Graham_Wright
Well-known member
Are they always on the exhaust valve(s)?
Decompressors
- Thread starter Graham_Wright
- Start date
rotrax
Well-known member
No, some, like the Royal Enfield Bullet and many competition two strokes trial bikes were comression release valves with their own integral poppet valve. These were fitted into what was effectivly a second spark plug hole.
My direct experience of Marine Engines however leads me to believe they are always on the exhaust valve/valves.
My direct experience of Marine Engines however leads me to believe they are always on the exhaust valve/valves.
Hoolie
Well-known member
Sounds logical to me. Better that unburnt fuel and gases go out of the exhaust rather than being blown back through the inlet.
Plum
Well-known member
Graham_Wright
Well-known member
The reason behind the question follows;-
I bought a used Fischer Panda generator with a declared fault. There was water contamination of the oil in the Farymann diesel engine. Suspecting the cylinder head gasket (in ignorance), I dismantled the head. There is no gasket but there is a copper ring that looked fine. The two red O rings that connect the water circuit were shot and have been replaced. All fine but!
The valve push rods are operated by a cam on a shaft in line with the crankshaft i.e. "fore and aft". The valve rockers are on a shaft at right angles to the crankshaft. To connect, the push rods follow a 90° twisted route . They are enclosed in a protective shroud. This shroud detaches when the head is lifted. The push rods then become free and the original arrangement is lost. The conundrum commences.
The rotation of the crankshaft is counter-clockwise when viewed from the power output end (i.e. the generator coils). There is a decompressor. In my very limited experience, decompressors operate on the exhaust valve(s). If I arrange the pushrods to achieve this, the firing sequence is wrong - inlet, exhaust, compression, fire.
If the decompressor operates on the inlet valve, the sequence becomes correct. The engine is rotating in the correct direction (after exhaustive checking).
Am I suffering a mental block?
I bought a used Fischer Panda generator with a declared fault. There was water contamination of the oil in the Farymann diesel engine. Suspecting the cylinder head gasket (in ignorance), I dismantled the head. There is no gasket but there is a copper ring that looked fine. The two red O rings that connect the water circuit were shot and have been replaced. All fine but!
The valve push rods are operated by a cam on a shaft in line with the crankshaft i.e. "fore and aft". The valve rockers are on a shaft at right angles to the crankshaft. To connect, the push rods follow a 90° twisted route . They are enclosed in a protective shroud. This shroud detaches when the head is lifted. The push rods then become free and the original arrangement is lost. The conundrum commences.
The rotation of the crankshaft is counter-clockwise when viewed from the power output end (i.e. the generator coils). There is a decompressor. In my very limited experience, decompressors operate on the exhaust valve(s). If I arrange the pushrods to achieve this, the firing sequence is wrong - inlet, exhaust, compression, fire.
If the decompressor operates on the inlet valve, the sequence becomes correct. The engine is rotating in the correct direction (after exhaustive checking).
Am I suffering a mental block?
Boater Sam
Well-known member
No, you are quite sane, unlike the guy who designed the Farymann engine.
Plum
Well-known member
With this valvetrain arrangement it is possible to incorrectly swap the pushrods so the exhaust cam operates the inlet valve and vice versa. Get the valve operating sequence right and check you have not completely swapped the two pushrods as they may not be of equal length. Maybe the designer went against convention and arranged the decompressor on the inlet valve.
Www.solocoastalsailing.co.uk
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Graham_Wright
Well-known member
The push rods are identical in length. Unfortunately, once the head is lifted, the shroud covering the push rods moves with it and it is impossible to determine the arrangement as the rods are then free.
Graham_Wright
Well-known member
I have this morning received from Fischer Panda UK another more detailed workshop manual with a view of the push rods showing they are indeed as I described - decompressor operates on the inlet valve.
I can now complete the rebuild.
Thanks to all for the help.
I can now complete the rebuild.
Thanks to all for the help.
Stemar
Well-known member
Why on earth would anyone do that?
Graham_Wright
Well-known member
Although not conventional, why not?
In this case, maybe because the decompressor assembly fits more easily with the rest of the engine components but it does not seem impossible to use the exhaust valve lifter.
Bran
Active member
The decompressor on the Farymann engine is to aid starting as these engines were originally designed to be fitted to plant equipment like cement mixers, vibro plates etc. You turn the decompressor knob, swing the starting handle to get the engine turning as fast as you can and then the decompressor returns to normal operation after a few turns.
Bilgediver
Well-known member
Many small Volvo's have the decompressor on the inlet valve. This is the forard valve on the head. However what may cause confusion as we discussed some months ago is that when you look at the air cleaner it appears to be opposite the aft valve however the port is actually angled forard and leads to the forard valve. The exhaust port on the exhaust manifold is also opposite the aft valve to which it connects. One reason to avoid stopping using the decompressor is that there is a reverse flow into the inlet valve which has been known to carry carbon from the combustion space back into the seat and promote valve leakage.
