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Classic Engines, Modern Fuel

Comments on Topic: Stopping the engine

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Submitted by Anonymous

Hi Paul,

Last summer, I took the habit to stop the engine using the procedure described in your book at the bottom of p 78. The ignition is switched off whilst the engine is still running at middle RPM, openning the throttle to wipe the cylinders with fresh air. I never had a hot restart problem this summer but I also changes other things. Anyway, the efficiency of the method is not the point of my topic.

When I parked the car the last time in my garage for winter, I also did it. Useless but it was a habit. Two weeks ago I wanted to start the car for the first sunny days. It never started. Just caughing on one cylinder. No compression on 3 cylinders. The leak test revealed 3 leaky exhaust valves. The fix came from the forum. Hammer and wood block on the valves stems. It did the job and the car eventually went on. I never had such a problem with any car. So it made me remember the last switch off procedure. And my hypothesis is that the fresh mixture sucked during the switch off procedure washed away any oil on the valves stems and even on the exhaust valves stems and seats. hence the leaky valves with a little oxyde during winter stop.

What do you think of it ? Of course, the cure is easy. Don't use this procedure when you park the car the evening and especially the last stop before a long period.


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Submitted by The Author

Hi Laurent,

The problem with pressing the throttle as the engine runs down is that it will allow “raw” petrol into the cylinder. As the engine cools this will condense, potentially washing the oil of the cylinder walls. However, even though the engine will run for many cycles, it will vent the unburned petrol in to the exhaust. When it eventually stops there will only be 1 cylinder’s volume of petrol in a cylinder. Given the relatively small volume of petrol, I would not imagine this would be a serious issue.

However, to your valve problem. The petrol that is vented will end up “washing” the exhaust valves but I cannot think of how this could be worse than 700C hot exhaust gases at removing any oil from the stems. The only mechanism I can think of is that during the time the engine was running down, the exhaust valves cooled sufficiently to allow petrol to condense on them. This may then have worked its way up the valve stems. Again I cannot imagine how sufficient petrol managed to get into the stems to cause the valves to stick. Even if it had washed the oil away, unless there were any remaining residue you would just have unlubricated valve stems which would not stick sufficiently to stop them closing when the engine was restarted. In any case, this is exactly what happens with the inlet valves all the time. They are "washed" with petrol during every induction stroke.

The other thing I cannot understand is that you said 3 cylinders had exhaust valves that were stuck open. This suggests they must have become stuck as the engine ran down. When the engine is stopped simplistically, you will have one cylinder on each stroke, therefore only 1 cylinder will remain with its exhaust valve open! To me this suggests there were other reasons why the valves were sticking. One question, does your engine burn a lot of oil with a smoky exhaust? If the valve stems are very worn, you will get burned oil on the valves and valve stems which can cause the valves to stick. For me I think this offers a better explanation of your problem.

A better way of running the engine down for storage is to put a switch on the petrol pump. Switch the pump off and leave the engine running until it stalls through a lack of petrol. This both clears the float bowls and means there is virtually no residual petrol in the cylinders.


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Submitted by Anonymous

Thank you Paul for your opinion. It make sense.

No, my engine does not burn oil. The valves seals were changed on december 2015. Before, it was burning oil. But many seasons since. An no smoke at all.

The valves wer not stuck. I could see them moving up and down when rotating the crank handle. There was even a gap at the stems. But I did not measure the gap. I should have. They were only leaky. So maybe only the seats were oxydized.


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