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Old 05-27-2011, 07:34 AM   post #1 of 16
_yerhuckleberry
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Help Problem with getting JD STX38 to run

This stx38 has the Kohler 12.5 single cylinder engine & a black deck. I forget what year it is. It was running well this spring, then sputtered & died last week. At first it was turning over but backfiring, & it was flooding badly with fuel leaking via the exhaust. I thought it was a needle seat problem so put in a new kit & made sure carb is clean with all jets clear & other parts operational. Seems to have fixed extreme flooding issue. Also put in a new plug (& confirmed good spark) & a clean air filter (also removed filter during "start" to ensure wasn't starving for air).

So, it has fuel, spark, & air; but it's still turning over without firing/starting, though it backfires nearly each time after engaging the starter 4 to 10 seconds, & it often backfires multiple times in succession before the bendix spins back out to disengage the starter. Did that make sense? I'm sure I've probably left something out.

I'm working without a repair manual & so far haven't found what I need to know online, so I'm hoping someone here can help. Thank you. I appreciate whatever bits of help can be offered.
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Old 05-27-2011, 02:28 PM   post #2 of 16
ttcfan4476
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Default Re: Problem with getting JD STX38 to run

sounds like a sheared timing key
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Old 05-27-2011, 04:07 PM   post #3 of 16
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Default Re: Problem with getting JD STX38 to run

I agree with flywheel key.mine would sheer them often,1.74 and away it would go.They seem to be bad for that.
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Old 05-27-2011, 09:59 PM   post #4 of 16
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Question Re: Problem with getting JD STX38 to run

Thanks guys (if I''m not being too presumptuous?). WindowGoblin, my condolences on recent loss of your rocks.

Once the cowling is off, what's next? Is this simple enough someone could give me a basic walk-through on where to locate, how to confirm, and how to repair; or will I need to buy a shop manual (which would be less of a financial hardship and theoretically less of a time restraint later on) in order to get this done? I assume some of the repair might be self-explanatory, but this isn't one I've tackled before. Or maybe there is a helpful link that will more easily break it down for me? Sorry for all the questions.
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Old 05-27-2011, 10:12 PM   post #5 of 16
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Default Re: Problem with getting JD STX38 to run

With the shroud off take the nut off that holds the flywheel down. Once you have it off look at the cut in the crank and flywheel and see if the keyway is keeping them aligned. If not the key has sheared and the timing is off as a result.
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Old 05-27-2011, 11:23 PM   post #6 of 16
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Default Re: Problem with getting JD STX38 to run

Quote:
Originally Posted by BOSOX View Post
With the shroud off take the nut off that holds the flywheel down. Once you have it off look at the cut in the crank and flywheel and see if the keyway is keeping them aligned. If not the key has sheared and the timing is off as a result.
Thank you, sounds good BOSOX.
Before I tear back into this tomorrow, anything else I should be looking for that could fit the bill? Same details would be hugely appreciated.
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Old 06-10-2011, 07:50 AM   post #7 of 16
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Default Re: Problem with getting JD STX38 to run

Well, at this point the flywheel/crank key is still in place. I checked the magneto and pickup coil, as well as gap. Still have spark. Battery and ignition switch are recently new. Since we've had to borrow my dad's STX38, I swapped the coil between them before starting to buy more new parts that can't be returned. His still starts and runs with no difficulty using my potentially bad part, and mine continues same symptoms with his apparently good part. According to the Kohler manual for "engine cranks but will not start" the problem should be a faulty spark plug or a faulty ignition module. Doesn't make much sense to me but could the ignition module be weak enough to not start one engine, but strong enough to start another?
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Old 06-10-2011, 08:52 AM   post #8 of 16
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Default Re: Problem with getting JD STX38 to run

One thing to check is the air vent hole in the gas cap, is it clear.

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Old 06-10-2011, 08:59 AM   post #9 of 16
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Default Re: Problem with getting JD STX38 to run

and how about your gas. Is it possible that you have water in your gas?
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Old 06-10-2011, 09:36 AM   post #10 of 16
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Default Re: Problem with getting JD STX38 to run

The gas cap vent should be fine. Having no problem with fuel to the carb and I've loosened the cap to be sure.
As for water in the fuel, my initial thought was "no" since the fuel was fresh and mower was running well before it sputtered to its present state, but I know my wife isn't always vigilant about putting gas cans away. Also, we've had a lot of rain recently, so even though a capped can left out in the rain shouldn't have taken in any water, I can't say with absolute certainty that the fuel is 100% dry. I'll ask my wife if she added fuel shortly before it stopped running, though I think the same fuel went to the tiller which still runs fine.
How much/little water would it take in fresh gas to completely kill the engine? Would adding fuel drier be sufficient or should I purge her to rule this out? If water is an issue, shouldn't it fire up for at least a few seconds when using starting fluid or am I all wet?
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Old 06-10-2011, 03:51 PM   post #11 of 16
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Default Re: Problem with getting JD STX38 to run

OK, nothing I like better than a project. Let's see if we can get this figured out for you.

