make sure the plug is good on the back cylinder and that it is firing through the plug. If so, then run a compression test and see what that cylinder reads. Check the compression on both cylinders for comparison purposes.
There was another thread on here recently about rising oil level and oil in the breather of an engine. I'll try to find it and post it for you. I think it had some good instructions that may relate to your issue.
Here's one link that discusses issues that may be helpful to you, although a different engine:
http://www.6x6world.com/forums/engin...-fuel-oil.html
Here's another link:
http://www.6x6world.com/forums/engin...series-ii.html