Topic: Overflow tube spewing oil. (On to my back tire.........)

[slabbedask]

Just changed the oil and filter on trusty ol' Honda-san. (CB 750F '76) The engine is a fresh-built 836 and I'm using a 10W-40 fully synthetic oil. Filled the correct amount of 3.5 litres, but when hot, oil is spewing out of the overflow tube, that Honda in their wisdom has placed just in front of the rear tire! Had a very interesting launch at a traffic light just now.....

I've experienced this before the engine overhaul too, only not as much as now. It seems that the problem stops when the oil level in the tank is somewhat at the bike's own liking....
Anyone encountered this? What could be the reason? All the engine breathing tubes are intact, this was the first thing I checked....

[vern401]

Does it only do this after an oil change?
Or once it stops spitting and you fill it back up does it start spitting again?

[MCRider]

Just changed the oil and filter on trusty ol' Honda-san. (CB 750F '76) The engine is a fresh-built 836 and I'm using a 10W-40 fully synthetic oil. Filled the correct amount of 3.5 litres, but when hot, oil is spewing out of the overflow tube, that Honda in their wisdom has placed just in front of the rear tire! Had a very interesting launch at a traffic light just now.....

I've experienced this before the engine overhaul too, only not as much as now. It seems that the problem stops when the oil level in the tank is somewhat at the bike's own liking....
Anyone encountered this? What could be the reason? All the engine breathing tubes are intact, this was the first thing I checked....

The only thing like an overflow tube that is in front of the rear tire is the vent coming off the transmission part of the crankcases. If that is the nipple to which you refer, first that is not an overflow but rather a breather. It should have a black rubber hose on it that connects to the nipple on the bottom inside of the oil tank. That nipple is part of a metal tube which runs inside the tank up to the breather/condensor knob at the top of the oil tank.

I know this from experience, if you leave that "overflow" at the back of the transmission open to the atmosphere it will eventually under a mystery of conditions, drench your tire with oil.

[larryblevinson]

You might want to look at the thread I generated with my post "Massive Oil Leak." While my problem cleared up by draining and replacing the oil to the correct level, I would think that overpressure would have the same or similar impact. Good luck.

[slabbedask]

Thanks for the tips! It seems that MCRider's hit the head of the nail here.
On my bike, one of the PO's have mixed up the breathing systems a bit. The crankcase vent nipple is connected to the drain hose from the small breather box next to the oil tank! It all looks natural, until I just found a small diagram in the K8-manual that I downloaded via this forum a while ago. (Great service, invaluable help!!!)
So now I've got some hose rerouting to do.....
The only thing I wonder about now, is that I can't remember any breather knob on top of the oil tank....

BTW, one of the tubes from the breather box is meant to be connected to the air filter box. On my box, there's no place to connect it! I got the filter box from the states via eBay, as a PO had installed pod filters. (That now resides at the city garbage dump somewhere...... )
Experience gained from working on this bike tells me that it's a kinda mix'n match of many different bikes though, so I guess it's had a few "creative" previous owners......