It is appropriate that we discuss this topic after we have explored the topic of octane and premium gas as we did in the last article.

Most people know you can decarbon an engine while it is running by introducing water into it. It’s controversial because folks either consider it a used car lot hack similar to putting banana peels in the differential, or because they do it incorrectly and see no result. But it is not a bad thing to do. It’s a very effective thing. And there is a wrong way to do it, as with so many things.

A water tune is called for when a compression test indicates some very low cylinders and a subsequent leakdown test shows the reason to be heavily-carboned valves. Hondas don't like a lot of carbon buildup and often respond by their valves leaking. I see this very often and it usually happens when premium gas has been used over a long period. Of course, other things cause higher than normal carbon buildup also. Fuel additives, incorrect air/fuel mixtures, overuse of the choke, and oil consumption in the combustion chamber are some of them. If on the other hand a leakdown test does not point to carbon as the cause of low compression, then one or two checks more will determine whether some engine top end work is needed.

Here is the correct water tune technique. Get an old glue bottle, cleaned of glue or anything else inside. It should be transparent or at least translucent. Add to this a 6-inch length of clear hose that will fit tightly on the end of the bottle. You also need at least one carburetor syncing vacuum adapter (probe), and make sure the hose you are using fits snugly on this probe.

Warm the engine to operating temperature. Shut it off and install your sync probe. Fill the glue bottle 2/3 full with water. Attach bottle and hose to the sync probe and fold the hose completely in half or pinch it with a hemostat to prevent water entering the engine when it is started. This is important. Uncontrolled entry of water into the cylinder can result in cylinder hydrolock that can bend a connecting rod. Don’t discount this danger.

Restart the engine. Put a fan on it to keep it from overheating. The hose is still bent in half. Raise engine rpm to a minimum of 3,000 rpm and carefully unkink the hose partway to the point that dribbles of water are sucked into the intake manifold. The engine rpm is kept high because you want to elevate engine heat somewhat to maximize the steam conversion, and because the engine won’t be able to idle while you’re doing this. Keep the rpm up, and carefully limit water intake. You want dribs and drabs to enter the cylinder, not a stream. Slowly. Watch the water level in the glue bottle. After a quarter to half inch of the water has gone into the cylinder, pinch the hose shut again and give the engine a few revs to dry out the cylinder. Then shut the engine off, move your sync probe to another cylinder (remembering to replace the previous cylinder's vacuum screw) and repeat the procedure for each of the other cylinders. When all the engine’s cylinders have been treated, it’s a good idea to ride the bike a bit afterward to ensure no water vapor is left in the cylinders.

What does the water tune do? What happens is the water hits the combustion chamber and instantly turns to steam, very effectively knocking loose the carbon that is adhering to the piston crown and valves. It's a classic procedure that mechanics have know about for a long time. I helped a student do this on his Harley-Davidson Sportster many years ago and carbon visibly flew out the exhaust and speckled an onlooker's trousers. It really works. Many have asked why simply spraying water into the carburetor entrance using a plant sprayer wouldn't do the same job. It won't. Water intaked this way is already atomized—it has too much air mixed with it—it is too finely atomized to provide the shock we want the combustion chamber to receive.

The water tune isn't shade-tree. But neither is it a cure-all. It's basically a method for reversing formerly bad engine maintenance that allowed the engine to accumulate carbon excessively. The procedure can be very effective, even though it can't replace, and won’t be as effective as, a proper valve job. Recheck your valve clearances after the water treatment because when significant amounts of carbon are removed, the valve clearances will change, they’ll tighten up. A repeat compression test is a good idea also. You want to gauge how effective the procedure is, and you want to know if the engine actually needs rebuilding. The water tune won’t overcome a worn engine.