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I have a 1978 Honda CB400TII that inadvertently turned into a project bike. After a month if getting the engine apart and back together I finally started it and 3 things happened:
1: Oil started leaking from the gasket around the bottom of the cylinders. And not just a little. It was enough for it to start pooling on top of the crankcase and drip down the bottom. It's not leaking on all sides just the right and left sides. I replaced the gasket when I put it back together so I know the gasket isn't bad. The only thing wrong I can think of is that the old gasket was stuck on pretty good and I couldn't get all of it off but I did for the most part. I'm hoping that maybe just putting a bit more torque on the bolts will fix that problem.
2: While the bike did start and idle, when I rolled on the throttle even a little it would sputter and die. When I got the bike it had trouble idling but would be fine when I had the throttle open, but now I've reversed the problem. The only thing I can think of is that I replaced the handle bars to make them lower and for some reason it made all the cables tighter and to get the throttle cables onto the carb I had to take off about a third of an inch if the cable housing. I did get the engine to rev after about a minute of leaking and letting the engine warm up, but...
3: Smoke started to come out of the top of the exhaust headers. I'm really hoping it was just the paint I put on them was curing (or even burning off as long as it wasn't the actual engine that was smoking) that is made for high heat or just the oil that might have been dripping on the them burning off.
Any advice would be helpful!
 

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Were there any o-rings or packings in the bottom of the cylinder block? A lot of these old Hondas used the stud holes as oil passages and they would have an extra o-ring around those. If one was damaged or missing that will leak pretty good.

Header paint will smoke when you first run the bike.

Edit: Looks like just 2 large o-rings around the cylinder sleeves.

 

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well i don't know anything about hondas, but i can tell you you need to have all the old gasket off or you wont get a good seal. don"t try tightening it you could brake the bolts and then have a real headache. sounds like you need to check your carb (may be running to rich if i'm thinking right) or your timing might be off too as for the smoke i really wouldn't worry about it unless its coming out of the pipes
 
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