Heat Risers

Note: If the engine is in good condition the heat riser should not block up. It's mostly when it gets worn and starts to burn excess oil that the carbon starts to build up in there.

The pre-heater tube that runs underneath the intake manifold -- is this designed to atomize the fuel? Is that its primary function?

Very important item.

The thinner horizontal pipe under the inlet manifold uses exhaust gas heat to stop the vapourising fuel in the airstream from cooling the mixture so much is causes the water molecules in the airstream to freeze. If this happened, a blockage would develop directly under the carby and the engine will eventually sputter to a stop until the ice melts.

Humid days with temps a little above freezing are worst for icing. If the temps are well below freezing then the air is very dry anyway, and with summer temps the cooling effect of the vapourising fuel is not suffient to cause icing, but even in these situations the heat riser has an important function - it keeps the fuel vapourised in the long inlet manifold, otherwise it can reform droplets and that makes for a patchy mixture inside the cylinders.

On engines with twin carbies sitting over the heads, you don't need a heat riser, as the inlet airstream is drawn straight into the hot heads, so no ice can form.

The manifold pre-heater tubes are the two tubes that run from the muffler on either side and then connect at the bottom of the intake manifold directly under the carburetor, right?

You've got the description right.

I've got the earlier one-piece manifold, and yours should be the three piece manifold.

The pipe which rises from the exhaust manifold and traverses across under the induction manifold is the 'heat riser' (I'm using the US description -- here we usually call it the manifold pre-heater. There is also the carby pre heater pipe, so it can get a bit confusing -- I'll mention this one again later). The function of the heat riser is to warm up the inlet manifold and prevent icing under the carburetor -- it's VERY important to have this working or the engine will suffer icing and stumbling on cooler days.

When the engine has been warmed up, this heat riser should be sizzling hot at the ends near the heads (finger burning hot), and the main inlet pipe above should be warm to touch. If it's working well, it may be hotter at one end and just very warm at the other -- depending on the muffler design. This is normal -- the original muffler design has a one-way flow of exhaust gases through the heat riser, so the "up" side will be a little hotter.

The heat riser gets it's heat from the exhaust gases from #2 (right rear) cylinder. The hot gases travel right to left through the heat riser, and, on the 'original' type muffler, there is a pipe on the left side which curves around the front (front is front of car) of the muffler and enters the front of the muffler opposite one of the two exhaust pipes. This arrangement ensures that the right side of the pipe is at high pressure and the left is at low -- making for a good flow of hot gas. It should therefore be hotter on the right than the left, and quite hot to touch where it rises from #2 cylinder.

Some after-market muffler manufacturers take license with this design, and leave off the curved pipe on the left side, having the left side connect direct to the exhaust manifold for #4 (left rear). This means that you get a reversing pulse of gases back and forward through the heat riser as #1 and #4 exhaust valves open both sides are at high pressure. Not the best arrangement, but it will work OK in warmer climates where the need for extra heat is not so great. If this is your arrangement (when you said 'warm at both ends but cool in the middle' it made me think of this possibility) -- consider changing the muffler for one with the curved pipe attached to it if you notice ice forming on the manifold just under the carby, or perhaps even if it is just very cold and 'dewy' there after a run (engine thoroughly warmed up).

Icing usually shows up as serious stutters or lack of power when you accelerate after the car has run for a short time. It won't show up the moment you start the car, as the fuel vaporising in the manifold takes a bit of time to cool it sufficiently to form enough ice to restrict the manifold. It usually wont happen on REALLY cold days (snowy days) as the air will be drier anyway. Temps of about 5-15c and high humidity are the most likely times for icing, if the heat riser is not working right.

If the heat riser is only luke-warm or even cold, it's been blocked by crud in the exhaust gases and needs to be reamed out whilst you have the muffler off.

The other problem with some aftermarket mufflers is that they sometimes don't bother to even drill out the heat riser fitting at #2 cylinder, so no hot gases at all! Probably not a problem for you though, you said it was at least 'warm'.

I bought one muffler years ago that had a fiber washer (#2 exhaust manifold to heat riser) with NO hole in it! Punching out the hole fixed that problem anyway.

Assuming that the heat riser tube may be plugged, how does one go about correcting the problem? It looks to me like I'd have to remove the muffler; or is there a way to remove the tubes from the intake manifold? It's obvious how they are removed from the muffler. I hope this isn't the problem -- looks like a big job.

Removing the heat riser for mine (one-piece with the inlet manifold attached remember) involves removing the carby, tinware around the heat riser ends, and the inlet manifold and heat riser together. It's quite a fiddle. Yours may be a bit easier with the three pieces.

I read someone's advice a while back about how to clean out the pre-heater tubes -- it sounded like you have to remove the intake manifold to do it.

Yes -- you have to poke a stiff wire through it at a minimum. Shouldn't be a problem for you though-- you have a fresh engine, not a smoker. Keeping the engine in good condition and well tuned is the best prevention for clogged heat risers -- less crud in the exhaust. Old oil burners tend to clog the heat riser.

Cleaning it out if it's blocked is 'any way you can'. There's a procedure for it on our Web site -- you won't be able to do the "final" ream described there -- upending and soaking it overnight with carburetor cleaner, but just a ream-out as described (if needed) will get good results.

