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I have a Craftsman LTX1000 lawn tractor. I think it is about 7-8 years old. I bought it used this year and am having a throttling issue. (engine: Briggs & Stratton Small Engine Model 407777/0121-E3 20HP OHV I/C Intek Platinum).

When throttling engine to full it only runs about 1/3 power. When I take cover off to access air filter the carburetor shaft is exposed and the engine speed cranks back up. I can run the engine at low throttle and even half choked but when I go to full choke and full throttle there is very little change. There is no way it will actually cut grass. I have tried everything I know how to do:

- Suspected bad gas so drained and refilled
- Changed the oil
- Replaced the air filter
- Removed the muffler to see if there was build up of soot. There was none. I even ran engine with muffler off and still had the same issues.
- Checked the throttle cables. They are moving freely. No broken springs or anything.
- Cleaned all the fuel lines
- Replaced the fuel filter
- Sprayed carb cleaner into carburetor shaft and even removed top plate near carburetor (head gasket?) and cleaned that. I am not sure what this was but it had a gasket and several cavities
- Replaced the spark plugs and checked gap (.040). This helped as the engine cranks much better.
- Tried running with gas cap off in case the small hole was clogged. No difference.

One other strange thing I have noticed since I had the mower. Every time it gets to the last bit of gas before running out the engine suddenly runs really high for about 15 seconds before running completely out. It did it again today - was trying to fix it when it suddenly started running great only to find out it just ran out of gas.

Any ideas? I am about as far as you can get from being a mechanic and I am about to pull what hair I have remaining out my head! Please help!

First, easy quick thing to do is..... Go out and unplug one of your spark plug wires and try to start and see what happens. Then plug it back in and try the other wire. Come back here and tell us what happened.

Fish said:

First, easy quick thing to do is..... Go out and unplug one of your spark plug wires and try to start and see what happens. Then plug it back in and try the other wire. Come back here and tell us what happened.

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Okay - I unplugged first wire and engine started up and seemed to run stronger. I plugged it back and unplugged the other and the engine would not start - just turned over. Also, I didn't mention in my other post that engine does smoke a bit from the muffler. Smoke is black and is not overly thick but smoking enough that I should mention it.

Well that tells you which side you need to look at!!!!

You will have unburnt fuel residue, when one side is not working.....

Pull the valve cover off of the unworking side, and that will likely tell us what we need to know....

Fish said:

First, easy quick thing to do is..... Go out and unplug one of your spark plug wires and try to start and see what happens. Then plug it back in and try the other wire. Come back here and tell us what happened.

Click to expand...

Fish said:

Well that tells you which side you need to look at!!!!

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So, since I changed the spark plug does this mean I need to change the wire next? Or is there something else to look at? Sorry, I am a mechanical neophyte

No, your trouble likely lies under the valve cover held on with the 4 bolts....

Pull cover off...a little gas spilled out. The valves look clean. There is some oily/gas dripping off valves. Is this what you mean by unburnt residue? Took pic:

Garicha said:

Pull cover off...a little gas spilled out. The valves look clean. There is some oily/gas dripping off valves. Is this what you mean by unburnt residue? Took pic: View attachment 23140View attachment 23140

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Not sure what to do at this point...can anyone else weigh in? Thanks!