I've got a lawnmower with a Kohler Command Pro 27 engine. When started cold, it only runs on one cylinder for about 5 minutes. Once it warms up the other cylinder will start firing. Even after starting firing, it will start missing again if put under a load until the engine get's up to it's normal temperature. I've been messing with this for a while but the last week it started backfiring. This happens when it's running at normal temp and put under a load.

I've replaced both spark plugs, checked that both coils were correctly positioned and tight. The spark plug is firing on both cylinders.

Any ideas of where to look next?

 

Could be deposits on the valve stem and guide keeping a tight fit until things warm up and expand. These engines are a bit prone to valve/head problems. The valve seat could also be coming out. If there is dirt and grass/debris caked on the head, chances are a valve is the problem. The heads overheat quickly and have these problems when the cooling fins get clogged w/dirt and debris.

 

Is there any way of checking that short of pulling the head. As far as the fins go, they are clean. I blow the mower off every time I finish. I haven't noticed any unusual build up.

 

You can do a leakdown test and if the valve is acting up at the time it will show. You can also remove the valve cover and see if the valve sticks as you push it down and let it return, or if it doesn't come up as far as the other valve.