Hello, I am new to this forum and am in need of some advice. I have a 1974 CB750 with some rather high miles on it 94K I believe. It leaks oil from the left side of the engine at a fairly steady rate, during and after riding. The previous owner said it had to do with some O ring seals. It looks like its coming from the head gasket to me, although the bike has good compression and power.
Does anyone know what could be causing this and how I should go about fixing it? Also would I have to pull the engine or could I leave it in the frame?
There are rubber o-rings on both sides of where the cam chain runs through the center of the block. Could be a leak from the head gasket in that area as well but may not efect compression if the leak is not at a piston. Oil is pumped to the top of the motor. Bad news is, either way, you are going to probably pull the head to fix and should replace the head gasket anyway. Maybe do a valve job and all while your'e at it. Top end will be like new. Unless your frame is cut, you'll need to pull the motor. Not that bad on these bikes. Looks like it's leaking pretty bad.
When you get your manual and the head off, with the cam and rockers off, check the side play on the valves to the guides. There should be specs for this in the book. You may want to remove the valves and have them at least reseated and lapped in the head. They are probably not bad if you still have good compression. Get some new valve seals as well, they are cheap. It depends how far you want to go with it like rebuild carbs, hoses, just how much you want to spend. Also, when you get this thing apart, it's not a bad idea to get a Honda and Clymer manual. I've found most of these manuals leave something out that the other one will have. I've got several manuals for my bikes and glad I did.
Seconding this. You will want to pull the head. When you replace the gasket, I recommend getting a honda one OR if you have a bore job on the engine, a cometic gasket. Make sure you get every last piece of the old gasket off or you will just end up doing it again.
New valve seals are a good idea but hold off till you can pull and check the valves. You might want to get a spring compressor for the valve springs, they have about 150 pound of force. may as well measure those for spec too and check your keepers for bending.
As for the oil leak itself, it is almost positively the orings.
The ride IS the adventure. The destination is just to get gas!
Yeah, all that re-run said above. Don't skimp on gaskets and seals, try to get Honda or you'll be doing it all again soon. Do it right the first time. Also check out the cylinder walls for scoring and carbon ridge at the top. The ridge is normal but if you end up boring or at least honing the cylinder walls, you'll need a good set of rings or they will never seat right and will burn oil. I'm probably going to fast here but if I were going that far on it, I would do it all, just my opinion.
I appreciate the advice as I haven't worked on a Honda motorcycle before (honda cars mainly) but they seem pretty straight forward. I just finished restoring a 78 XT500 Yamaha and although the engine is a beast, I could remove it myself, no problem. The CB motor on the other hand is going to be another issue, that will certainly require help. I just love the way that the engine dominates the look of the CB.
I already have a Clymer from the previous owner, which I found to be very comprehensive, but like yourself, I prefer to have several manuals handy.
Thanks again for your advice!
You might want some help getting it out but if you get stumped along the way, one of us here should be able to walk you through it. I always did like the engine style of the SOHC 750's. Pretty simple but Honda really had a winner with them.
I had a similair problem when i used semi synthetic my feet got cover it was behind the carbs after i had gone 40 miles i since used 10/40 and cured the leak. I heard the s/s oil acted like a flushing fluid as this was not around when the bike was made.
The oil issue is true but only if you have a dirty engine. I switched to synth and the bike runs perfect. Crappy oil will cause sludge that builds around seals and while the seals still go bad, the sludge stills seals things until the worst time.
If you leak of a good synth oil, you have bad seals.
The ride IS the adventure. The destination is just to get gas!
Im glad i found this post. I have 2 CB750 engines that leak like crazy. The first one i pulled out of the bike last summer. Road to the coast (about 150 miles) and my pant leg was soaked with oil. Swapped the engine and now the 2nd engine is doing the same thing. I almost possitive its the same problem. Top end overhaul here we come! :/
I know this is an old post, so I was wondering if you had resolved the issue as I have a '78 cb750k with the exact same leak; although mine is not quite as severe as yours. It's a much slower leak, and I'm still able to ride the bike; I just end up with some oil splash on my lower pant leg, probably every 3rd or 4th time I ride, and it happens much more on high way travel. I've had a few people look at it and tell me it's at least a 6 hour job, and I obviously won't know the exact cause of the leak until I get down in to the gasket. I'm just wondering how you resolved the issue, and how much time, parts, etc. it took for you to fix it?
These bikes absolutely should not leak oil. They were designed to leave the old oil leakers behind. I once has a Matchless 500, single cylinder with vertical split crankcase and the oil came out almost as fast as it went in.
I know I am going to get flamed, for this, but. Oil leaking from the o-rings between the head, and cylinder is common on these bikes. However yours is excessive, and should be repaired. On most a little oil leaking, and just making a little mess on the fins can be prevented by taking a piece of cotton rope, (like clothes line rope) and putting it between the fins, where the head, and cylinder meet. This will keep your bike, and pants leg clean, untill you really need to remove the top end.
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1976 CB 750-A X 2
1977 CB 750-A X 2
1977 CB 750-K X 2
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
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