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Although I have a single carb, with cone air filter, I think the principle should be the same. I have an 850 mk3 with a reed valve (Yamaha xs650 version, I think from HNW) in that pipe, between the case and the oil tank. I have a hose on the second outlet on the oil tank which is directed under the seat and down and round to the end of the swingarm, where it vents to the atmosphere.It was set up this way when I first got the bike, but without the reed valve. I had all the crankcase pressure symptoms. I added the valve and a miracle occurred. So, I don't think it needs any negative pressure oil tank side of the valve to work. Atmospheric pressure seems sufficient. That little valve, which I knew nothing about before joining the forum, has been the single greatest improvement I've made to the bike.
Although I have a single carb, with cone air filter, I think the principle should be the same. I have an 850 mk3 with a reed valve (Yamaha xs650 version, I think from HNW) in that pipe, between the case and the oil tank. I have a hose on the second outlet on the oil tank which is directed under the seat and down and round to the end of the swingarm, where it vents to the atmosphere.
It was set up this way when I first got the bike, but without the reed valve. I had all the crankcase pressure symptoms. I added the valve and a miracle occurred. So, I don't think it needs any negative pressure oil tank side of the valve to work. Atmospheric pressure seems sufficient. That little valve, which I knew nothing about before joining the forum, has been the single greatest improvement I've made to the bike.
