Decompression Sickness Yanmar Diesel

[Cliff Carter]

I received an unpleasant surprise today when I went to start our 1977 C30 with a YSE12 Yanmar engine, no compression at all. I had run the engine three days before for approximately an hour with no problems. I shut the engine off after docking and it was fine.I checked the following to ascertain the problem but so far no luck.1. Checked decompression lever - Neg.2. Engine cranks over too fast and easy.  Crankshaft and rod still attached.3. Fuel to the engine. OK.4. Removed valve cover and valve train operating  to the eye as it should.5. Checked dip stick and no water in oil.6. Suction on the intake pipe on the down stroke. OK7. Some back pressure can be felt through the top of  of the oil filler tube on the down stroke.My next task will be to remove the head and hope I have a bad (very bad) valve. This will also let me view the piston top.Is there something that I am missing that is simple and causing this problem?The engine is a single cylinder 12 horsepower unit which has a little less than 800 hours on a rebuild by previous owner.Any suggestions would be appreciated. ThankxCliff CarterPlusOneC30 #598

 

[johnnystarliner]

an idea or two

hello,When a motor spins freely like you are describing it can be due to being worn out however more likely an "event" happened that is causing your problem.Motors do not generally just wear out overnight however some problems might of occured while you last ran it or when you tried to start it,that could be causing your problems.1)When you turn it over do you hear anything other than the motor spinning? Perhaps you have a faulty decompression valve.2)Another item would be if the fuel injector is stuck open. Any diesel motor should be equiped with a small piece of clear fuel line on both the inlet fuel line and the return fuel line. This allows you to see if you have any flow and or air. If you have clear line you would see mega bubles, most (likely on the return side) if the injector is stuck open. This is not a common problem, but I have seen it happen.3) Perhaps your motor has a butterfly valve on the intake manifold or near the air cleaner,instead of an injector pump shutoff, to stop your motor. Is the valve open? If your motor could not draw air it would have little to compress. Again not likely, but has happenes.4) If possible check your valve timming. It may have jumped so the exhaust is opening too soon. Also a tight valve could cause the problem so check valve lash.5) To dtermine if you have too much crankcase preasure (a sign of a holed piston or washed rings) put a piece of paper over your open crankcase vent. Hold one corner down so it is like a flapper and crank the motor(WATCH YOUR FINGERS,TOES,NOSE AND LONG HAIR WHILE DOING THIS ).On the piston compression the paper should be drawn down or at least only flutter lightly while on the piston return it should blow out a little.These are just a few thoughts, good luckjohn

 

[John K Kudera]

Diesels

I have been told diesels will not burn valves, but what can happen is a piece of carbon is lodged on the valve seat, causing it to stick open.

 

[Cliff Carter]

Problem Found!

I took a closer look at the valve train today and discovered a broken intake valve spring. The valve was opening with the rocker arm however on its return is was stopping about a quarter of an inch or less from the rocker arm.Thus the valve was not closing completely. So "its off with it's head "rather than just replacing the spring using a spring compressor. Might as well get the valves done and re-seated and replace the exhaust spring at the same time.Thank you for your suggestions.Cliff CarterPlusOne