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I have a John Deere LT 155 with a 15 hp Koehler engine. It

Customer Question
will start and mow for...
I have a John Deere LT 155 with a 15 hp Koehler engine. It will start and mow for about 2 minutes, then it begins to sputter until it finally dies. It will usually restart, but will sputter and die when you try to mow. It has a new fuel filter. If you allow it to cool, it will start and mow a short time again before it sputters and dies.
Submitted: 8 years ago.Category: Small Engine
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Answered in 7 minutes by:
7/28/2009
Technician: James W, Small Engine Troubleshooting Expert replied 8 years ago
James W
James W, Small Engine Troubleshooting Expert
Category: Small Engine
Satisfied Customers: 3,708
Experience: 7 years as Mechanic & Parts Manager for Brother who has Owned Lawn & Garden Repair shop for 35 years
Verified
Check one thing first..

Start running, then when it starts to sputter, loosen and remove the gas cap. It may not be venting correctly. If this makes it work, replace the cap with a new one..

You have several possibilities.



Just read all of my answer before you act.



1. Todays gasoline is not the same as it was just a few years ago. There have boon changes in gas formulas. DO NOT USE REGULAR 87 OCTANE GAS! Use a good quality, good brand 89 octane fuel. DO NOT USE CHEAP GAS.



2. One of the problems you seem to have is a phonomenon called "VAPOR Lock".

This is caused when fuel vaporizes in the fuel line and doesn't allow the proper amount of gas to get to the carburetor. This is usually caused bu the fuel ine getting too close to a heat source such as the muffler of the cylinder head or even sometimes the engine block. Also cheap fule of 87 octane fuels will cause this to happen more often. Check all the fuel lines and make sure that they are away from heat.



As mowers are operated, they tend to accumulate grass under the engine shoud. We call this a "Birds nest or mouse nest." These nests will keep the engine from cooling as they block the flow of air to the head and cylinder. Clean this grass out several times a year.



3. Another issue can be a fuel issue in the carburetor.

As engines sit or get older, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems.

Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.

More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carb. It Must be removed from the engine, cleaned very well, blown out with compressed air and reassembled using a NEW carburetor rebuild kit. ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carb too. Dirt and water from a dirty fuel tank will also plug up the carb. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carb kit.



4. Make sure that you install a new Air Filter. DO NOT CLEAN AIR FILTERS always REPLACE. Replace air filters no less often than every 20 hrs of use. Plugged air filters will cause many problems.



I will also provide an Over heating template to try to, but I think that is a longer shot than what I said first..

What could cause the overheating problem?



Overheating can be attributed to two things something getting to hot or something not getting cooled properly.



Lets take the situations that would cause something to get too hot first.



Engines can create extra heat by having the engine run too lean....(Not getting enough fuel) Fuel is not only used to create the power in an engine but it is also used as a coolant in the combustion chamber. If it runs too lean, then the explosions will create more heat and then there is also not enough fuel to help cool the engine.



What would cause the engine to run too lean?



As engines sit or get older, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems.

Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.

More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carb. It Must be removed from the engine, cleaned very well, blown out with compressed air and reassembled using a NEW carburetor rebuild kit. ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carb too. Dirt and water from a dirty fuel tank will also plug up the carb. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carb kit.



Ok now lets see what could cause the engine to have insufficient cooling.



When small engines run, they build up heat as a natural byproduct of combustion.

Just as a wood burning stove gives off heat so does a combustion engine. Different fuels will produce different amounts of heat. Coal for instance burns much hotter than wood.....a hardwood such as oak or hickory burns much hotter than softwoods such as soft maple of cottonwood. The same can be said for gasoline. 87 octane gives off much more heat than 89 octane of premium. That is because 87 octane explodes and all of the heat is transferred to the piston in one very fast burst. It cannot be transferred through the piston and cylinder to the air fins where the air from the cooling fan takes it away fast enough to keep the engine running as cool as we would like. On the other hand, mid-grad 89 octane and premium fuels do not explode...they burn in a much slower manner and the added time aids the engine in cooling., so the engine runs much cooler. This is due to the antiknock compounds that have been added to the fuel. The tiit-knock compounds job is to slow the burning process and to eliminate the explosions that result in the knock (or pre-ignition) that occurs with regular gas. The problems we are having with the gas these days is that they keep making it cheaper and cheaper by not adding as many additives as they one did. These additives are very expensive.

So as a conclusion, we do not recommend using 87 octane in ANY SMALL ENGINE. We would prefer that only good Brand (NOT CHEAP DISCOUNT STATION) gasoline is used in all 4-cycle engines and Premium MUST be used in ALL 2-cycle engines.



Another situation that causes the engine not to cool well is that grass builds up under the engine cover....we call this the birds nest affect.....When grass is built up under the shroud or cover as we call it, it blocks the flow of air around the cylinder and block of the engine. This prevent the engine from cooling because it cannot take away the heat that was created during combustion. In many cases this grass build-up gets so hot that it can even create a fire under the cover.



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Customer reply replied 8 years ago
I drained tank and put in 91 octane fuel and went to start, and now it seems to be blowing unburnt gas out the exhaust. An ideas? Thanks Monty
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James W
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Category: Small Engine
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