The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Rangerjack on 25.10. 2022 11:01
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G’day all,
Have been having some right cylinder spark plug fouling (LH cylinder fine) chased the usual suspects carby settings and plug heat ranges, to no avail. Finally got hold of a compression leakage test set and had a look at the LH cylinder first, rock solid. RH cylinder leaks badly, interestingly the leak is not through the inlet or exhaust valves but blowing through the oil scavenge return line into the oil tank. The small return line that normally pours oil back into the oil tank under normal operations.
Bit puzzled but I think the rings are bad and it’s blowing through the crankcase, however is it possible that a blown head gasket could give a similar indication?
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G’day all,
Have been having some right cylinder spark plug fouling (LH cylinder fine) chased the usual suspects carby settings and plug heat ranges, to no avail. Finally got hold of a compression leakage test set and had a look at the LH cylinder first, rock solid. RH cylinder leaks badly, interestingly the leak is not through the inlet or exhaust valves but blowing through the oil scavenge return line into the oil tank. The small return line that normally pours oil back into the oil tank under normal operations.
Bit puzzled but I think the rings are bad and it’s blowing through the crankcase, however is it possible that a blown head gasket could give a similar indication?
There are oil drain holes through the barrel, sealed by the head gasket, that carry oil from the exhaust valves back to the crankcase, so the answer is yes I think.
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G’day all,
Have been having some right cylinder spark plug fouling (LH cylinder fine) chased the usual suspects carby settings and plug heat ranges, to no avail. Finally got hold of a compression leakage test set and had a look at the LH cylinder first, rock solid. RH cylinder leaks badly, interestingly the leak is not through the inlet or exhaust valves but blowing through the oil scavenge return line into the oil tank. The small return line that normally pours oil back into the oil tank under normal operations.
Bit puzzled but I think the rings are bad and it’s blowing through the crankcase, however is it possible that a blown head gasket could give a similar indication?
There are oil drain holes through the barrel, sealed by the head gasket, that carry oil from the exhaust valves back to the crankcase, so the answer is yes I think.
G'day Jack
Same for the pushrod tunnel. Lift the head and give the gasket, head and barrel faces a good inspection. Then have a good look at the bore.
Cheers
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Thanks Musky and Kiwi, will lift the head and check out the facings and gaskets. Cheers Jack.
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Pressurising the cylinder tells you there is a leak somewhere, and with a good crank seal and rocker gaskets the only way out is up the scavenge pipe, through the pump and into the oil tank. It is what you would expect, dry pistons and bores will always pass air at high pressure. If the head gasket has failed, there is also a theoretical path to the sump via the pushrod tunnel and cam followers. If the test is repeated with oil in the bores and the leak lessens, it is more indicative of a bore sealing problem, rings/piston/bore.
To check the head gasket involves almost as much dismantling as removing the barrel to inspect the bores, pistons and rings, so you may as well go the whole hog for the sake of a new base gasket.
Swarfy.
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Pressurising the cylinder tells you there is a leak somewhere, and with a good crank seal and rocker gaskets the only way out is up the scavenge pipe, through the pump and into the oil tank. It is what you would expect, dry pistons and bores will always pass air at high pressure. If the head gasket has failed, there is also a theoretical path to the sump via the pushrod tunnel and cam followers. If the test is repeated with oil in the bores and the leak lessens, it is more indicative of a bore sealing problem, rings/piston/bore.
To check the head gasket involves almost as much dismantling as removing the barrel to inspect the bores, pistons and rings, so you may as well go the whole hog for the sake of a new base gasket.
Swarfy.
Just a thought, but if the breather is in the right position can this provide a path for air to pass to the atmosphere?
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Correct Kiwi. But what are the chances of that? If the breather was in the correct (open) position and it still vents entirely through the oiltank, that suggests a blocked breather. The breather spends the majority of its time shut, so I happily disregarded that scenario. Guilty as Charged and caught out by someone who is still following the plot.
Leakdown tests are more useful to diagnose gasket/head casting failure on water cooled engines. A simple compression test confirms the initial symptoms and lets face it, compared to GB's tank engine posted today, the BSA Vertical Twin is a simple beast and a barrel off teardown is no big deal for peace of mind.
Swarfy.