind the "chimney sweep" end with the torch, wick some solder in the there. Reverse the drill and go at it again.

I probably spent a good 2 hours auguring mine out, but when I was done I had piles of black carbon powder all over the floor and the airflow through the riser was VERY free. I started out with my cordless drill, but when the battery started running out, I switched to a corded one. The chimney sweep auger seemed to work better at higher speed of the line powered drill. You could shove the drill in and out fast and "feel" the blockage getting cleared away.

Here's some pictures:

A clean, square cut done with the 3" carbide disc.
An "angle" cut, again done with the 3" carbide disc.
The "Chimney Sweep" end. The fuzzy "blob" you see down the cable a bit is solder. It was a messy solder job, but it didn't matter. I only created this cable for the purposes of this pic (after I did my own heat riser).
Here's a "used" Chimney Sweep cable. You can see that it has unraveled a bit, but it was still very effective.

Copyright 2000; John S. Henry

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