Problem with my Kohler CH20S

Newbie here from the U.K. so hope someone may be able to help. I've been using a Scag Turf Runner with a vee-twin petrol (gasoline) Kohler for 20 years. Engine details attached.

Yesterday I started it and was just about to put the deck into gear and she suddenly stopped. She wouldn't start so back in the workshop. Took both plugs out and put one back in the HT lead and earthed it and cranked the engine. Unfortunately I was looking closely for a spark and I was literally showered in petrol. Long story short, I checked the dipstick and petrol was very nearly up to the top, way past the full mark. Today I pulled the sump plug and got over a gallon of mainly petrol mixed with the engine oil out of the engine. Somehow petrol has been getting into the sump - lots of it !

It appears that this engine has a pulse style fuel pump (not come across these before) Apparently it sucks fuel out of the fuel tank on the downward stroke of a piston and then pushes it into the carburettor on an upward stroke of a piston. Presumably there are some valves in the pump which aren't working as they should and herefore allowing a lot of petrol into the sump.

Or is it possibly nothing to do with the pump and instead it's a damaged needle valve or a stuck or damaged float in the carburettor ?

It's all I can come up with - does anyone else have any other ideas please ? TIA for any help.

Most common cause is the float valve not sealing allowing the carb to overflow and fill the cylinders and then leak past the rings into the crankcase, or in some cases the gas will run down the breather tube from the air filter housing into the sump. the fuel pump could be a possible cause but doesn't seem to be common.

You can take one of those auto brake bleeder vacuum pumps and connect it to the fuel pump pulse line and while covering the small mesh vent screen on the fuel pump, put a small amount of vacuum on the pulse line and see if it will hold vacuum. If it looses vacuum then bad pump.

ILENGINE said:

Most common cause is the float valve not sealing allowing the carb to overflow and fill the cylinders and then leak past the rings into the crankcase, or in some cases the gas will run down the breather tube from the air filter housing into the sump. the fuel pump could be a possible cause but doesn't seem to be common.

You can take one of those auto brake bleeder vacuum pumps and connect it to the fuel pump pulse line and while covering the small mesh vent screen on the fuel pump, put a small amount of vacuum on the pulse line and see if it will hold vacuum. If it looses vacuum then bad pump.

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Thank you for that. Yes, I think you are correct and that is what is happening because today I did a little experiment. I took the pipe off the pump that goes to the crankcase and substituted a short piece of pipe in it's place. I then took the pipe off the pump that goes to the carb. and substituted another short piece of pipe in it's place. I left the pipe from the fuel tank still attached to the pump.

I then alternatively sucked and blew down the pipe that I had substituted for the pipe that goes to the crank, thereby mimicking the action of the crankcase pressure. The result was that small amounts of petrol were emitted out of the pipe that was replacing the one that normally goes to the carb. And I didn't get any petrol in my mouth and so I reckon that the pump is working as it should. We've had 28mm of rain already today in my part of the U.K. so I reckon I will be back in the workshop tomorrow investigating the carb. :wink:

Raining here today also. Expecting 1.5-2 inches today into tonight, which would translate to 37-50 mm for you.

There have been some developments. Today I attached a piece of pipe to the intake pipe that is normally fed by the pulse pump. I then put a squeezy bottle containing petrol onto the other end of the pipe, inverted the bottle and applied pressure. There was petrol (gas.) streaming out of the carb, proving that the float arrangement/needle valve wasn't working as it should. So I took the top of the carb. c/w with the pipe and held it over some water in a dish and then pressurised the pipe by blowing into it. Sometimes I could get bubbles out from under the carb. top, sometimes the float was obviously pushing the needle valve up onto it's seat as it should.

So I dismantled the needle valve and it looked O.K. but there was some crud in the bottom of the float chambers and so I cleaned everything up and put it back together again and repeated the test with the top back on the carb. This time, when the float had risen sufficiently it closed the needle valve and I couldn't pump any more petrol into the pipe. I think I will try and start it again when I get a couple of spark plugs that I have ordered but I am beginning to think that there was some crud that was stopping the needle valve from seating properly. And if I'm honest, I don't think the fact that the needle ivalve s spring loaded helps the situation.

The spring in the center of the float valve is what they call an anti bounce valve. When mowers are hauled on a trailer or used over rough ground the bouncing can cause what is called float bounce, and this can cause the carb to overflow and full the crankcase with gas as the mower is being hauled, or can cause flooding when operating on rough ground.

ILENGINE said:

The spring in the center of the float valve is what they call an anti bounce valve. When mowers are hauled on a trailer or used over rough ground the bouncing can cause what is called float bounce, and this can cause the carb to overflow and full the crankcase with gas as the mower is being hauled, or can cause flooding when operating on rough ground.

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Mmmm. That makes sense. Thank you. It just felt, to me, as thought the spring was almost overiding the floats. In other words, I wondered if, when the floats were rising was the spring in the valve being compressed rather than the needle valve being pushed up into it's seat ?