426 crankcase breather question....


14 replies to this topic
  • Ztard

Posted June 07, 2009 - 06:09 PM

#1

In addition to MX, and trail I use my 2002 YZ426 for supermoto as well. Here is my issue. I have a catch can on the crankcase breather coming out of the top of the valve cover it's a 2.5 oz bottle, and at the end of a 12-4 practice session it was over flowing. The oil level was right in the middle of the range, I use 10-40 Maxima race blend conventional oil.

Some history: I recently rebuilt the top end, new base, head gaskets, and a run of the mill valve job, and a new oil pump. Before the rebuild the crankcase breather had a few drips of oil, and maybe every 4 trackdays i needed to drain and clean it. Nothing like what I am seeing now.

Would switching to a thicker oil help?

Why is there so much pressure in the crankcase?

If you need more info please ask. Any help appreciated:worthy:

  • grayracer513

Posted June 07, 2009 - 07:43 PM

#2

There are two sources of pressure in the crankcase. One is the pumping pressure caused by the bottom of the piston displacing air as it runs down the bore. This is a net zero, since piston reverses direction and goes back up.

The other is combustion gas blow by caused by the compression rings leaking. If this is excessive, it can cause the problem you are having. If you did not hone the cylinder, or assemble the rings correctly, or if another problem came up, the rings may never have seated as they should have.

Double check yourself on the oil level.

  • Ztard

Posted June 08, 2009 - 04:42 AM

#3

I did not re-hone the cylinder; I must have inadvertently lined up a couple of the rings when I put it back together. The vent hose is puffing like my exhaust…..looks like I messed when I put everything back together……..really not looking forward to buying another set of gaskets and repairing my error. Should I get new rings, and crosshatch, or can i get by? What kind of damage can i do leving it this way?

Thanks Gray

  • grayracer513

Posted June 08, 2009 - 06:19 AM

#4

The rings are probably not reusable. If left this way, the piston will develop a deposit of combustion varnish down the sides, the oil ring will become stuck and start leaking, the oil will get contaminated with burned and unburned fuel, and the piston may seize.

A set of rings will cure the problem if the cylinder is in good condition.

  • OUTERLIMITS

Posted June 08, 2009 - 09:16 AM

#5

I did not re-hone the cylinder; I must have inadvertently lined up a couple of the rings when I put it back together. The vent hose is puffing like my exhaust…..looks like I messed when I put everything back together……..really not looking forward to buying another set of gaskets and repairing my error. Should I get new rings, and crosshatch, or can i get by? What kind of damage can i do leving it this way?

Thanks Gray


Where exactly is your catch bottle in relation to the top of the valve cover where the vent hose begins? Reason I ask is that I have always taken my vent hose out of the airbox on bikes, routed it upwards, and capped the end with a small Uni clamp on filter. Any bike I did this to tended to always dump oil into the airbox by the stock routing, as routed downward it can never return to the valve cover. On my WR it is routed up through the hole in the frame by the steering head and the filter sits near the top of the radiator, zip tied to the guard. Never lost a drop of oil this way and yes this vent pulses in and out as it should.

  • Ztard

Posted June 08, 2009 - 10:54 AM

#6

I like this idea of using gravity. I like to see a photo of your set up if you have one. My catch can is mounted on the frame, about 6in below the valve cover.

  • grayracer513

Posted June 08, 2009 - 12:40 PM

#7

While there is merit to the concept of mounting the catch can higher, don't overlook the point:

Before the rebuild the crankcase breather had a few drips of oil, and maybe every 4 trackdays i needed to drain and clean it. Nothing like what I am seeing now.


The catch can has been where it is without filling up in a day. It is normal to see air and oil from the breather, and puffing at low rpm is normal, too. 2.5 ounces of oil lost through the crankcase vent in a day is not, and the cause of that will not be cured by relocating the breather.

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  • OUTERLIMITS

Posted June 08, 2009 - 01:15 PM

#8

Agree, something more must be going on to have that kind of a change. Still, once corrected, better to have the oil get back in the engine rather than on the ground or in a can.

  • grayracer513

Posted June 08, 2009 - 01:21 PM

#9

Still, once corrected, better to have the oil get back in the engine rather than on the ground or in a can.

That's a simple matter of letting the catch can drain itself:

http://www.thumperta...ad.php?t=606654

  • Ztard

Posted June 08, 2009 - 02:15 PM

#10

While there is merit to the concept of mounting the catch can higher, don't overlook the point:



The catch can has been where it is without filling up in a day. It is normal to see air and oil from the breather, and puffing at low rpm is normal, too. 2.5 ounces of oil lost through the crankcase vent in a day is not, and the cause of that will not be cured by relocating the breather.


You don't think the new oil pump has anything to do with it? Don't take this the wrong way, but to your point it did not do this before the topend rebuild....why would replacing the rings and new crosshatching fix it? The bike makes real good power, and does not burn oil.

BTW I checked my oil tonight, and it is just showing on the stick. ie low at this point

  • grayracer513

Posted June 08, 2009 - 02:41 PM

#11

You don't think the new oil pump has anything to do with it? Don't take this the wrong way, but to your point it did not do this before the topend rebuild....why would replacing the rings and new crosshatching fix it? The bike makes real good power, and does not burn oil.

BTW I checked my oil tonight, and it is just showing on the stick. ie low at this point

You ran the engine for 60 seconds before you checked the oil, right?

The oil pump has nothing to do with it. Oil blows out the breathers excessively because either there is too much of it up there, or there's too much blow by in the crank case. If you did the rings without honing the cylinder, or misassembled them, or if the cylinder was damaged, and should have been replaced, then that was the cause.

Even if there were two missing oil gallery plugs in the cam box, there would not be an excess of oil pumped there sufficient to cause that much oil loss.

  • Ztard

Posted June 08, 2009 - 03:08 PM

#12

Yes I ran the bike up and down the street for around 10min before I checked it. When I had the bike apart the cylinder still had crosshatch marks in it and the bottom end checked out, piston, crank, rings all looked good. If you recall I had some minor head work to clean up the cams and cam journals...could the machine shop have left these cam plugs out? Where in/on the head are they located? In the cam covers? I appreciate the help, at this point I'm thinking the rings may be not lined up as directed in the manual....So i guess my question is will a new set of ring, or properly aligned rings fix this issue? Thanks

  • grayracer513

Posted June 08, 2009 - 03:33 PM

#13

The machine shop would not have removed the plugs for any reason. The missing plugs scenario was hypothetical.

You can get a more conclusive analysis of the matter by having a leak down test done. This test will immediately find leaking rings, if that's your problem, and if it is, new rings and a proper hone job will fix it.

  • Ztard

Posted June 08, 2009 - 03:47 PM

#14

Thanks:thumbsup: Any idea what kind of leakdown #'s I should see...good vs bad?

  • grayracer513

Posted June 08, 2009 - 06:19 PM

#15

There are a number of variables, and some disagreement on that. The two biggest things it depends on are the construction of the tester and the size of the bore. Bigger bores can leak more and be considered good because of the greater ring gap.

Most automotive publications call for <10% for a good fresh engine (which yours should be), and <20% for one that's serviceably healthy.

Here's an article by Mobil:

http://www.mobiloil....kdown_Test.aspx





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