There are two types of test for leaks. On a 4 cycle engine you do a cylinder leak down test. On a 2 cycle engine you do a vacuum/pressure test. This would be called a vacuum/pressure test. You should test vacuum also. Sometimes a seal will hold pressure but won’t hold vacuum.
When the piston travels upwards it will still create some vacuum in the crankcase, that’s what draws in the fuel and air mixture from the carb in the first place. Most of the vacuum will be used for just that- as it’s the path of least resistance, but you can have a crank seal leaking enough to draw extra un-metered air through it
Just worth mentioning, this is called a pressure test, not a leak down test (which is totally different and used only for 4 strokes). Also you need to do a vacuum test too as a pressure test won’t test the validity of the oil seals
Now how would you find a leaking crank seal using this method? On the clutch side, since the case cover is sealed and you can't open it without draining your oil and coolant. I'm guessing you would leak down the engine and if nothing obvious is leaking from the outside and you still have a leak, it would lead you to thinking the crank seals are bad and you would then tear the bike apart?
If you have lots of blow by through the crankcase breather hose then you know some is getting down there somehow. If you clamp the breather hose with a pair of vise grips during a leakdown and it fixes your issue, then you need to investigate further. Depending on your machine they all design the powervalve linkage differently. Some machines will have small parasitic loss through the powervalve linkage, but on this yz they are separated by a proper linkage seal. If you loosen your oil fill cap too while you have air going in you may hear bubbling or hissing too if your RH crank seal was bad. A few ways to do it but a leakdown will for sure find it
Great video. Remember to set piston at the bottom of the stroke. Also I would have thought the valve on the gauge should be open. Otherwise your just measuring the pressure in the gauge.
Thanks! But I’m convinced it doesn’t matter where the piston is in the stroke, as no set of rings are going to make a 100% air tight seal. If a constant 5psi is on the bottom than a constant 5psi will be at the top in a stationary engine. If it weren’t, when you apply pressure the the bottom side of the piston you would expect it would rotate it and force the piston to TDC and it would stay there, but that is not the case. You close the valve to see how much leakage over a given time period there is. If you keep the valve open you’re just feeding it a constant pressure, which is fine for when you’re diagnosing where a leak is, but you need to see it hold a charge of air by itself with no more supply. And of course you’re reading the gauge on the unit, it’s exposed to the same atmosphere as what is in the engine, not sure what you mean there
Yes- but you need to keep in consideration how the power valve system is actuated on your specific machine. Usually with a less than tight system you will get erroneous bleed down through the power valve linkages and down into the crankcase/oil your oil fill hole. Hope this makes sense. A big leak out there I would definitely suspect wet side crank seal.
You’ll see it in the video- was like a 2” rubber hose coupling that I hose clamped a progressive step down of plumbing fittings to a small end cap. Kind of a hokey set up but it worked- you need to clamp externally on the YZ250’s exhaust port. A normal expandable rubber frost plug won’t work in the YZ’s oval shaped exhaust port
Coold video. I was wondering why you didn't introduce the air from the spark plug hole but then realized the reeds would block the air. So to check to see if your reed valve housing is leaking I'm assuming you would have to introduce the air from the carburetor?
Working on my jet ski. Is this test compatible if you have more than a single cylinder? Blew my engine and don’t wanna have that happen again, my current best guess is water filled carbs, and the seals from each cylinder I’m a little skeptical on
turdpike thanks! Not relating to the test but found my PTO crank seal came loose. No wonder my piston looks like a metal foundry. And just to think my dumbass almost looked over this as a dust cap or something.
turdpike haha 😂 just getting into the hobby and being my own mechanic. First top end rebuild, love learning this stuff as long as I don’t cost me any more money. ✌️✌️
I have a kx 250 98 that’s had a top end rebuild and it’s still not producing the power it’s supposed too I believe,I’m considering getting a leak down test done ,if they have a leak somewhere will that cause a decrease in power or not necessarily ?
100% it would cause a decrease in power if say it had a raging leak in the RH crank seal, or like major head gasket leak. A leak down test is very informative
@@turdpike sorry just asking I'm doing this is school today and we got told you have to take the cover off right side of bike and cover the power valve with like Plato like substance. Lol and I'm struggling haha can't get it to build pressure so I'm check out RU-vid on lunch lol
@@thewicketd7817 hmm, what kind of bike? If it’s a YZ like this with this style of cylinder, there is no good way to stop the hose (left side) from leaking, so you can just clamp it. The RH side on the power valve linkage itself where it goes down into the lower half of the engine it has a seal on it. Unless that seal is leaking badly, the RH side doesn’t need to be touched in my opinion. And you’d know if that seal was leaking by losing a bunch of air via the transmission vent.
I’m not convinced it does, the whole cylinder is pressurized to the same level. To me it doesn’t actually matter where the piston is in it’s stroke, but I could be wrong?
turdpike I could’ve wrong too, maybe that’s only if you’re pressurizing from the spark plug hole, so the transfer ports are open and the crank case pressurizes. Idk if it’s different doing it through the intake
