Hi,

New owner of a 1980 KZ750. Starts and runs great but I noticed a slight oil leak from the top of the head, right below the valve cover, on the outside surface of the screw hole. Kinda difficult to explain so I have attached a picture of a nearly identical engine with the exact oil leak areas circled in orange. I would be grateful to hear thoughts and opinions on how serious this is or common/uncommon. Thank you!

 

Congrats on the KZ750. Mine was an '81 and my second bike. Rode it hard and it never complained.

The oil appears to be leaking from the "half moons" of the valve cover gasket. I don't think this is serious or uncommon. When/if you pull the valve cover, clean the surfaces and then put a little dab of yamabond or similar liquid gasket on the half moon part of the gasket when putting back together. If the gasket is in poor shape, get a new one and still use the liquid gasket on the half moons.

 

Thank you so much for responding! I really appreciate your time. You are spot on with the half moons being the problem. Sonce I am now home from work I took an actual close up picture. It appears as if the leak is below the gasket area. The oil is coming from the bottom edge of the raised half moon on the outside of the screw hole. It is coming from exactly where I circled in the close up picture. Can the metal be cracked??

The second pic is her in all her glory, my first bike. Pardon the grime but I still need to give her a good wipe down lol. But I'd love to hear any more advice if you have any. Thanks again!

 

Sorry, pics added now.

 

I don't think those half moons are metal. They are probably very brittle rubber that is cracking. I searched Dennis Kirk and found this:
https://www.denniskirk.com/vesrah/ca...prd/147388.sku

Not sure if this is the correct part for your bike, but probably something similar.

I highly doubt that engine metal is cracked. It's an old bike with old gaskets. It will leak! When it stops leaking, check for oil!

That's a very nice looking vintage KZ. Where did you find it? I've been searching for years to find the same as my first two bikes ('77 Suzuki GT380 and '81 KZ750LTD) but the only things that turn up are bucket-o-parts or chopped up 'cafe racers.' Yuck!

Advice? If you're going to ride it at any speed, the first thing I'd change are the tires unless they are fairly fresh. Old rubber gets hard and loses its stickiness. They will have terrible traction and are MUCH more susceptible to failing. Take a safety course. Get decent gear and use it. Spend time on forums, etc researching how to do maintenance. There's a wealth of information out there and lots of older riders who know older bikes and love to share their knowledge. That bike may be overdue for lube and/or replacement of bearings where ever anything spins or pivots (wheels, steering stem, swing arm). Check chain and sprockets (can you get some close-up pics of the sprockets). Carefully check the cable ends and replace if there is any fraying. Lube them.

It may be time to check valve clearance. That's a little bit of a project for a new owner. But when that's done, replace those half moons and other valve cover gaskets to fix the oil leak. Do your research. Get a manual. Have parts ready before disassembly if possible as finding parts for 37-year old bike may be an occasional challenge.

 

I can't thank you enough for responding and helping me out. After seeing your picture and reading your explanation I went in the garage and looked again. You nailed it on the head! These half moons are actually rubber inserts between the head and valve cover. They fit so tight I thought it was just the way the head was designed. I tapped the head with a fine tip phillips and got a nice ting ting sound, then tapped the half moon which resulted in a very satisfying thud thud. Rubber!

A quck look on ebay confirmed the above mentioned, see attached picture. Totally replaceable, inexpensive, and no serious issue, especially out of an old bike just like you said.

I always wanted to learn to ride but it never seemed like the right time. Fast forward 10 years and I now have a garage to keep a bike as well as a very understanding wife who supports anything I choose to do. Not to mention, a perfect place to learn, a one way in one way out development where I live with freshly paved streets and no traffic at all.

One night I was looking at the local for sale adds and the bike was listed. All of a sudden the stars aligned as the seller lived less than 10 mins away. The fire I needed was lit under my *** as I knew this was my chance. I immediately called my neighbor who owns 3 bikes and many hot rods to come check it out with me as he is very knowledgeable and I didn't want to get stuck with a junker.

We went to see the bike the next day and after a short test drive, before I knew it, I was following my neighbor back to my house. Much easier than getting a trailer and tying her down lol. I have seen prices for these all over the map so I can't tell how good of a deal it was but I picked the bike up for 650 cash in west Jersey near PA. It has just over 20k on the odometer and the previous owner swears it has not turned over.

My neighbor who drove the bike home for me said it runs well and shifts well. Told me the clutch feels good and the brakes feel good as well. In the spring we plan on giving it the once over as far as lube etc., just like you said. He did tell me, just as you did, the tires feel a little bit hard but seem OK for now. I plan on taking the class as soon as it warms up before I even think about getting on it. Already purchased an armored jacket, armored gloves, and full face DOT and Snell approved helmet. I don't care if I look like a storm trooper lol.

The valve cover seal and timing will be done in the future most likely with my brother who is very mechanically inclined. Not rushing that though as I know thats a job. So far I mainly did some cosmetic things. Took 0000 steel wool to some corrosion on the handlebars and rear springs, re-painted the master cyclinder due to corrosion, and replaced the cracked turn signal lenses.

The previous owner put a new exhaust on it with basically no baffling, it sounds mean as hell lol. I am pretty sure the bike was repainted also, but I dig the color. I already picked up an extra starter, brake switches, and a carb kit. The previous owner gave me all the manuals and stated he regularly serviced it, providing me with parts receipts. I am trying to learn as much as I can and will take everything slowly. Like you said though, I am glad the bike is unmolested and traditional looking. There is just something awesome about these vintage models and most people say KZs will run forever.

I have also attached pictures of the sprocket, the best I could do. For now she remains topped off with stabil and put away in the garage. Come spring time, I really hope I start to embark on this hobby and enjoy it just the way my father did. Once again, thank you so much for your time and information. I sincerely appreciate the help you have given me and willingness to share your thoughts and valuable opinions. It does not go unnoticed. Many many thanks!

 

For $650, I think you did very well. Twenty thousand miles sounds realistic. If that bike had 120K on it, it would show. The chain has just a little surface rust and looks pretty good. I should have said pics of the front and rear sprockets from the side so we can see wear and notching on the teeth, if any. From what you show, they look pretty good.

You're welcome. Glad to help. Keep in touch here with updates if you don't mind. I'll follow along.

 

IDRIDR has hit on the head. Those half moon plugs will leak if not installed properly. A thin coat of high heat silicone sealant should do the trick. I use Dow Corning RTV-736. Its orange in colour. If you are going to replace / re-mount these, t would also be a good idea to spring for a new valve cover gasket, too.

As for the timing, if you know what you are doing, it should only take an afternoon and a few beer to get it done

 

Here are a coupe of pics of my 1984 ZN750 LTD engine that I rebuilt. (Had no choice... there was a hole in #1 piston) The bike was given to me gratis and has been sitting in a barn for 11 yrs.

 

Nice looking cleanup, Jester.

Here's another safety thought. Replace those old rubber brake lines. You don't want one of those failing. Service the calipers and master cylinders.