Sabb GG loss of compression

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Falmouthgypsy1970

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Ok folks so… after a couple
Of months of trouble shooting and rebuilding my sabb GG 1970 engine… me and boaty went from Darthaven to Weymouth for a pit stop on our journey… she loved life finally on the water after a number of
Years… had to be engine only as no
Wind so
She didn’t feel the
Wind in her sails for her 4.5 tonne weight 🤦‍♂️ stayed at Weymouth and now shes
Not starting… injectors are spitting fuel, new batteries, lack of compression on wind up by hand… is it a simple case
Of winding and winding until she starts again? Or another cylinder head problem after the time she’s been stood
For? The exhaust did get cloudy after I accidentally got caught in the Portland race and
Run on full power to get no where 🤦‍♂️ we made it through and to Weymouth but now nothing… ideas?
 
can increase compression temporarily by putting a small amount of oil in the cylinder. the decompression lever is at compressed setting?
 
The two most common causes of no compression are leaks past the valves or piston.

Ensure first :- the valves are not held open by 1. Too tight adjustment or they have 'bedded in' after rebuild enough to be held open. Not an unusual situation.
2. The manual decompressor is fully releasing.

If the above are ok, or have been rectified if they were culprits, it will be the piston/cylinder area.

It could be a cracked head, piston or cylinder, but unlikely.
 
rotrax said:
The two most common causes of no compression are leaks past the valves or piston.

Ensure first :- the valves are not held open by 1. Too tight adjustment or they have 'bedded in' after rebuild enough to be held open. Not an unusual situation.
2. The manual decompressor is fully releasing.

If the above are ok, or have been rectified if they were culprits, it will be the piston/cylinder area.

It could be a cracked head, piston or cylinder, but unlikely.
Click to expand...
As it happens… I did leave the decompressor open over night it turned out… it does seem harder to turn so partially compressed…

The decompressor valve was slightly loose… so have tightened it up… possibly too much? Possibly piston has dropped out?

Just done fuel change as diesle had diesel bug - on my other post / diesel man told me to put in old diesel… atleast now he’s made 80 litres out of me 😂😂
 
I had one of these engines on my boat about 30 years ago. I seem to remember it being bullet proof and not much to go wrong with it. If you have been fiddling about with the decompression lever I reckon that's a start of where to look for problems. My lever was quite sensitive to being in the right position exactly before it would start. Good luck 🤞
 
38mess said:
I had one of these engines on my boat about 30 years ago. I seem to remember it being bullet proof and not much to go wrong with it. If you have been fiddling about with the decompression lever I reckon that's a start of where to look for problems. My lever was quite sensitive to being in the right position exactly before it would start. Good luck 🤞
Click to expand...

ISTR seeing de-compressors with a roll pin/split pin somewhere in the mechanism. This could have fallen out or sheared?
 
38mess said:
I had one of these engines on my boat about 30 years ago. I seem to remember it being bullet proof and not much to go wrong with it. If you have been fiddling about with the decompression lever I reckon that's a start of where to look for problems. My lever was quite sensitive to being in the right position exactly before it would start. Good luck 🤞
Click to expand...
Right see… this is what I’m thinking…. Apart from my cloudy watery fuel situation… so… with the lever… how sensitive are we talking? It come loose on her maiden voyage after 3 years of being on hard standing… manual says it had to be very precise…
 
penberth3 said:
ISTR seeing de-compressors with a roll pin/split pin somewhere in the mechanism. This could have fallen out or sheared?
Click to expand...
This is intriguing… roll or
Split pin? Where would one of these be? I took top cylinder head off to pistons… something did fall out of that case… to the point of no return… bilge pump currently non functional as electric bloody problems aswell… least if my worries atm… but I think it was from inside the lub oil chamber…
 

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Falmouthgypsy1970 said:
Right see… this is what I’m thinking…. Apart from my cloudy watery fuel situation… so… with the lever… how sensitive are we talking? It come loose on her maiden voyage after 3 years of being on hard standing… manual says it had to be very precise…
Click to expand...
You can usually feel if the lever is touching the valves, if it is the thing will never start. Mine was very wobbly but I got used to it after a while.
Yes good idea to sort the fuel.
 
Falmouthgypsy1970 said:
This is intriguing… roll or
Split pin? Where would one of these be? I took top cylinder head off to pistons… something did fall out of that case… to the point of no return… bilge pump currently non functional as electric bloody problems aswell… least if my worries atm… but I think it was from inside the lub oil chamber…
Click to expand...

The pin I referred to could be anywhere between the external lever and the valve, connecting two parts of the mechanism. Photo shows you've got the cover off, so operate the decompressor and see if it depresses a valve on each cylinder. It's as easy as that.

BTW, this cover is not the "cylinder head".
 
penberth3 said:
The pin I referred to could be anywhere between the external lever and the valve, connecting two parts of the mechanism. Photo shows you've got the cover off, so operate the decompressor and see if it depresses a valve on each cylinder. It's as easy as that.

BTW, this cover is not the "cylinder head".
Click to expand...
Or see if the engine will fire up without the cover, if it does the problem is obvious.
 
Last edited:
You might want to check the tappet clearances, if you haven't already, just to make sure something hasn't settled/gone outright of adjustment and they're holding a valve slightly open, causing the lack of compression.

Beware of flooding the engine back through the exhaust. (Risk of hydraulicing and bending conrod or other damage.) If you are turning it over repeatedly without it starting, turn the cooling water inlet off until it starts.
 
Falmouthgypsy1970 said:
This is intriguing… roll or
Split pin? Where would one of these be? I took top cylinder head off to pistons… something did fall out of that case… to the point of no return… bilge pump currently non functional as electric bloody problems aswell… least if my worries atm… but I think it was from inside the lub oil chamber…
Click to expand...
you can run the engine like this - the decompression lever and cover is off the engine = valves compress.
 
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