My Spitfire has poor compression on number two cylinder. 100 psi as opposed to 125ish on the other three.
Not a huge problem at the moment, but does make cold starting a bit tricky.
I understand that poor compression generally narrows it down to head gasket, valve stem seals or piston rings.
Any advice on how I might work out what it is? There's no water in the oil, and no oil leak from the head.
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Head gasket usually leaves pressure in coolant system even when engine has cooled, or pressurises cooling system quickly after start up.
Valve seals usually means blue smoke at start up when it's been stood a while, or when you lift off the throttle. Valve seals not all that likely to give a loss of compression.
Piston rings usually just smoke, but you can IIRC check for this by injection a bit of oil into the cylinder then re-doing the compression test - the oil can seal the piston rings, and bring the compression back up.
You have however missed a leaking valve. Check the valve clearances to see if one has closed up. I can't think of an easy way to check if an exhaust valve is burnt out and passing though (other than all the other checks being negative!). Shouldn't be too arduous to get the head off this engine though and fix the valves up if that's what it is.
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RichardW
Is it illogical? It must be Citroen....
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Tell tale sign of a gasket failure is when two adjacent cylinders have similar low compressions - ie they are leaking into each other across the small gasket bridge in between.
Your compressions are not very poor, as when a valve has actually burnt out or the gasket gone.
Another cause can be gummed up piston rings, that don't expand and seal the bores properly, or a broken ring.
The oil test works well in my experience, and should help you pinpoint either bores or valves. If you suspect sticking rings you could try pouring in something that will ungum them. There might be proprietary products. I hesitate to recommend something so crude as parafin, but you could try injecting a syringe into the low cylinders, turning the engine over slowly a few times,and leaving overnight. Do it just before an oil change.
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Thanks for the advice guys. I'll try the oil test and check the valve clearances.
Taking the head off would be simple enough, but I'd like to be able to work out what's wrong before I do it.
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