Tractor mower.
I have been bothered for a few years by it burning oil, running rough intermitently and oil sucked into the air filter.
It seemed better if I allowed the oil level to drop, rather than top it up, naughty I know.
Compression seemed good, between problems it ran well enough, but...
Today I set about it, to earnestly find out what was wrong - I had sort of convinced myself that the dipstick might be the wrong one, maybe too short. I got as far as pulling the carb off, then the metal box engine breather. That was bolted into a rectangle cast into the block and seemed to have a one way valve, then a baffle. The rectangle in the block had a larger hole at the top, then a smaller one right at the bottom. The breather was full of oil, as had been the rectangle behind it in the block and the pipe leading up to the air filter.
Other than the sheer amount of oil where it didn't ought to be, I found nothing obviously amiss. I was about to build it back up, when I thought I wonder if the small hole in the bottom of the rectangular casting, might be intended as an oil drain for what was thrashed up to the breather, so just to be sure it was clear, I poked some thin wire down it. It had to drain back down to the sump. It didn't seem to be obstructed, so I gave up and built it all back up for a test.
Anyone know if that small hole might have been a drain and might have been the cause of my issues?
Along the way I spotted that there was no gasket between head and intake manifold, so I made one from some cornflake box and that who ever had built it, had managed to cross thread one of the two nuts, bolting the air box to the carb manifold. This left a massive gap on one side, such that much of the air would bypass the air filter.
The test was fine, no smoke and the best it has ever run, but I do wonder if it might be because the engine breather was clear of oil and none being forced up to the air filter or whether the drain might have been blocked after all. Place your bets gentlemen...