A wire 'bottle brush' like a battery terminal brush might work, an old screw driver as an auger (bit difficult around the bends), or you could try the last resort method -- heat the heat riser by blowing an oxy-acetylene flame through it until it starts to glow, then switch off the acetylene and blast through it with pure oxygen! You'll get a volcano of carbon sparks out the other end (take suitable precautions). John Henry (and others) say this works -- never tried it myself. As I said, the three piece manifold should be easier to do. Hopefully this won't be necessary.

According to a few people here, the only method with which works is to fray the end of an old clutch cable so it bites, and spin if in a drill, and it's VERY slow. Once you have a hole you can fill it with carb cleaner and soak for a day or two, then it cleans out easier.

I've got a '73 Super 1600cc. She runs just fine around town, fine getting on the highway, fine on the highway, but as soon as I slow down, ie. get off the interstate, engage the clutch, she stalls out on me. I usually pop the clutch and get her running again. Most of the time though, she won't even idle after I get off the highway. It usually takes a few minutes for her to be back to normal. I've tried adjusting the carburetor (Solex PICT 34), and adjusting the timing (009), to no avail. Am I just not adjusting it correctly?

Sounds like manifold icing. Even in warm weather, if the riser tubes that heat the manifold are blocked, you'll get ice in the inlet, due to venturi effect (the mixture is forced into a smaller tube, it moves faster and cools the inlet). You really need to take off you muffler, and try to clean out the riser tubes.

Easy to check: when it acts like it wants to stall, feel the inlet under the carb. If its frosty, or really cold, you have blocked risers.

(This is only really relevant to the bug that has run on the highway for a while.)

I took a swipe at the preheater tubes with a piece of emory cloth. It appears that they are constructed of carbon steel and were painted the same yukky cream color as the car! VERY rusty, so I'm going to sand them real good with emery cloth and paint them with corrosion-resistant paint -- maybe silver, I'll see. That will be easier with the fan housing off.

We talked about removing the center section of the intake manifold to get to the front left nut on the alternator pedestal. It can be done, but to do it you have to disconnect the preheater tubes from the muffler.

I told you that the pre-heater tube is not plugged. I was sure relieved to find that out. I did so by sticking the garden hose on one end -- and the water (together with a lot of rust and carbon) came gushing out the other! Then I finished cleaning and painting the intake manifold. Reconnecting them is a chore.

I tore into the muffler to try to get those pre-heater tube connections bolted together, with nice gaskets between the flanges. The connections were mis-aligned, and there was no gasket between the exhaust header and the pre-heater tube (there is supposed to be a metal/fiber gasket under each side of the heat riser pipe).

Those gaskets are the ones with the 'restrictor' holes in them. They are subject to full exhaust heat -- maybe 700-800C, and are made of asbestos or other similar heat resistant fibre.

It puzzles me a little that you had trouble lining these pipes up, as the heat riser is three piece on yours. Oh! I'd forgotten that the three-piece manifolds still have a one piece heat riser, so the "fiddle" to get them to fit is just the same for both of us.

"Doing it all up loose" before tightening anything is about the only way to do it I guess. It might be partly the muffler design too if it has been made "not quite right" the heat riser hole near #2 cylinder could well be off line. Does yours have the left side heat riser bolted to a separate curved tube which entered the front (front is front of car) of the muffler, or does it bolt up to a fitting which is part of the #4 manifold? If so, these are more likely to cause fitting problems (and don’t provide as much heat riser heat either -- because the hot gases surge back and forth with the exhaust pulses on #2 and #4, where the original type has a one way flow front #2 to the lower pressure inside the muffler.)

There's bound to be a problem with fitting it then --vbthe match is never exact, and with this type both attachment points are fixed to the manifold pipe, making them difficult to move. The "correct" type has the left side attached to the separate tube which curves down to the muffler, so this is easier to move a few mm to fit it to the heat riser.

The type you have is supposed to be OK (for heat), but because it has an oscillating pulse instead of a uni-directional pulse from #2, it can block up with carbon etc. more easily as the engine ages and more gunk comes out the exhaust. And since BOTH ends are at high pressure, the amount of gas moved through the pipe has to be lower than with the "correct" muffler arrangement, so in cold conditions, when you most need it, your type can sometimes be marginal in providing enough heat to the manifold. I wouldn't worry about that part of it though -- it got through last winter so it should be producing enough heat for that engine. Just the "fit" is the problem.

My main worry in this regard is the gaskets. I'm sure the gasket material I have won't cut it temperature wise, and I suspect it would be difficult to cut gaskets out of material that would (if it's even available). Given the dearth of parts from wrecking yards, if you happen to run across a source for these, please let me know. Also for the little tin pieces that bolt around the pre-heater pipes.

Mid-America Motor Works, BFY Obsolete Parts, Aircooled.Net, etc. I'm sure will have them -- they are a common replacement item. The tin bits would be harder -- almost certainly a wrecker item.

I pried the flanges apart and slipped the new gaskets in -- duck soup. The left one went together nicely; the forward-most bolt on the right gave me a little trouble, but I used a little "friendly persuasion" and got it in. Now it's all buttoned up, with new gaskets, and I feel much better about it.