Icing Manifold. AIR-TECH

Many people are coming in complaining that their motors are running rough in the cold weather. This is the result of the heat riser being plugged. If you have a stock carburetor or a single carb set up this can be caused by icing that takes palace under the carburetor. Feel the manifold under the carburetor after you have been driving for awhile, if it is cold to the touch, or if you see frost around it, this is your problem.

What does icing have to do with the performance of my motor? Here’s what is happening, the gas and air are going down the intake at a high rate of speed creating a wind-chill, causing the sides of the manifold freeze. To put it another way, if it’s 100 degrees outside and I dip your hand into gas, thenI have you hold your hand out the window and we drive down the road at 100 mph, does your hand get cold? Hell yes it does. Once the manifold freezes, it will draw the moisture (gas) out of the mixture and ball it up on the sides of the manifold. It eventually will run down the side and make it to the combustion chamber. Now, raw gas does not burn well, it’s the fumes (or mixture) that burns. To put it another way; The smaller the droplets of fuel mixed with air, the better it will burn. The reverse is also true; the bigger the droplets, the worse it will run. With icing you’re having large droplets run down instead of a nice air/fuel mixture, making it very inefficient.

Now that you know what the symptoms are, here’s how you can figure out if that’s what’s happening to you. Start the car and it might run good for a while. Then, as you keep running it, it starts to run worse, lacking power. It may be coughing through the intake and won’t idle and has a flat spot from hell. Shut the motor off and feel the intake just below the carb. Is it cold or frozen? If so, it is its icing. Let it sit for hour with the motor off before you turn it back on and it will run fine for awhile again before it starts acting up. Most of my customers come in and buy tune up parts which they don’t need. Why? Because these are also symptoms of bad ignition parts. Then they pull the spark plugs and they’re black. Why? Because raw gas doesn’t burn, it’s the mixture that burns.

So how do you correct the problem? If you are running a header, more than likely the heat riser stand on the header has not been drilled into the exhaust pipe. I have also seen where the hole in the header is so small that it doesn’t let enough of the exhaust to travel up. Make that hole as big as you can. The header will have to be removed in order to drill these. Now, if they’ve already been drilled or if you are running a stock muffler, then the heat riser tube is probably plugged. It is easier to clean this tube out if it’s off the car, but not necessary. If you are going to clean it when it’s still on the car, you must remove the exhaust. To clean it, you need a 16” piece of clutch cable, a drill and a thick pair of gloves. Spin the cable into the drill so the cable wraps up as it spins in the manifold. Hold the drill with one hand and guide the cable with the other. Go slowly so the cable doesn’t whip around and hit you. NEVER REVERSE THE DRILL! If you do, the cable will get stuck; you will never get it out and have to throw the intake manifold away. Repeat this procedure on both sides until you can blow air freely through the heat riser tube. This should solve your problem.

I have also seen many times that it is impossible to clean these tubes out. If that is the case, go on the hunt for a good used manifold where the heat riser is clean or cleanable. I personally have had no luck with the new aftermarket manifolds with the push in tubes. For some reason, they just don’t transfer the heat to the base of the carb.  On some of the two barrel carb set ups, even with the heat riser clear and working, the manifold can’t get enough heat. I really don’t know what to tell you here at all. That is why I love dual carbs with aluminum manifolds, you don’t need a heat riser. The heat comes off the head and travels right up the manifold.

So there you have it. Ice should be made in the fridge, not your intake manifold.