base gasket leaks

[SuperRat2110]

I was looking at the "transmission cavity" and I'm kind of confused how it works? When the cases are sealed properly how does that cavity get used as a breather if the crankshaft seal fails?

[ossa95d]

The transmission, clutch, and right side crank main bearing all use the same oil for lubrication and therefore share the same cavity. I'm sure you already know how a two stroke engine works. Very simply put, as the piston rises it creates a negative pressure in the crank case. Fuel and air are pulled into the crank case through the carburetor. If either of the crank seals are bad the rising piston will draw either air and/or oil through the seals depending on which side it is. This unmetered air or oil will raise heck with your fuel/air ratio creating a lean or rich mixture. As the piston is forced down by the combustion process it pressurizes the crank case and the fuel/air mixture is forced up through the transfer ports to the combustion chamber. If either seal is bad fuel/air mixture will escape creating a less than efficient process. If that fuel air mixture escapes through the right side seal into the transmission cavity it will escape through the transmission vent. We are replicating this dynamic by performing a leakdown test that pressurizes the crank case. I hope this is helpful.

[thrownchain]

You pour oil into the clutch cavity, which also has the crank in it, which shares the oil with the tranny cavity. Hence they are all connected.

[SuperRat2110]

I'm just going to take your words for it. My brain isnt computing. But on a positive note, I resealed the case and made sure to get it all. Just did a leak down it's a pass!

[Bullfrog]

Another way of describing the leak "connection" between the crankshaft cavity and the transmission cavity is to note the following:

- The clutch side crankshaft seal IS a separator between the crankshaft cavity and the clutch cavity (inside the clutch cover).
- There are cast-in holes which connect the clutch cavity with the transmission cavity. This is what allows "filling the transmission" by pouring oil into the clutch cover filler hole.
- So . . . if there is a leak past the clutch side crankshaft seal into the clutch cavity . . . that is "effectively" a leak into the transmission cavity too . . . and the leak pressure can escape through the transmission vent.

Hope that helped.

Ed

[SuperRat2110]

Ed, thanks, I get how the the clutch cavity and transmission cavity are connected, but the way I see it is the cavity where the breather is is sealed off from the rest of the larger cavity where the gears and oil are. There must be a passage to it but I'm not seeing it.

[Dale]

Ed, thanks, I get how the the clutch cavity and transmission cavity are connected, but the way I see it is the cavity where the breather is is sealed off from the rest of the larger cavity where the gears and oil are. There must be a passage to it but I'm not seeing it.

Yes, there is a passage. There is one small section in one of the case halves that does not extend out to mate up with the other case half. This creates a slot once the two sides are assembled and provides a path up to the vent.

[SuperRat2110]

Is that passage through the other case because I see there is a hole on the mating side of the other case.

[Dale]

Is that passage through the other case because I see there is a hole on the mating side of the other case.

Yes, it is the left case half that has the section that creates the slot. This slot begins an indirect path up through several chambers for air to escape out of the vent. If the path were direct oil would spew out of the air vent all of the time.

[ossa95d]

The good news is that your leakdown test was a pass! Good work!

[SuperRat2110]

Yeah, it is good news. I'm hoping to have the bike put back together by the end of the weekend. But I'm not holding my breath.