Author Topic: Engine pressure leakage testing  (Read 492 times)

Offline Rangerjack

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Engine pressure leakage testing
« on: 25.10. 2022 11:01 »
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?

Online KiwiGF

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Re: Engine pressure leakage testing
« Reply #1 on: 25.10. 2022 11:16 »
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.
New Zealand

1956 A10 Golden Flash  (1st finished project)
1949 B31 rigid “400cc”  (2nd finished project)
1968 B44 Victor Special (3rd finished project)
2001 GL1800 Goldwing, well, the wife likes it
2009 KTM 990 Adventure, cos it’s 100% nuts

Online muskrat

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Re: Engine pressure leakage testing
« Reply #2 on: 25.10. 2022 19:50 »
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
'51 A7 plunger, '57 A7SS racer now a A10CR, '78 XT500, '83 CB1100F, 88 HD FXST, 2000 CBR929RR ex Honda Australia Superbike .
Australia
Muskys Plunger A7

Offline Rangerjack

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Re: Engine pressure leakage testing
« Reply #3 on: 25.10. 2022 21:14 »
Thanks Musky and Kiwi, will lift the head and check out the facings and gaskets. Cheers Jack.

Offline Swarfcut

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Re: Engine pressure leakage testing
« Reply #4 on: 26.10. 2022 09:27 »
  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.

Online KiwiGF

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Re: Engine pressure leakage testing
« Reply #5 on: 26.10. 2022 19:41 »
  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?
New Zealand

1956 A10 Golden Flash  (1st finished project)
1949 B31 rigid “400cc”  (2nd finished project)
1968 B44 Victor Special (3rd finished project)
2001 GL1800 Goldwing, well, the wife likes it
2009 KTM 990 Adventure, cos it’s 100% nuts

Offline Swarfcut

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Re: Engine pressure leakage testing
« Reply #6 on: 26.10. 2022 20:00 »
 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.