Now, you mention the Kohler manual. Did you download the repair manual off the Kohler site? They one I got is well over a hundred pages long and is an Adobe .pdf file. Very informative.

Step 1. When you crank then engine over, is there compression? Take the spark plug out and put your finger over the hole while cranking. Don't get shocked! This engine has a compression release which will lower the compression a bit, but you will still feel the compression stroke.

Step 2. When you turn the key on, do you hear an audible "click" coming from the carburetor? There should be a wire connected to a solenoid on the bottom of the carb. This is a setup to prevent gas from entering the engine when the key is shut off. No click? Report back and we'll address this issue next.

Step 3. Do you have a shut-off valve on the gas line? If so, shut it off, remove the wire from the fuel solenoid on the bottom of the carb, and remove the solenoid (which holds the carb float bowl on). Is there a rubber "cap" on the plunger? There should be, so if not it may have come off and gotten stuck in the fuel passage. Catch the gas that comes out of the carb in a pan or bowl and see what it looks like. Water? Debris?

Step 4. Read in the manual about the cam/crank timing. This could be the issue. Also, the compression release may be stuck/broken.

This is a place to start, so let's see what you find out from here.
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Old 06-10-2011, 05:07 PM   post #12 of 16
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Default Re: Problem with getting JD STX38 to run

Brad you do such good work.

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Old 06-10-2011, 10:59 PM   post #13 of 16
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Default Re: Problem with getting JD STX38 to run

It's great to have your help, Brad. I'm still working on some of your questions but wanted to get the one's I can presently answer out of the way.

Alright, first the manual. The one I have is the service manual from Kohler's site in .pdf format for the CV12.5s, spec 1270. On the back cover, it is identified as "FORM NO.: TP-2339-D".

There is compression. I forget the measurement but will check again for confirmation. It did seem a bit low but I also measured the compression of my dad's "running" stx38, which was only slightly higher, for whatever that is worth. Granted, my tester is an older inherited tool (not the thread-in type), but the previous owner wasn't the sort of gentleman to keep inoperable equipment in his kit so hopefully it is close to accurate.

This carb is a Walbro without a shutoff solenoid - just the float bowl. You suggested inspecting the fuel I catch from the bowl for debris & water. I'm not sure what to look for to identify whether there's any water present.

Speaking of the cam/crank timing, are you referring to (sections 9 & 11 if we happen to be looking in the same manual) the timing marks on any of the shaft gears in the crankcase, or something else?

On my way out to make use of whatever daylight still remains. I plan also to recheck the spark, measure resistance from the ignition module lamination stack to the high tension lead, and disconnect the kill wire to be redundant in checking function of the coil. Since my knowledge and understanding are limited I hope these last two, that were mentioned somewhere in the stacks of what I've recently read, won't be a waste of time.
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Old 06-11-2011, 05:31 AM   post #14 of 16
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Default Re: Problem with getting JD STX38 to run

OK, so far, so good. You have the book/file that I was referring to. That is a great reference.

You have compression. It will be less at cranking speed than you might expect due to the compression release. This could possibly be a problem area, but let's address this a bit later. Want to start outside the engine and work inwards, hoping to find the problem at the most likely point, with the least amount of digging.

If you drop the carb bowl and there is indeed water in there, you'll be able to see it. A good receptacle would be a glass jar. If there is water it will be separated from the fuel. It will look clear and the fuel will be colored. If you have some water, I'd take the fuel line off and drain the gas tank completely to clear that out as well. Due to the fact that you already had the carb off to replace the needle/seat, additional water in the bowl will indicate it's presence in the tank.

Have you checked the oil level? You mentioned a substantial flow of fuel going thru the engine and out the exhaust. This is not a good situation. Some of that will find it's way into the crankcase, and will cause problems. If the crankcase is over-full, time for an oil/filter change to prevent damage and see if that is adding to the problem.

The kill wire could certainly be an issue, if it's worn through. Also, make sure the wiring harness is secure on the key switch. I "doubt" that the coil is bad, since you swapped yours with your dad's and the issues stayed with the tractor, not the coil.

Lets see where this takes you and you'll either be mowing or digging deeper! Also, if others have some thoughts/ideas, please jump in.

Good Luck.
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Old 06-15-2011, 05:00 PM   post #15 of 16
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Default Re: Problem with getting JD STX38 to run

Fuel is good. Compression is 60. The oil level was just over "full". When I changed the oil & filter there was some fuel in the oil which may have been due to the needle seat issue. There hasn't been any more fuel dripping out the exhaust.

Nothing is different when I disconnect the kill switch at the coil. Resistance through the pickup coil read 9,060 ohms. Spark is white hot and consistent. The only thing I could find in the manual about timing with this model is: "automatically controlled by the module. Therefore, other than periodically checking/replacing the spark plug, no maintenance, timing, or adjustments are necessary or possible with this system." Am I overlooking something else with the timing?

It'll still flood if kept choked while cranking. The backfiring only seems to be happening when not choked. Vacuum at the carb seems okay, but I'm not sure I could tell if it were just a little low as opposed to drastically reduced.

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