For engines where one or more cylinders are low go round the cylinders again putting a teaspoonful of oil into each immediately before testing it, this is a 'wet' test.
If a low cylinder increases significantly from the dry to the wet test, i.e.
Emptying the carbs would prevent that, but if you do the wet test some oil is likely to get in anyway. On the pukka kit there are two pressure gauges, one either side of a restriction, with the 'inlet' gauge connected to a compressor via a control valve, and the 'outlet' gauge connected to the cylinder. put it in 4th gear with the handbrake firmly applied to prevent the compressed air pushing the piston down.
Piston is set to TDC on the compression stroke i.e. The control valve is adjusted to give, say, 100psi on the first gauge, and the second gauge indicates leakage from the cylinder.
Compression Testing Crankcase Breathing Dating the Engine Dipstick!
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In the beginning of 2012 and throughout 2013, I would often feel flat, and at times, it was a challenge for me to get out of bed in the morning.
From the Workshop Manual high-compression engines (8.8:1) should be 160psi and low compression (8.0:1) 130psi for 18G engines.Having the piston at TDC basically checks the valves, head-gasket and the top of the bore, but if you rotate the crank to move the piston down the bore, and get a sudden decrease in pressure on the output gauge, then damage to the bore at that point is indicated.Originally the MGB used the same breathing method as the MGA.In a dark garage, clipped to each plug lead in turn and pointed at the appropriate valve with the rocker cover removed, by turning the adjuster back and fore you should be able to freeze the valve anywhere from fully down to fully up and so see if it is sticking partly down or not.It will help to raise the back of the car relative to the front during this test, to put the engine fully horizontal, to reduce the amount of oil running down the back of the engine with the cover off.The general rule of thumb is that anything more than 10% difference from the lowest to the highest warrants investigation, and in reality they should be closer than that under normal circumstances.