Breathe in and breathe out
Had a similar experience on the 850 recently. Fitted a Mikuni for all the good reasons and had to find somewhere to put the oil tank breather. I used a 500ml BP plastic oil bottle with a 1/2 right angle plastic retic joiner fitted into the screw cap. This catch bottle sits in front of the battery.
I fitted a clear plastic hose to the top cap joiner and ran a hose under the battery tray down between gearbox plates and exiting alongside the oil filter. I used clear because I wanted to see what vapour was coming out. The inlet from the oil tank is the same original hose, but this connects to a hollow brass tube slighly less shorter than the bottle height. The brass tube is extensively cross drilled to allow gas pressure out if the catch bottle starts to fill up. The brass tube is inserted into the upper side of the catch bottle and gives a good anchor to the inlet hose.
After about 1,200Kms, I checked the contents of the catch bottle. Not much but typical black slime. The clear hose hardly had any water vapour or oil mist in it so it seems a fair solution. But, while this was a OK substitute for the original airfilter connection, it also raised the questions of a) increased blow by pressure from worn piston rings and b) blow by contamination of the oil in the tank.
It appeared that after changing with fresh oil, it did not take long for it to get fairly black. My thought was that the piston ring blow by is pumped up there by the big engine breather hose, this narrows down to 7mm internal bore at the oil tank and blasts dirty vapour into the oil. Next, it seems while Norton may have accounted for the volumetric displacement of crankcase air when new, the increase in pressure and contamination when the piston rings are worn in has not been adequately catered for.
At the same time I had a persistent small undetectable leak from the cyclinder head area under hard riding. A good mate, a real expert on Commandos advised that porous heads and barrels do occur.
2 things seemed obvious - divert the engine breather from the oil tank to prevent contamination and add a second relief breather to the engine, logically the cylinder head. I added the 2nd breather a 1/4 hose to an air line angle brass fitting to the inlet rocker box cover. Again I used a clear hose to see what was coming out of it. Low and behold, you wanna see the shitty goo coming outta there! Grey slimey crud (oil water vapour emulsified)
This head breather hose now runs over the top of the Mikuni, into the rubber ring on the frame junction bracket, around the left of the battery and sneaks down through the left footpeg plate to exit in front of the rear hub sprocket giving the chain a little extra moisture.
Having removed the original breather to the oil tank, the diversion now runs down to exit alongside the oil filter, I sealed off the existing tank junctions and drilled a very small hole in the underside of the tank cap and in the top to increase atmospheric compensation. This works fine, no vapour traces at all on the cap itself.
Results so far - Oil in the tank is much cleaner, demonstrable pressure from both crankcase and head breathers and remarkably NO OIL SPLATTER from the head / barrel anymore. A clean dry Commando.
Mick
Perhaps they needed a breather down both sides (and higher up and nearer the front :shock: )
