A perplexing problem

Ken52 · #1 06-17-2016, 10:20 PM

The tractor is an Cub Cadet 1862, (Kohler 18hp Magnum) with about 1800 hours on it. I've owned it for around 15 years, and for the last several years, it has had a problem. It starts and runs perfectly, until it gets hot. I can mow for about 10 - 15 minutes, then it immediately loses power and starts to miss (sounds like it's running on one cylinder). I have to shut off the PTO and idle it down. After about 20 - 30 seconds the miss disappears and it idles perfectly. So I increase RPM and start mowing again until it repeats in another 5 or so minutes of mowing.
Here is a list of what I've done to it:
1. new ignition coils (the second one is a genuine Kohler part
2. new fuel pump
3. electric fuel pump
4. cleaned fuel tank and lines completely
5. cleaned and adjusted carburetor several times
6. cleaned exhaust valves and ensured adequate clearance
7. several fuel filters
8. spark plugs (tried both Champion and Autolite)
9. different carburetor (both are Walbro)
None of the above changes made any difference whatsoever. That is, after each change, the engine would again exhibit the same identical symptoms. The airflow is good, cylinder fins are clean.
I've searched forums but I've not been able to find any ideas that have made a difference.
Anyone have experience with something like this?

J-Mech · #2 06-17-2016, 11:13 PM

Ever replace the fuel lines and filter? How about the screen in the tank?

Have you cleaned the intake valve stem and guide? It's rare, but they can stick. Did you clean the exhaust valve guide? How tight is the exhaust valve stem to the guide?

I know you've replaced a lot of parts, but keep in mind that issues like this are usually much simpler.

zippy1 · #3 06-17-2016, 11:30 PM

Are you sure the screen in the fuel tank is good? And the tank is clean?
Sounds very close to an issue I had not that long ago...

Ken52 · #4 06-18-2016, 10:33 AM

I appreciate your comments, Jonathan and Todd.
I've pretty much ruled out fuel supply issues. I haven't replaced the fuel lines, but I can pump a lot of fuel with electric fuel pump through the new filter. And I have flushed and cleaned the tank and lines.
I did remove the exhaust valves, cleaned them thoroughly, and even took a few thousandths off them to be sure that I had plenty of clearance when hot. I'm running on the high end of the tolerance for clearance now.
I looked at the intake valves, but they looked really clean and clearances were good.
One thing that I forgot to mention in my original post is that, the last time I used an infrared heat sensor on the engine, the LH cylinder was running about 100 degrees F hotter than the right. IIRC, the temps were around 450F and 350F. However I haven't checked temps since I found out that the carb had a plugged orifice. Now that the carburetor works the way it should, I expected that the temps might be more even. I'll check that later today just to be sure.

dvogtvpe · #5 06-19-2016, 10:19 AM

it is difficult to get temps at both cyls the same with one carb. but that is a pretty big difference. ever check for leaking intake gasket or head gasket?

Packard V8 · #6 06-19-2016, 02:26 PM

You've done everything I'd have suggested, but the symptoms are still those of a fuel supply issue.

My 2135 had exactly the same symptoms and required removing the tank, cleaning and drying, new fuel line from the tank to the pump and from the pump to the carb.

Assume you've verified the gas tank cap vent is clear?

jack vines