312-A Kohler Magnum 12 troubleshooting advice

[davem1111 2,161]

So, the 1987 312-A that I've had for quite a few years (my first horse) has been running badly lately. It blows black smoke when choking it to start, runs rough then smooths out a lot with the choke open. However, it just starts coughing and dies after about 10-15 minutes (yeah, it's been bloody hot lately, so that could be a factor).  

 

1. No, I'm not running non-ethanol gas in it right now, but I have tried to switch to that later in the season with all my tractors so there's no ethanol in them over the winter.

2. OEM fuel pump died a few years ago and I replaced it with a CarBole 12v pump. I don't have any reason to suspect this - yet.

3. Filter seems good, need to check the screen on the tank valve, and check the gas cap.

4. Suspecting engine wear, I bought a compression tester.  Getting about 70 psi.  This is "cold", I haven't tried to warm it up before checking.

5. Was reading about "leak down" tests, wondering if I should be trying that.

6. The circuit board with the indicator LEDs is in bad shape and I had to do some soldering/splicing a while back to get good connections, maybe something is not good electrically.

 

Any and all advice welcome.

[squonk 43,811]

My first thought is if this is a Magnum I have seen many coils go bad on these. @Racinbob just went through this.

[ri702bill 9,419]

1 hour ago, squonk said:

My first thought is if this is a Magnum I have seen many coils go bad on these. @Racinbob just went through this.

I second that. Use an inline spark checker. Start it up cold, when it dies, see if you have spark on the hot restart. Most likely no spark. Once the coil cools off, it will provide spark.

Very annoying.

[953 nut 59,984]

3 hours ago, davem1111 said:

1. No, I'm not running non-ethanol gas in it right now, but I have tried to switch to that later in the season with all my tractors so there's no ethanol in them over the winter. Doesn't take too long for the ethanol to do damage. A new fuel hose, filter, and a heavy dose of Barryman's B-12 could be the answer.

2. OEM fuel pump died a few years ago and I replaced it with a CarBole 12v pump. I don't have any reason to suspect this - yet.

3. Filter seems good, need to check the screen on the tank valve, and check the gas cap.

4. Suspecting engine wear, I bought a compression tester.  Getting about 70 psi.  This is "cold", I haven't tried to warm it up before checking. With a compression release cam that is good compression.

5. Was reading about "leak down" tests, wondering if I should be trying that. Couldn't hurt but doubt it is needed.

6. The circuit board with the indicator LEDs is in bad shape and I had to do some soldering/splicing a while back to get good connections, maybe something is not good electrically. Those lights have nothing to do with your engine.

 

[Racinbob 11,929]

4 hours ago, squonk said:

My first thought is if this is a Magnum I have seen many coils go bad on these. @Racinbob just went through this.

 

The symptoms are a fair amount different from mine. The black smoke thing and coughing never happened to me. But it definitely could be the culprit. 

[Rick3478 440]

On first read, it sounds like over-rich fuel/air mix.  Black smoke when choked is a normal thing, due to rich mix making lots of soot, but if it doesn't clear up when un-choked, it may still be running rich.  If the carburetor has never been apart, a good cleaning and adjustment may be in order.  The needle valve wears shorter with age, which raises the float level and fuel/air mix gets richer.  And I would be remiss to not mention the air filter, which acts like a choke if it is clogged.

[Rick3478 440]

Quick thought about that CarBole fuel pump.  Make sure you have the low pressure version.  2 to 3 PSI is plenty.  The higher pressure automotive versions can overpower the needle valve and flood out.

[peter lena 9,272]

@953 nut  have had zero / fuel issues since changing out all related hoses ,and regularly  adding an ounce or two , of STA BIL  to all my fuel , fuel filters are clean / clear , fuel bows are clean / clear , what is that telling you ?  have also  detailed all related linkage areas  for functional  movement ease . cables move / slide  with a  light spring assist , super lube  hydraulic  oil in all cables . repetitive problems let you get after  every detail , eliminate and move on . pete

[953 nut 59,984]

6 minutes ago, peter lena said:

regularly  adding an ounce or two , of STA BIL  to all my fuel

I am a firm beleaver in an ounce (or two) of prevention being worth a pound of cure. Every can of non-ethanol gas that comes in my shop has two ounces of Sta-Bil and two ounces of Berryman's B 12 in it.  A couple of years ago I had a stroke and none of my small engine equipment was fired up for a year, some sat for two years, but I have had no fule related issues with any of them.