Re: what to know about 706

Posted by Owen Aaland on March 20, 2007 at 08:53:40 from (65.165.246.14):

In Reply to: Re: what to know about 706 posted by Kelly C on March 19, 2007 at 21:30:36:

Burned exhaust valves on the IH six cylinder gas engines are almost always caused by carbon buildup on the valve stems. This is why IH went to using low ash oil in these engines. There also was a change in valve stem seals but I believe most if not all the 706s were built with the later style. The valve guides need to be machined to accept the newer seal. Installing the newer seal on a guide that is not machined for it can cause more oil to get into the guide that if the original style seal is used.

Oil leaking past the valve stem seals is cooked on the valve stems leaving carbon deposits. as long as engine load conditions remain somewhat constant this is not a problem. Increased load on the engine will cause valve temperatures to rise which causes the stem to expand. The added carbon deposits on the stem can cause the valve to hang open for a fraction of a second longer than normal but that is all it takes for the exhaust gasses to ruin the valve. Under load the valve is running at tempuratures that are nearly red hot. A missfire caused by a valve hanging open means that the oxygen was not used up on the combustion stroke. This oxygen can then burn the metal in the valve. It is not unusual to see a valve that looks like someone took a cutting torch to it.

(Cub Cadets can suffer from the same problem when the guides get worn. The engines dies under load and will not imediately restart. Within 30 seconds you will hear a little ping as the valve closes and the engine will restart.)

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