Small Engines (Lawn Mowers, etc.): Kohler engine, snow blade, kohler engines

Question
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Followup To
Question -
Hi Marc,

I have several 10-16 hp K-series Kohler engines with a common problem.  They start okay, they idle okay.  The problem starts when you rev them up and put a load on them like cutting grass.  They will then start to jerk and shake and start slowing down and lose power.  If I didn't know better, I'd say they were trying to mechanically sieze.

If you release the load and drop the rpm, they will instantly recover and idle fine.  It has been awhile since I messed with them so I don't remember all I did but I believe I changed the points, condenser and coil on two of them with no improvement.  Carburetors are clean and there is no water in the gas.

One person told me then engines needed to be rebuilt as this was symptoms of worn out engines.  I don't understand his logic.

Thanks,
Mike
Answer -
Typically, the symptoms of engines being worn out are low compression and/or mechanical noises caused by excessive clearances.  Is it possible the equipment the engine is attached to is the problem and not the engine?  If the engines are not worn out and the problem is related to load, it's likely there is a problem with the high speed circuit in the carb.  The problem will either be an adjustment or a blockage causing a lean condition which will not provide enough fuel for the engine to support a load.  If you have several engines with the same problem, where did these engines come from?  Where they all from the same type of equipment?  Is the problem on the same piece of equipment?  Let me know what you find.  Hope this helps.

Marc:

There is no question these engine have a lot of use on them.  Yes, they have some mechanical sounds and they will smoke a little.  The engines are all on Cub Cadets.  One is used with a snow blade and the other two(used to be three) are lawnmowers.  All came from different locations.

I might add that when the engines start losing power they blow some black smoke out the exhaust.  If it was going lean I don't think it would do that.  I described it as seeming like it was trying to sieze.  I could also describe it as suddenly lugging (straining) at high speed like the timing suddenly overadvanced.

At least one of these engine had the points, condenser, coil replaced along with cleaning the carb. Took the head off and scraped the carbon out. The valves are loose enough in the guides I don't think they are sticking (I forgot to add, they will sometimes backfire out the exhaust when it loses power).  I initially thought it was the exhaust valves sticking.

Thanks for your time,
Mike

Answer
If the valves are loose (they are only supposed to have so much clearance) you may have a compression and/or oil consumption issue.  As for having black smoke and it backfiring out of the exhaust, those are opposite conditions.  Make sure you are really seeing black smoke.  Backfiring is caused by excessive heat created during combustion due to a lean condition and this causes further burning in the exhaust system when the hot gas exceeds the flashpoint of the unburnt vapours on the side of the exhaust valve opposite the combustion chamber.  This doesn't happen with a rich condition.  Nonetheless, if you hear mechanical noises, it is time to consider a rebuild.  The timing is fixed on these engines and neither advances or retards with RPM, unless there is an ignition system problem such as the point gap changing or the points plunger sticking.  It is definitely a good investment to rebuild these Kohler engines as they will give many hours of services afterwards.  Hope this helps.