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I have a 316 with a rebuilt Kohler KT17 series 2 engine. This last snow storm I noticed the tractor losing power as it warmed up. I mean I let run for about 20 minutes before I started blowing snow and it seemed fine for the first 15-20 minutes of blowing snow, but then the distance it threw the snow slowly became less and less. It burns oil so I thought I'd put more in. I thought I'd try to put more oil in with the tractor idling as slow as it goes. Yeah, I know, bad idea.

But when I took out the oil dip stick (the big one not the little one showing the actual oil level) and I couldn't believe how much pressure was coming out of the hole. It reminded me of a farm tractor exhaust. The little puff of smoke that comes out every time a cylinder fired. I then remember the blocks are pressurized while running (I think) but not that much and that intermittent. It smelled like exhaust so I wondered if there is blow by or blow back (when rings aren't containing the pressure of the combustion).

Should there be so much pressure in the block while running?

Thanks, Eugene.
 

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Eugene,
The crankcase should maintain a weak vacuum through action of the one-way breather valve. A downward piston stroke would tend to pressurize the crankcase, but the valve opens exhausting the crankcase gas. An upward piston stroke creates a partial vacuum in the crankcase as the breather valve closes. The overall result is a net vacuum unless there is excessive blow-by or a defective breather valve. Opening the oil fill tube during engine operation is not a standard procedure (it is a good way to get a face-full of oil), and the proper way to measure crankcase vacuum is with a water manometer. The Kohler service manual describes the manometer and it is easily constructed with some clear tubing from a hardware store.

You indicate the engine was rebuilt. Did you do the rebuild, or do you know if it was done properly? Is it fully broken in--how many hours since the rebuild? The engine bogging down as it heats up could be from any number of causes. Does it run hot, and does it miss? A lean carb setting, poor timing, and plugged cooling fins can cause overheating. Does it run better with a bit of choke? Try adjusting the carb and points. Check for air leaks at the intake manifold and carb gaskets. Look for a leaking head gasket(s) when the engine warms up. Disassemble the breather to check for proper operation of the valve.
Harold
 

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To expand on the breather and the relationship to the experience you had: When the pistons come down displacing the air in the crankcase, with the dipstick out, the breather can't do it's job as the pressure is released through the dipstick hole. Now, when the pistons go back up, the air is drawn in through the same place. The vacuum that should be present is never developed, instead the air drawn in is expelled on the next down cycle. As you may have noticed a lot of oil can accompany that! There is always a lot of oil in the air inside the crankcase as it 'leaks' past the rod and main bearing when running. The crank counterweights then throw that all around, and the squirt holes in the rod upper half contribute filling the area with droplets and mist. That's how the cylinder bores and camshaft lobes get lubed.
 
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