Yanmar ysb8 strange problem.

[Rappey]

Engine has been rebuilt (apparently) Everything inside and out is brand new . The owner has seen videos of the engine running on the bench after rebuild. Im happy to believe it.
Engine is fitted in boat but no fuel system or electrical has been connected.
I connected up what needs doing and cranked it. Also hand cranked and there is lots of compression.
Once i finally got it bled i cranked. It fired maybe 2 or 3 times and stopped . Im getting closer to it running
But maybe not !
So i cranked again and instantly noticed the engine sounds completely different as it turns over? Hand cranked to get a feel and there is zero compression. Huge rapid audible suck on the inlet side but absolutely no resistance from compression. The head is right up against bulkhead so no chance of anything without removing the engine.
What could have happened ?

 

[Wjj]

I'm not familiar with the ysb8 but a bit of googling says it has a decompression lever. That'd be the first thing to check. I had a similar issue with an engine losing compression and it turned out the exhaust valve was stuck open with a lump of head gasket in it that had somehow escaped, so hopefully its nothing like that

 

[Rappey]

Ive felt the decompression lever while rotating the engine and could feel the valve "doing its thing" . It feels correct.
Exhaust valve stuck open is also what i thought but not sure why.

 

[PabloPicasso]

Perhaps removing the rocker cover might allow you to see whats happening?

 

[earlybird]

I think that you do need to remove, at least, the rocker cover. Since the engine has a horizontal cylinder is the "bulkhead" that you refer to actually a non load-bearing fore and aft partition? If so, would cutting an access hole in it, if possible, be a lot easier than engine removal? Several "ifs" but useful for any future problem as well.

 

[Rappey]

The head is against the side and the bottom of the ¼ berth and a bulkhead also joins at same spot. Couldn't be in a worse place.
I can barely see the rocker box cover.
I was more after theories of what may be causing the problem out of curiosity.
Will go out to boat for one more look before i give up as it seems more trouble than its worth.

 

[garymalmgren]

Re: Exhaust valve stuck open is also what i thought but not sure why.
To loose compression like that it would have to be the inlet or exhaust valve stuck open, a completely burnt valve face or totally disintegrated piston rings.
Re Huge rapid audible suck on the inlet side but absolutely no resistance from compression.
The de-compressor activates the inlet valve. The suction that you are hearing could be air entering the cylinder via a decompressed inlet valve.
Re: Will go out to boat for one more look before i give up as it seems more trouble than its worth.
This sounds like you have not bought the boat yet and are thinking or giving up because of this problem.

How much work is involved in moving the engine forward enough to remove the rocker cover and check the de-compression mechanism?
I can do that with my YSE 8 in 1 hour. maybe it is not as hard as you imagine.


Re; I was more after theories of what may be causing the problem out of curiosity.
As I mentioned there are very few reasons for a total loss of compression and a malfunctioning de-compressor seems the most likely cause.

gary

 

[Rappey]

a malfunctioning de-compressor seems the most likely cause.

The engine has been fully rebuilt and has ran on the bench. That would eliminate valve seats i would have thought as its all new.
The rebuild was not by me so i can only go on what I've been told. It has 4 new engine mountings, new starter and new alternator, new fuel hoses, a bag full of new engine instruments and new sender units are in the engine. That alone is a fair amount of money so would assume the internals have had £££ lavished on it as well. The engine has been painted red so that must mean it's all new ?
You can feel the valve on the decompression lever so i know its not that and it was turning over as it should with compression, until it fired a few times and then zero compression. You can feel if the valve is touching the decompression lever, or when it makes contact .
The owner of the boat wanted me to get the engine running then sell the boat for him.
The engine would have to completely come out of its cave to get to the head and there's not really enough money in it to make it worth my while, but I'm very curious about why its gone like it has. !
It looks a large heavy iron lump compared to the much physically smaller aluminium1gm.
The boat is a centaur and quite nice. Was thinking around 4k running. Going to struggle to get over 1k as a non runner.
I guess if I'm bored i could take it out