valve stem oil seals, cb340/4

[dickyta2]

Has anyone had any issues with aftermarket valve stem oil seals?

   I`ve just rebuilt a 350/4 motor, including new pistons and a rebore  and I`m getting smoke coming out of the breather pipe and a small amount out of the exhaust...it does seem to be running fairly well though!!!

[Trigger]

It is most probably not smoke, it is oil vapour from the oil cooking because it is hot

[dickyta2]

Okeydokey, cheers for that Trigger :-)

[Trigger]

Your engine breather hose should go to your air box. If it is not vapour maybe, someone did not rebore your barrels to the correct spec.

[dickyta2]

Theres no airbox on it I`m afraid!!

[royhall]

Not all 350/4's had valve stem oil seals on the exhaust side, just taper top guides. On the exhaust side the pressure in the port tends to push the oil back up the valve stem so it's unlikely to be the exhaust side. Seals are fitted to the intake as the suction of the descending piston pulls the oil down the stems so if one is displaced or worn out the engine will burn oil and cause smoke. Hope that makes sense. That said, stem seals won't be the cause of vapour from the breather hose that's usually crankcase pressurisation from worn bores or a bad rebore. Or as Trigger says overheating engine, if your running pods you may be way too lean and that will cause the overheating.

[Bryanj]

Always got vapour but not liquid from 500 and exhaust guides gone gives clouds of blue smoke

[royhall]

Always got vapour but not liquid from 500 and exhaust guides gone gives clouds of blue smoke

As Bryan says if your guides are shot you will get smoke regardless of seals or not. But I am assuming they were checked when you rebuilt the engine. The CB750F2 as an example is notorious for knackered smokey exhaust guides. How many miles has it done since the rebuild?