Topic: Dang oil leak-'78 CB750K

[denward17]

Took off #4 exhaust just to have a peek at the exhaust port, looks normal to me.

I see nothing that is wet inside.

Opinions?

[HondaMan]

Yep, that looks OK from here: if the valve guide was leaking then the bottom of the valve quide would look slightly wet or have a dark buildup (black carbon) after sitting a while, from the typical leak I'm attempting to describe.

[BenelliSEI]

Not that. The one I had showed a black, wet oily valve penny when I looked in….

[denward17]

Finally got my new battery and ran it today for ~25 mins in the garage with a fan.

Didn't mention this before, but I recently changed the oil/oil filter; I went from 20w50 to 15w40.  I did overfill about ~1/2 quart, that was removed before starting.

I did spot some weeping on the oil galley plug at the front of the cases, so ordered a new o-ring and installed today before running it, see pic.

So after running in the garage today, I see no oil leaks.  Little too chilly for me to ride, ~36 degrees F.

So, what do I think fixed it?

I had removed all valve cover screws (about 4 days ago) and applied thread sealant compound, re-installed and let cure.  After studying it a bit, all valve cover screw location bottoms were open to atmosphere, and one was just over and slightly back from that 6mm end bolt that was getting wet with oil.

OK, I'm assuming I fixed it, a ride in the garage is different than a ride on the open road, but in the garage I did let it idle up near 4K RPM for a while.  Oil pressure at idle was ~15 psi when I was ready to turn it off, so I know it got warm.

Waiting for more favorable weather to go for a ride, will update again after a good ride.

[BenelliSEI]

Excellent news! I have a micro drool on one side of the SC. Will pull those screws and do the same.

[BenelliSEI]

Edit to add:
All valve cover screws on the front were open to atmosphere on the bottom, not sure they could leak there, but went ahead and took all of them out and put thread sealant on them.

I’ve got a “micro drip” on the SC project. Will try this next. Thanks!

[denward17]

Edit to add:
All valve cover screws on the front were open to atmosphere on the bottom, not sure they could leak there, but went ahead and took all of them out and put thread sealant on them.

I’ve got a “micro drip” on the SC project. Will try this next. Thanks!

Hope it is the fix!

[BenelliSEI]

🫰🫰🫰

[HondaMan]

I did spot some weeping on the oil galley plug at the front of the cases, so ordered a new o-ring and installed today before running it, see pic.

Wow, that's a rare one! I don't think I've ever seen one of those plugs leak before.
In contrast, I have seen them leak from the main oil galley (needing a new O-ring) on the engine's left side, above the alternator. Then the alternator gets really oily and can weep out the 3 screws by the "HONDA" logo and often out the bottom of the alternator cover's gasket - especially if the cover has scratches on it. Strangely enough, about half the bikes (CB750s) where I've seen this also had bad starter-clutch rollers, slipping a lot - yet in other bikes (like the 500/550, et al), these same rollers are bathed in oil but work fine.

[denward17]

First ride after oil leak fix, I think it was the valve cover screws.

Rode ~40 miles today, had a nice 70 degree F day and had to go.

I don't see any oil weeps anywhere, so hopefully will hold up.

Obligatory pic included....

[BenelliSEI]

Dennis….. That is really good news! Lucky you; this is my driveway today, at 10F. No riding until April……

[denward17]

Another ride today, ~62 F degrees, ~40 miles covered and no oil leaks, hopefully will last a while.

[Mark1976]

Another ride today, ~62 F degrees, ~40 miles covered and no oil leaks, hopefully will last a while.

   62°,  well aren't you the lucky one, bike looks great...
I wish, we were like 42° today and that's warm for this time of the year. Its our January thaw. Maybe 47° tomorrow, that's a heat wave for us, still not even close to warm enough to ride, we still got 6" of snow on the ground.

[Stev-o]

First ride after oil leak fix, I think it was the valve cover screws.
I don't see any oil weeps anywhere, so hopefully will hold up.

Good news and bike looks great!

[MauiK3]

Yes, good news.
Bike looks great.
Dogs love the snow and cold!!

[denward17]

^Thanks Guys!

[BenelliSEI]

Excellent news!

[PeWe]

Aftermarket valve cover gaskets have often the holes not matching the head and cover.
In front of cam chain  between 2-3 and rear between 1-2.
The off front holes can make the cover bolt to fit tight. When tighten it, gasket will tear apart. A  piece of it can fall inside or just create a leak.

Important to adjust all holes before assembly with a sharp knife or hole punch.
This cover most engine cover gaskets.

Honda CB750 gaskets (light green) have no alignment issues what I have seen.