Topic: Rebuilt CB400F motor - oil burning issue on #1 cylinder.

[Flyin900]

Recently finished a full restoration of a 1975 CB400F with the cylinders honed and new OEM Honda rings installed and new valve seals on the intake side of the motor. The OEM valve guides on the exhaust will not accept a valve stem seal in their design from the factory.
Compression is 135-130-130-130 on #1 to #4 cylinders with only 150 miles on the motor. The head was fully tank cleaned and the valves were all cleaned and lapped. The work was done at a machine shop that I have used on a variety of past projects with no issues.

The #1 cylinder is clearly oil fouling the plug and burning about a pint of oil so far in the 150 miles of initial riding. The oil is evident in the exhaust stream with some visible smoke once the bike has fully reached operating temperature.

My thoughts are issues with the exhaust guide or possibly ring issues on this cylinder with something missed or out of spec when the honing was done or valves were done.

Another thought was has anyone found an issue with the rubber seals that go around the oil control orifices situated in the top of the barrels leaking oil into the combustion chambers. I used an aftermarket gasket kit for the rebuild and everything seemed to fit fine, yet I realize Honda seals are sometimes the best option for the slight differences in size or quality specs.

The other three cylinders spark plugs look fine with only #4 running a little rich. I checked the float level on #4 and it's too high on that carb.

Thoughts, areas to check and feedback is always appreciated.

Cheers,

[calj737]

A bit early on the miles for a break-in to have concluded, but since it’s related only to a single cylinder, I’d start to wonder as well.

First guess would be exhaust guide. Compression says rings are probably good, but is the bike smoking while riding or idling? Does it only smoke when revved if you’re at idle? Have you tried to perform a leakdown test? That would be definitive for an out of spec component since the leak would point you to guide, seat, or rings. Won’t show up usually as an oil control o-ring, so that might also answer your question. 

[1952paul0]

Recently finished a full restoration of a 1975 CB400F with the cylinders honed and new OEM Honda rings installed and new valve seals on the intake side of the motor. The OEM valve guides on the exhaust will not accept a valve stem seal in their design from the factory.
Compression is 135-130-130-130 on #1 to #4 cylinders with only 150 miles on the motor. The head was fully tank cleaned and the valves were all cleaned and lapped. The work was done at a machine shop that I have used on a variety of past projects with no issues.

The #1 cylinder is clearly oil fouling the plug and burning about a pint of oil so far in the 150 miles of initial riding. The oil is evident in the exhaust stream with some visible smoke once the bike has fully reached operating temperature.

My thoughts are issues with the exhaust guide or possibly ring issues on this cylinder with something missed or out of spec when the honing was done or valves were done.

Another thought was has anyone found an issue with the rubber seals that go around the oil control orifices situated in the top of the barrels leaking oil into the combustion chambers. I used an aftermarket gasket kit for the rebuild and everything seemed to fit fine, yet I realize Honda seals are sometimes the best option for the slight differences in size or quality specs.

The other three cylinders spark plugs look fine with only #4 running a little rich. I checked the float level on #4 and it's too high on that carb.

Thoughts, areas to check and feedback is always appreciated.
the only answer is of course going back over your build.
Cheers,