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How much blow-by is normal?

4077 Views 5 Replies 4 Participants Last post by  busydad
my CB400T runs great, the intake, and exhaust is stock, jeting is 2 sizes up on the primary, I have no complaints there...

when I changed the oil last week, I took the drain plug off (engine warm) I let it drain for a bit and then I gave it a few kicks to see if more oil would come out.

To my surprise, there seemed to be a lot of air blowing, it sprayed oil about 2 feet away. Also, there seemed to be more blow-by on one of the cylinders than the other.

lately it started leaking at the seam between the upper and lower case, so now I'm thinking maybe there is too much pressure in the case.

2 years ago when I got the bike, I rebuilt the top end with new rings (honed the cylinders) I used all new gaskets and torqued everything to specs, compression was good on both sides (although it took a few more kicks to reach max pressure on the RH cylinder)

i'll check compression again, but then what? head gasket? if I replace the head gasket, should I pull the cylinders off and check the rings? do I need to replace all the gaskets?...

any advice is welcome

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oh and I forgot to say, after changing the oil, I started the bike forgetting to put the filler cap back on and I got sprayed head to toe...
That kind of spray would be normal. The CB400t has a 360 degree crankshaft, meaning both pistons rise and fall together. As the pistons go down, the crankcase is pressurized. As they both go up, the pressure is reduced. With the oil cap off, the hole will ingest air as the pistons rise, and spray air as they fall....

I don't think you have any issue there....

The 350/360/450 dohc have 180 degree cranks. As one piston goes up the other is going down. This means the relative volume in the crankcase remains the same...hence no blast from an open oil fill cover...
Blow-by is actually oil leaving the engine case due to leaking rings. it shows up as a fog exiting the crankcase vent and lots of oil on the engine.

This is where all of that compression goes when your engine only makes 120 PSI on the compression gage.

I don't think that is what you're experiencing.
Everything you describe sounds normal. Richard's explanation is spot on. It's good to check the compression, the head gasket isn't known to go bad, they will seep some oil as they age. It's not uncommon for there to be seepage at the case seams also with age. You might re-torque the case bolts and check that the leakage you see isn't coming from the starter O-Ring and spreading out.
thanks guys,

I never thought of the volume change with the 2 pistons moving together, it makes total sense. now I also understand why people put a filter on their breather when using pod filters, I always wondered....

one less thing to do this winter and more time to finish the KH, horray!!!
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