John deerestx 38 stops running

[pawandmaw]

I have a john deere stx 38. It has a kohler 12.5 command engine. The engine numbers on the motor are worn off . The mower runs good then quits after running 5 to 10 minutes. I put a new fuel line on, the other had a small hole. The carb is clean the engine runs smooth. then quits. I check spark plug after it stops the spark plug has plenty of fire. The oil level is good. Once it cools down it starts right up.
Thank you for your help
Paw

[bgsengine]

check the fuel pump - they like to have the check valves fall out of place when it gets hot and fuel flow stops, vapor locks until it cools down. have had 2 or 3 of them in the last couple years, and a couple where the fuel pump diaphragm ruptured and dumped gas into the oil, so make sure and check oil too.

[StarTech]

Try squirting a fuel in the carburetor throat when it quits to see if it is a fuel related problem but I would lay odds on it being an ignition coil problem; although, a spark failure is possible. Just because it has a good spark when not under a compression load doesn't mean it is firing good when under compression with a fuel load. Sounds more like the electronic trigger in the coil is failing after the coil is heated up by the engine.

BTW is this a black deck or yellow deck STX38? At least all the 12.5hp engines used the same coil but there differences in the other engine parts that would need to be narrow down if needed... there is about 10-12 versions of the CV12.5 that were used on the yellow deck version.

[KE4AVB]

check the fuel pump - they like to have the check valves fall out of place when it gets hot and fuel flow stops, vapor locks until it cools down. have had 2 or 3 of them in the last couple years, and a couple where the fuel pump diaphragm ruptured and dumped gas into the oil, so make sure and check oil too.

Going to be a little hard to do since they are gravity fed systems on both the yellow and black version of the STX38.

[Deere2me]

I have a john deere stx 38. It has a kohler 12.5 command engine. The engine numbers on the motor are worn off . The mower runs good then quits after running 5 to 10 minutes. I put a new fuel line on, the other had a small hole. The carb is clean the engine runs smooth. then quits. I check spark plug after it stops the spark plug has plenty of fire. The oil level is good. Once it cools down it starts right up.
Thank you for your help
Paw

Check the fuel ( anti-afterfire) solenoid, they can die /act up intermittently. Also the wiring/ground.

[pawandmaw]

This mower has the black deck on it Starfire 1 thank you for help narrowing it down, I plan on just putting a new coil to start. Brian this mower has the old style walbro carb on it, there is no solenoid under the carb. But next question is there an easy way to test the three prong regulator, or just get a volt meter out on the regulator when it is running to test it. then what voltage should it read ?? Also I left the gas cap off and had no effect. And yes this is a gravity fed carb,that is one thing I have learned the old fuel lines can collapse after running a while, but this one had a small pin hole from rubbing on the frame so I already replaced it with a new gas filter.

[bgsengine]

voltage regulator test

First, start engine and measure battery voltage while engine running at full throttle - approximately 3600 RPM if the battery voltage reads absolute minimum 12.8 (full charge) to as much as 15.4 volts DC (Amazing the battery had enough to start engine, then!) , the regulator is just fine and charging system is working.

If not, then you test at the MIDDLE TERMINAL for 12.8 volts or better. If there is, regulator is fine, problem in machine's wiring.

If not, then you want to switch multimeter to AC VOLTS , and connect the black and red probes, one each, to the two OUTER terminals (those are AC volts) and you wanna see around 38 volts or so (At least in the 30V range) , and if there is, the alternator is good, regulator is likely bad (unless the problem is a bad ground - regulator body needs to be grounded.. I have seen some engines come in where they had a jumper wire missing from regulator body to engine block - if the blower housing is plastic.)

Otherwise if you do not see at minimum 28 volts AC output from the alternator (and mind you, you will want to unplug the connector from regulator and measure across the 2 outer , usually white, wires to verify - shorted regulator can pull those volts way down.)

[Deere2me]

This mower has the black deck on it Starfire 1 thank you for help narrowing it down, I plan on just putting a new coil to start. Brian this mower has the old style walbro carb on it, there is no solenoid under the carb. But next question is there an easy way to test the three prong regulator, or just get a volt meter out on the regulator when it is running to test it. then what voltage should it read ?? Also I left the gas cap off and had no effect. And yes this is a gravity fed carb,that is one thing I have learned the old fuel lines can collapse after running a while, but this one had a small pin hole from rubbing on the frame so I already replaced it with a new gas filter.

Kohler has a website where ya can, get this, download a service manual.....I know, who'd a thunk it, right! http://www.kohlerengines.com/manuals/landing.htm

[pawandmaw]

Well the second thing I should have done was check compression, the first is a manual. But the compression is 72 to 75 pounds. This explains why it would quit after 5 minutes or a little bit longer when it was cold. The valves are 7 and 11 for Kolhers correct?? Since they are not adjustable and I have not removed the valve cover yet what are the chances of saving an old Kohler???
Sorry I wasted your time , but I will say I did learn how to test a regulator now. Thank you Brian
Paw

[KE4AVB]

Well the second thing I should have done was check compression, the first is a manual. But the compression is 72 to 75 pounds. This explains why it would quit after 5 minutes or a little bit longer when it was cold. The valves are 7 and 11 for Kolhers correct?? Since they are not adjustable and I have not removed the valve cover yet what are the chances of saving an old Kohler???
Sorry I wasted your time , but I will say I did learn how to test a regulator now. Thank you Brian
Paw

We all got to start somewhere and if you learned from us then it wasn't a waste of time. We also benefit from users posting and asking questions too as makes us think and sometimes we learn something new too.

With hydraulic lifters there is no specs for clearance that I know of. As for compression is at the low end even on an engine with a decompression system. If it was Kawasaki that would be the minimum specs. Only a leak down test would pin point if it is rings/cylinder, bad head gasket, or valve related problems. The leak down test is the non-invasive way of testing compression problems.

Considering the age of most of the STX38s I have seen it is probably due for a rebuild. As for is it worth it which sounds likely as the engine still runs depends on the rest of mower condition or if your just planning on selling the engine.