[FlLiving]

The Exmark has sat for a while after running well. It has a Kohler Command 25 that I have replaced 1 head on, a few seals and such in the past 3 years. It ran today for a good 5 minutes idling, went to mow and it made it 20 feet before surging and shutting down. Restarted and idled fine, behavior repeats.

The last time it acted like this it needed a new head (other side last time), and another time the carb was full of junk. I opened the oil fill cap when it was idling and a huge amount of exhaust pressure is vented continuously. When I say a huge amount, you can feel it blowing from a good 2 ft away. Is this indicative of the head being bad?

Thank you in advance.

 

[Ridin' Green]

The oil cap seals the crankcase which is what gives you full pressure lubrication. It should have plenty of pressure normally.

Sounds like a fouled carb to me, especially after sitting for some time. try some Sea Foam or Star Tron in the fuel and run it for a while. I've gotten several engines that acted like yours to run great just by doing that.

 

[Richard Martin]

The oil cap seals the crankcase which is what gives you full pressure lubrication. It should have plenty of pressure normally.

Pressure inside of a crankcase is the last thing you want. When the manufacturers speak of full pressure lube they're saying that there is an oil pump and oil passages for the oil to get to the various parts that need it. If you have pressure inside of the crankcase then something is wrong. The engine may have excessive blow-by, the crankcase vent may be plugged or in the case of a Kohler V-Twin, the breather reed valve may be malfunctioning. Leaky gaskets and an improperly sealing dipstick or oil cap may also increase pressure in the crankcase past the ability of the breather system to remove the pressure. Once the engine is started you don't want any way for extra air to enter the crankcase. Almost all 4 stroke mower engines have a crankcase breather. Even the old Briggs single cylinder 3 hp engines had one.

One of the problems associated with excessive crankcase pressure is a leaking governor shaft seal.

I suspect that if the OP is NOT having an oil leaking problem then the crankcase pressure (or lack there-of) is fine.

 

[Ridin' Green]

That may be correct Richard, but it is the complete opposite of what my dealership head mechanic told me about how these small engines are designed to function, and the reason they have a seal in the dipstick lid.

 

[Richard Martin]

Oh, check the carb again.

 

[Richard Martin]

That may be correct Richard, but it is the complete opposite of what my dealership head mechanic told me about how these small engines are designed to function, and the reason they have a seal in the dipstick lid.

A splash lube engine does function that way in that they throw oil everywhere. But even their crankcases are vented to the intake manifold.

The reason his engine was expelling so much air is because it was also pulling in an equal amount of air through the open cap. Things happen very fast and although you can't see the air getting sucked in, rest assured, it's happening.

 

[Blade Runners]

Were the head gaskets installed and torqued correctly? I know a water cooled engine will pressurize the radiator when the head gasket is blown, not torqued, or installed improperly. Air from the cylinder compression stroke leaks under the head gasket and into the cooling jackets/ports in the cylinder block then into the radiator/cooling system causing it to bubble as if it is overheated, and in extreme cases, water will just shoot out of the radiator. Maybe the same is happening through your oiling sytem and pressurizing the crankcase. I'm not familiar with how the oiling system works so I am only venturing a guess but your symptoms sound very similar.

In any case I would strongly suspect what ever parts were changed, esp. the head gasket.

 

[Blade Runners]

Also, did you replace the headbolts when you replaced the head? Some head bolts arent made to be reused. They stretch and are actually made that way by design. The headbolts on my Yamaha quads have to be replaced anytime the head is removed due to this.

 

[smallengineshop]

I do mechanic work and it sure sounds like a carb to me.

 

[Blade Runners]

Can you please explain how a carb pressurises a crankcase?

 

[soloscaperman]

Please keep me up to date it's been over a year since I bought my 1998 Kawasaki ZX9R Ninja and when I take the oil cap off it blows air at me like a compressor. I am thinking PCV either it be a mower or Streetbike?? Bike still lays rubber if you peg it at 20MPH in 1st gear. (have an extended swing arm so it won't wheelie ;-)

 

[Richard Martin]

Please keep me up to date it's been over a year since I bought my 1998 Kawasaki ZX9R Ninja and when I take the oil cap off it blows air at me like a compressor.

The days of me arguing with people are over. I have a small engine certification but that's okay...

And yes, I do get the sarcasm. :laugh:

 

[blakdawg]

Been here or thier rings and cylinder wiped out. Common on a kohler. One cylinder runs hotter than the other and will fail over time. Compression test and leak down test. Did you look at the cylinder when you replaced the head? Good luck. I just put in a short block.