Engine compression

[Dampinky]

Gday guys.
I'm having a bit of trouble with my Jupiter. I was getting fuel in the oil. I have just done a engine compression test and one of my cylinders was down more the 10%. I did the small amount if oil in the bore and rechecked and it had gone up to match the other cylinder. (Meaning maybe piston ring)
Does anyone know what compression reading I should be getting.
I got
Cylinder 1 - 125 psi
Cylinder 2 - 135 psi
Cylinder 3 - 125 psi
Cylinder 4 - 110 psi after oil 135psi

Thanks for you time

[David Morris]

Hi,

Your results sound about what to expect to me. I have never got much more than around 140lbs/in. If you are using the original pistons, then the ring setup is not that great and anyway the pressure you get will depend on a combination of the ring sealing and the condition of the valves. The fact that things improved with a drop of oil shows that the rings/bores are a bit worn, but quite serviceable.

As for the fuel in the oil, I doubt that that is due to any leakage past the rings. If your car was a Javelin, then I would suspect fuel entering the sump via the mechanical fuel pump. But, don't Jupiters have an electric pump? How are you sure it is fuel in the sump? I am afraid that water in the sump is much more likely? I suppose that you might get fuel getting past the rings if there was far too much choke being used, but this would have to be a lot before you started noticing it in the sump?

All the best,

David

[Keith Clements]

The amount of compression also depends on crank speed and the compression ratio. The latter is affected among other things by gasket thickness, combustion chamber gas flowing, head shaving , piston top to gudgeon pin height and bore size.

Petrol in the oil is usually caused by a leaking float chamber needle valve which maybe due to an over pressure fuel pump . Look down carb and see if there are any drips with fuel pump switched on or ,with mechanical , hand priming.

If the plugs are black and not fawn then mixture too rich. If plug has crust on it likely worn rings. You can hear compression leaking past rings by listening down oil filler and past valves by listening down carb or exhaust pipe.

[Dampinky]

Thank you David and Keith.
I might have to get a fuel pressure regulator as fuel is coming straight from tank-pump to carbs.
It may just be running to rich. I haven't been able to set the ignition timing up properly as of yet so they may be a problem to sort

[Keith Clements]

set reg at a out 1.5psi