Topic: CB400 guys - Where are the regular leak spots ?

[greenjeans]

Helping a friend out with a 400F that has a pretty good leak.  It's not quite a leak when sitting still and not running.  It's not dripping or pooling on the ground.
However, when you ride it, it will put noticeable oil on your pants and in the area underneath the carbs.    The motor is clean so it makes inspection somewhat easy, but not really.
The head gasket appears dry as a bone - the fins around it are dry.   The valve cover gasket appears dry as well.   
I have not taken off the side cover gaskets yet, but it seems like the leak would be coming from the upper edges but can't really tell.   

What's the usual suspect?

[Jimray23]

Does the crankcase vent still have a vacuum hose on it? If not that could be your culprit

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Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[innovativems]

#1 is typically the rubbers the sit inside the head gasket.  If that's good( which is sounds like it is) then get out the baby powder. 


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[Bodi]

The classic 350/400 four oil leak is from the head gasket, at the extreme ends where pressurized oil leaks past the oil flow control orifices and out... but that would be rather obvious. When riding at decent speed it will blow back along the top cylinder block fin and spot your pant legs.
The valvegear cover seals with a o-ring type gasket and I have seen leaks from there. With no pressure - there's just splashing oil inside - unless the seal is really really bad it just seeps a little bit.
The crankcase vent will blow out oily "smoke", that's at the very top of the engine with its own little cover section that should separate larger oil drops. If your engine is tired and has a lot of blowby then you will get some oil deposited wherever it vents to. There was a hose to an oil separator under the airbox when Honda delivered the bike... that's not commonly still in use after 40 years though as it dirties up the air filter with oily blowby and the separator element disintegrates after a decade or two... most have been disconnected with the vent hose either hanging loose or removed: with no vent hose the oily deposit is on the top of the motor, fork top tube, and tank underside.

[chewbacca5000]

Valve cover is a big one on this bike.

[MoMo]

Valve cover is a big one on this bike.

agree with that.   The rubber gasket around the valve cover can get pinched during assembly. Never happened to me..   Yeah, right

[Bodi]

Yes, if that's pinched you will get a steady leak.
Honda glued the original gasket into the groove with some goo that's close to impervious to any solvent I've tried. Paint remover does get it out and the groove can be thoroughly cleaned. I use automotive weatherstrip adhesive to glue a new gasket into the groove - it can't be pinched when it's glued in place.

[SOHC4 Cafe Racer Fan]

#1 is typically the rubbers the sit inside the head gasket.  If that's good( which is sounds like it is) then get out the baby powder. 


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Roll her in baby powder and find the wet spot.   

[greenjeans]

Thanks guys - gonna take a run at it this weekend.