Oil Leak: Honda CB750K


Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Leaky seal

Q: I have a 1981 Honda CB750K with 54,000 miles on it. I took the bike to the Honda dealer to have the carburetors overhauled and was told that the cam chain needed to be replaced, so they did a complete engine overhaul. Four months and many parts problems later, I got the bike back. It runs great but has an oil leak at the drive sprocket. They have put two more seals on and they have both leaked. They glued the second and third seals on. The bike was not leaking oil when I brought it in. Everything is tight. The seal only leaks when the transmission is in gear and the shaft is turning. The people at the Honda shop say they do not have a clue. I was an engineman in the Navy for five years. I have never seen anything like this. —John K. Thompson/via email

A: If you hadn’t mentioned the top end overhaul I would suggest you have a burr on the final drive sprocket that is destroying the seal. Since you mention that the problem only showed up after the overhaul, I’m going to suggest a different cause. I wonder if, during the top end rebuild, somehow the engine breather became blocked? There shouldn’t be much pressure buildup in your engine, but with blow-by from the cylinders plus a little pumping loss it might be enough to produce some internal pressure in the engine. Since the engine and gearbox share an oil supply, the easiest place to relieve this pressure might just be the big seal at the final drive sprocket. Ask the Honda shop to test the engine breather and see if it is still working as designed. There is also the possibility that the automatic chain oiler is doing its job too well. There’s supposed to be a felt element built into the hollow end of the output shaft that is oiled by internal circulation. The drips from that are spun off by centrifugal force to supposedly lubricate the chain. If the felt is missing, or otherwise damaged, I can see where the volume of oil would increase drastically.

Note: I managed to get the schematics confused between SOHC and DOHC Honda CB750s, which made part of my answer to this question meaningless. There is no automatic chain oiler on the DOHC 750s. If the oil is coming from behind the drive sprocket it must be coming through the large seal. If the oil is coming from in front of the drive sprocket, it may be that the small O-ring seal behind the large washer holding the sprocket in place has failed or is missing. MC

2/4/2014 4:30:52 PM

You can ignore that last bit about the automatic chain oiler. I biffed my research and looked at a SOHC engine schematic. The DOHC engines didn't have an automatic oiler. If it's coming from the sprocket it has to be the seal behind the gear or the o-ring on the bolt holding the gear in place. Thanks to Jamon at SconaCycle in Edmonton Ontario for setting me straight.

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