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Our scag mower developed a strange problem when taken out of storage this year. By all appearances it runs very rich. The engine has about 300 hours. It was fully serviced, carb was cleaned and valve clearance was adjusted. I took the airbox off and I can see fuel misting out of the carb. It would use a 1/4 gallon of fuel in about 15 min of running. The first thing I did was to check if both cylinders are running by taking the spark-plug lead off. The engine appears to run equally rough on either of the cylinders. That tells me that the ignition is fine. I can see black smog coming from the muffler. Right away I took the carb off and checked the needle seal by apply compressed air to the fuel inlet. The seal seems to hold air when the float is top position. Here is what I suspect.

1. Fuel pump is defective and pushes too much fuel into carb?
2. The mechanic didn't do the valve clearance right. The poor fellow was nearly 80 and unfortunately passed away after a bout of flu in Feb. However, he was always very competent in the past.
3. One of the head gaskets is blown?

I am leaning toward third option. For one it seems like the fuel is misting out of the carb rather than being sucked in. Doesn't indicate that there some positive pressure may be entering the intake manifold? This could also explain excessive pressure that causes fuel pump to increase the fuel pressure.

Or could the old fellow make a mistake in valve adjustment? I only just ordered the leak down test kit. Anything else I can do to narrow down the problem in the mean time?

I think it is definitely high crankcase pressure. It blows oil out of the dipstick tube. That must be indicative of the blown head gasket. Should try to diagnose which side is the culprit or replace both of them?

You can easily verify the above by removing the breather tube that runs between the engine & carb.
No more blowback = Compression leak / blown gasket
Same blowback = valves

bertsmobile1 said:

You can easily verify the above by removing the breather tube that runs between the engine & carb.
No more blowback = Compression leak / blown gasket
Same blowback = valves

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I've disconnected both the breather tube and the tube to the fuel pump. Yes, the crank pressure seems to have normalized. It is very odd, because the compression reading is at 80psi on one cylinder and 170psi on the other. I've taken the valve covers off on both of them and there are no sticky valves. The valve train seems fine. It is got dark and I couldn't test the valve clearance but all valves looked to be closed at the TDC for the respective cylinders. The manual states that any pressure over 60 PSI is passable. I cannot understand why there is such a difference in psi between two heads.

small hole in piston cause it ate a small bolt from the carb butterfly, loose head bolt compression leak into crankcase broken ring, sticky ring.
Take the plug out of the suspect side replace the impulse line & breather and see if the problem persists.
Problem goes away then pull the head on the dud side.
I just did an intek twin with the valve seat loose in the head
Gave similar synptoms to your engine.
Thought it was a head gasket till I had a good look at the head.
Oddly enough it was the inlet which usually never gets paticularly hot.

bertsmobile1 said:

small hole in piston cause it ate a small bolt from the carb butterfly, loose head bolt compression leak into crankcase broken ring, sticky ring.
Take the plug out of the suspect side replace the impulse line & breather and see if the problem persists.
Problem goes away then pull the head on the dud side.
I just did an intek twin with the valve seat loose in the head
Gave similar synptoms to your engine.
Thought it was a head gasket till I had a good look at the head.
Oddly enough it was the inlet which usually never gets paticularly hot.

Click to expand...

Thank you Sir! I am on it. I suspect that the side that makes high PSI is the culprit.

There is some witchcraft going on here. I took the compression measurements today and it is 75 on one and 90psi on the other cylinder. I must have misread the gauge in the dark last night. The crankcase have been filled with gasoline overnight. I am draining it all now and will fill with fresh oil.

I feel like a complete moron now. Having discovered a crankcase full of gasoline I decided to take a closer look at the carburetor again. Again, I pressure tested the needle/seat and it holds 100psi when closed. Then I was about to put it back together when I observed that the main jet was missing!!! We had a mechanic doing the winterizing service and apparently he took the jet off for cleaning and assembled the carburetor without putting it back. He drained the gas tank too, so the machine stood w/o fuel until last week. I've check the oil before fueling it and it looked fine and at level. I can only assume that the fuel was just gashing past the jet-less tube and pouring into the heads. Eventually, it built up pressure up in the crankcase and causing all these wild symptoms. I can't be cross with the old fellow who serviced it for he has passed away in the winter. I can only be cross with myself for not noticing it earlier. Well, I ordered the new jet,so hopefully that will fix the matters

Ok, I ordered a new jet and assembled the carburetor. The engine run fine now. However, the morning after I found the crank case was full of gas. I think the needle needs replacement.

Or install a fuel shutoff valve.

I am curious as to how an engine with a fuel pump gets a crankcase full of gas overnight. Usually that's a gravity-feed system problem.

cpurvis said:

Or install a fuel shutoff valve.

I am curious as to how an engine with a fuel pump gets a crankcase full of gas overnight. Usually that's a gravity-feed system problem.

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I have a fuel shut off valve and I have been using it since the "incident" and there were no recurrences. The gasoline flows freely through the pump when the engine isn't running. This is a standard pressure driven pump. Do you think I need to replace it?

c5vettster said:

I have a fuel shut off valve and I have been using it since the "incident" and there were no recurrences. The gasoline flows freely through the pump when the engine isn't running. This is a standard pressure driven pump. Do you think I need to replace it?

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I don't see why you need it at all if gas reaches the carburetor on its own.

cpurvis said:

I don't see why you need it at all if gas reaches the carburetor on its own.

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Hmm, a good question. You don't reckon the field pump is broken?

The fuel pump is a simple diaphragm with a very light return spring and a one way valve on the outlet.
Nothing in there to stop the fuel flow and most will happily syphon if the end of the fuel line is lower than the level of fuel in the tank.
So as long as you are not hunting when the fuel tank is getting low then your pump is fine.

bertsmobile1 said:

The fuel pump is a simple diaphragm with a very light return spring and a one way valve on the outlet.
Nothing in there to stop the fuel flow and most will happily syphon if the end of the fuel line is lower than the level of fuel in the tank.
So as long as you are not hunting when the fuel tank is getting low then your pump is fine.

Click to expand...

Thanks! That explains how my crankcase got filled with gasoline.

c5vettster said:

Thanks! That explains how my crankcase got filled with gasoline.

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I don't think so.

Gas has to get by the needle valve and overfill the float chamber in order to get in the engine.