What is the "hot soak" valve cleaning method?

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Over time, carbon deposits and varnish can build up on the valves, piston crowns and intake tract. These deposits are usually greater when the bike is frequently used to make short trips, as the engine does not reach full operating temperature. Frequent idling and low-rpm riding, especially when the engine or ambient temperature is cold, can also contribute to advanced carbon build-up.

Some carbon on the valves is not harmful, but as the deposits accumulate they can cause poor performance. Valve surface area increases, creating turbulance and disallowing complete intake and exhaust of fuel mists, causing imcomplete combustion and further deposits. Eventually, valves may stick, or the sharp edges or hardened deposits may cause hot spots and damaging preignition.

Rather than waiting for an engine rebuild to clean the valves, a simple treatment process can be used every 15000 miles or so to keep the system relatively clean. Basically, a valve cleaning solution is fed directly into the intake manifold of a hot, running engine; this "hot soak" quickly breaks up deposits.

Carbon valves.jpg

For this job, a 3" length of 1/4" I.D. clear tubing, a 1/4" O.D. T-fitting, and a bottle of valve cleaner are needed. Cut a few pieces of the tubing off, and use the T-fitting to make a configuration like this.


After warming up the bike, kill the engine and pull the intake manifold vacuum hoses, then connect the T hoses to the nozzles.

File:Intake fittings.jpg

Put the other end of the hose into the bottle of cleaner,

File:Valve cleaner.jpg

and use a small vise-grip or clamp to close it.

File:Line clamp.jpg

Start the engine, and turn up the idle to 2K RPM. Slowly release clamp pressure, so that the cleaner is drawn up into the engine; do not let too much be taken in at once, or the engine will stall. It is normal to see a white smoke exiting the exhaust pipe.

File:White smoke.jpg

After several minutes, the engine will start to labor as the carb bowls become empty. For a longer interval, the free hose connected to the tank petcock can be reattached to one of the intake nozzles to refill the bowls. When the intended treatment time has been reached, open the clamp so that the cleaner stalls the engine; do not start the engine again for several hours.

Reconnect the carb hoses and pour the remainder of the cleaner into the fuel tank, if desired. When next starting the bike, the engine may cough and sputter a bit, as well as belch out some nasty stuff from the tail pipe; this is to be expected. Go for a ride, and fully warm the engine up, then change the oil. Smoother idling and better efficiency may be noted thereafter.

There are some dissenting opinions.

A. Bill H: My suggestion is don't waste your time doing this. It won't make a bit of difference and could create some kind of problem you don't have now. As Marcus Aurelius said, or words to this effect, "Do what is necessary and skip the rest; the greater part of what we do and say being unnecessary..."

B. Ian: Same thing, but much easier:

  • Take a bottle of Chevron Techron Complete Fuel System Cleaner.
  • Pour entire bottle into the tank.
  • Fill tank up with fresh gas.
  • Ride until the tank is completely empty in one shot.
  • Fill up tank again.
  • Engine should now be perfectly clean.

Or you could do it the harder way:

  • Remove carbs and spark plugs.
  • Spray cleaner onto valves through intake ports, when valves are closed.
  • Spray cleaner into spark plug holes when piston is at TDC.
  • Let soak overnight.
  • Change oil.
  • Reinstall carbs and sparkplugs.
  • Go riding.