Hi Paul, welcome to Outdoorking. I think that will be a Kohler Command CV15S, not 155. It should be an OHV vertical crankshaft single cylinder engine of 15 hp. Here is the full workshop manual for it:
http://www.kohlerengines.com/onlinecatalog/pdf/tp_2339_d.pdfUsually the best way to start an investigation like this if you have the workshop manual, is with the manufacturer's troubleshooting chart, which is on page 21 of the manual. It reads as follows:
Engine Will Not Idle
1. Restricted fuel tank cap vent.
2. Dirt or water in the fuel system.
3. Faulty spark plug.
4. Idle fuel adjusting needle improperly set.
5. Idle speed adjusting screw improperly set.
6. Low compression.
7. Stale fuel and/or gum in carburetor.
I assume you have checked obvious things like that the air cleaner element is clean, the idle speed screw is adjusted to give 1200 rpm idle, the engine is warm, the choke is completely open, and there is plenty of clean fuel in the tank. You also need to check the tappet clearances if your engine has adjustable clearance (some of the Kohler's do, some don't). Refer to page 137 of the workshop manual. Check that the flywheel key is in place and is not sheared (a sheared flywheel key will result in incorrect ignition timing).
Have you checked the vent in the fuel tank cap? Just loosen or remove the cap, and see if the problem gets better or goes away. Have you properly cleaned the fuel tank and fuel line? Water can cause this type of problem. Next, try a known good spark plug (usually a new one, though it isn't that uncommon these days for new plugs to be defective), and set the gap correctly to 0.040" (1.02 mm).
After that, if you have a Walbro carburetor, turn the idle mixture screw (there isn't one on the Nikki) clockwise until it bottoms lightly on its seat, noting how far you have turned it. If it is a long way different from 1.0 turns, try it at 1.0 turns, which is the standard starting point for making that adjustment. If the best idle turns out to be at a lot more than 1.0 turns, there is reason to suspect an obstruction in the idle system.
If you get that far and have not found the fault, it will be time to talk about additional tests. Please post some pictures of your engine and tell us specifically what is wrong with the idle (rough, varying speed, won't run as slowly as 1200 rpm, etc.)
