Topic: an oil leak ..can this be true?

[loonymoon]

Had some fantastic rides on my bike (1973 350F) this weekend - it's running brilliantly. Just a couple of questions though regarding oil leaks...

My bike seems to have two leaks... 
Not automatic oil changes like you get on some old bikes, just a few drips here and there.

One comes from somewhere underneath and just drips a couple every now and then. One guy said it's probably the sump gasket and easy to fix. I can do this next time the oil is changed.

The other leak is slightly more concerning, although still not a big deal (I hope). The bike very occasionally gets a black oily mark on the cooling fins on one side just below where the spark plug hole is, just above the step in the fins where they get narrower. I have a feeling this might be more of a job to fix, an ideas where this may be coming from?

Given that I ride relatively rarely it's probably not worth doing a big job unless the engine is likely to blow up! Any opinions?

[Jonesy]

Not super familiar with the 350/4, but based on what I've seen on my 750 and my wife's CB360 it seems that old Hondas like to have a new tachometer drive cable oil seal. I'm not sure where it's located on the 350, but it's one possibility that's an easy fix. On the bigger SOHC/4 motors there are o-rings in the mating surface between the head and cylinder to seal off oil passages. These harden over time (and heat) and become brittle, eventually developing minor leaks. To replace them requires pulling the head from the motor. Again, I'm not sure if the 350 is set up the same way...

This weekend I had a scare... I got to a bike show and looked down to find oil covering most of the top cooling fin on the front of the head. The sheath for the tach cable cracked around the metal coupling that fits into the valve cover and was hosing oil all over the place! A new one is on it's way from CMS.

If the bike is all cleaned up, look at it before and after a ride to try and determine the source of the leak. If you ride in dusty areas, sometimes the dust can form a trail in the oil leading to the leak location. Can you snag a pic of the cylinder head?

[MikeDeB]

The other possibility for the oil leak around the upper cylinder fins is that the seal for the oil gallery on that side is shot.  This happens on a lot of the older CB750's.  It's doing it on both of my 750's.  Usually a top-end job takes care of this.

[Bodi]

If the leak is near the 1 or 4 plug, this is a pretty famous 350/400 4 leak. The oil gallery feeding the valvegear goes up on both sides, there's a restrictor orifice valve between the head and the barrel sections. The restrictors have  rubber sleeve sealsl that don't work too well when new and age doesn't improve them. The head gasket only extends maybe 1/4" past it to fresh air. If the seal is good, the gasket is good, and the head and barrels are absolutely flat and smooth... no leak. Otherwise you get the seepage problem. Most of us just wad paper towel or something in there to soak it up (and eliminate oil spots on our legs and - worse- on the rear tire) and live with it. You can remove the head and replace the seals, have the head and barrels checked for warpage (and probably machined), replace the head gasket, then retorque after about 500 miles... if the leak is really annoying you.