puzzler Kohler k301 just stops

[38racing]

A friend has asked me to try and solve problem with his k301 on a cub cadet 129. (71-74 vintage). He said it lost power and at full throttle it would stop with a backfire.
What I found is that it starts right up and runs fine at full throttle for a few minutes until a surging starts and then it just stops and flames out the muffler as it stops. It will restart immediately and again run for a short period.
This is what I have done so far:
Remove fuel cap (I admit that was not the first thing that I did)
I removed air filter.
I checked for fuel flow out of filter. It is gravity fed and flow was good.
I put inline spark tester and spark remains on shutdown. I even turned key off and then spark goes away but turning key back on before it stops rotating brings spark back and it runs with spark until the problem shutdown.
After shutdown I loosened bowl nut and fuel came out.
I adjusted both mixture screws to initial setting 2 1/2 on the idle and 2 on the high.
Not liking look of his gas I drained all his fuel.
I put 400 ml of ethanol free gas and 200ml of seafoam in the tank.
At this point I thought I had it fixed because it ran for over 5 minutes (I think as I broke my watch band so no watch) but then it stopped.
I then went and worked on something else. Just before coming in for the night I started it again. It ran for 15+ minutes without stopping. I actually shut it off to come in the house.
My thought is either a sticking float (but is that consistent with immediate restart and fuel from loosened bowl nut?) or some debris that moves to block the main jet but immediately moves away when it shuts down? Not sure why I got the 15+ minute run after some seafoam exposure.
I guess I will see what I get tomorrow when I start it.

[Mek-a-nik]

Possibly a low float or an obstruction of some kind in the needle and seat or thereabouts. Both conditions allowing it to run until it gets ahead of the fuel supply.

[HondaG100]

New fuel filter and a good carb cleaning along with a new plug and you'll be good to go.

[38racing]

New fuel filter and a good carb cleaning along with a new plug and you'll be good to go.

I forgot to mention in my list that I put a new plug in it. And the filter flowed fine. I think the 15+ that I thought it run last night was exaggerated . I just ran it this morning and it just stopped after 13 minutes. I am hoping it's because it has used up the 600 ml of gas/seafoam. I will add fresh gas and see how long it will run. I am not keen on pulling his carb apart if I can avoid it.

[bgsengine]

Do a leakdown test and check head gasket and valves - Strangely enough I have seen this happen a few times - a couple times it actually was a valve problem - once it was a sticky (not completely sticking, just slowww to seat) exhaust valve and another it had a loose valve seat - and another *seemed* to be due to loose head bolts (leaking head gasket) - never really did definitively diagnose it as head gasket, could not understand why it would cause the same issue other than sucking in air and leaning out the fuel mix, but replacing head gasket evidently fixed the issue.

[38racing]

Do a leakdown test and check head gasket and valves - Strangely enough I have seen this happen a few times - a couple times it actually was a valve problem - once it was a sticky (not completely sticking, just slowww to seat) exhaust valve and another it had a loose valve seat - and another *seemed* to be due to loose head bolts (leaking head gasket) - never really did definitively diagnose it as head gasket, could not understand why it would cause the same issue other than sucking in air and leaning out the fuel mix, but replacing head gasket evidently fixed the issue.

Just got 13 minutes on fresh load of gas. just had to walk over and it restarted and at 7 minutes run time know. If it was leaning out wouldn't one expect some surging for a period before shutdown? there is just momentary governor back and forth as it shuts down. Runs fine right until shutdown. As you say I will need to check more stuff.

[bgsengine]

Check the points too - see if the pushrod is aluminum - they ended up getting wear spots to where they will intermittently "hang open" the points. (they have an updated pushrod that is steel part number 47-411-04-S, I believe) Also the points return spring (Spring steel) weakens over time as it heats up it gets even weaker (Look for any signs of melting plastic at the insulator, burned points, etc.) - to catch intermittent spark failure you'd have to be sitting right on top of it watching spark tester to verify otherwise.

I used to fix those engines had them through the shop at a rate of about 10 a day - everything from preventive maintenance to total rebuilds and shortblocks - and I doubt there's anything that has been known to happen on these that I haven't seen.. even to the point of having seen one with a "grenaded" flywheel (what a mess!) - I can still just about rebuild them in my sleep.. I only wish they still made them - my expertise with those was an extremely lucrative business... if they still made them I'd be retired by now.

I agree if it was carburetor problem it would rarely have restarted immediately - it *COULD* be vapor lock in the fuel pump (The check valves sometimes fall out and re-seat - may have to tear down the fuel pump - carefully because there are no longer any service kits, and replacements if you can even get them cost an arm, a leg and your firstborn.) and it *COULD* be a swelled fuel hose (rotted from inside and fuel pressure "balloons" the inner liner , plugging the fuel line until pressure is relieved) and it *could* be a restriction in the fuel tank or fuel lines causing fuel to be slow to re-fill the fuel bowl.. but again if it was going lean like that it'd have a more and increasingly pronounced surge before dieing out.

[thomd51]

I have seen this before and I think it's the exhaust valve sticking in the guide. the exhaust valve gets very hot and the stem expands in the guide and gets stuck.
that's why the seafoam made a slight difference. mystery oil would too. TOM.

[38racing]

I have seen this before and I think it's the exhaust valve sticking in the guide. the exhaust valve gets very hot and the stem expands in the guide and gets stuck.
that's why the seafoam made a slight difference. mystery oil would too. TOM.

Might hit it with some more seafoam. I have pretty much covered everything in previous bs... post. No fuel pump .very short gravity feed line and I have monitored inline spark tester. Last run sitting was over 30 minutes so I decided to try cutting with it. Cut for over 5 minutes but I had to shut it off to leave on other matters. Any meaning to the muffler flame on shutdown. I know I have had it sometimes on engines when normal fuel tank runs dry.

[bgsengine]

Any meaning to the muffler flame on shutdown. I know I have had it sometimes on engines when normal fuel tank runs dry.

when you shut the engine down there's no spark, so you have unburned fuel shooting into the exhaust and burning off on hot exhaust. See that often on your cheap basic "short run" mufflers on those older engines.

Like I mentioned earlier, there's a few things that can do that including valves hanging, breaker pushrod hanging, points or spark failure (condenser and/or coil, too) so I would check the ignition carefully (its easiest to access anyway) and give it a good tune up if you spot signs of burning contacts, and check the pushrod (with a magnet) to see if it is aluminum or steel.. then consider checking valves and head gasket area.