Dear Ole Mechanic;

I have a lawn and garden tractor with an engine problem.  The engine is a 10 HP Model K241 Kohler which has been sitting outside for a year or more with only the tractor hood protecting it.  The gas tank was very rusty inside until I did a very good job (I thought) of flushing, washing, and cleaning.  I pulled the carburetor off and took it apart.  It was full of crap--mostly rust with a little water.  The brass float was cracked and full of crud, so I got a new float to replace it.  I did a real good job of cleaning all the passage ways with an aerosol can of  carburetor and choke cleaner.  I also cleaned and gapped the points and installed a new spark plug--which has a nice hot blue spark.  My problem is that the engine won’t idle.  At mid to full-throttle, it pulls and runs great and does not smoke.  Thinking that I had missed something, I went through all of the settings and re-cleaned all of the passage ways in the carburetor again.  Still, there was no slow idle.  It would start to slow down to idle and keep getting slower till it died.  If the idle speed screw was adjusted a little faster, the engine would not slow down to what would be considered a slow idle. 

Figuring that I still had missed something, I had a mechanic friend of mine go through everything.  He said that I had done everything right, but the engine still wouldn’t idle slow.  With his help, we did a compression check and compared it to a good idling 10 HP Kohler engine that I have on another piece of equipment.  The hand-cranked, good idling engine had sixty pounds of compression, and the lawn tractor engine with an electric starter was within fifteen pounds at fortyfive pounds.Cub Cadet

What am I missing?  Why can’t I get that engine to idle? 

At my Wits End


Dear Wits End;

It really sounded like a vacuum leak to me, until you gave me the compression readings.  At that point, I had to hit the books--which you probably don't have access to.  The compression for that engine should be 120 PSI (Pounds per Square Inch) and should be no lower than 105 PSI, unless the engine has an ACR (Automatic Compression Release).  At least that is what I got from the only book that I have access to.  I will cover the ACR shortly.  It seems that you do not have a vacuum leak, just not enough vacuum for the carburetor to work right.  If the engine does not have ACR, then you are going to need to partially disassemble the engine to see if the valve springs are broken, the valves seating properly, the valves have the correct clearance, and the head gasket is sealing.  Since you said that there was no smoke with the engine running, I doubt that the rings are shot.

It is possible that you have one of the Kohler K241’s that has an ACR.  The ACR is a special internal engine device that acts with the camshaft to hold the exhaust valve open longer and reduce the cranking compression and effort needed to turn the engine over.  This ACR should work until the engine gets up to at least 650 RPM.  The ACR will lower the compression a lot.  It may be that at a higher RPM, the ACR is acting too long because its internal parts are worn or sticking.  It also could be that you are trying to get the engine to idle too slow.  The information that I have says that the engine should idle at 1,500 RPM.

If that does not seem to be the problem, then what I suspect is that today’s unleaded gas has gotten to the valves, and they are not closing properly or completely. That will require at least a partial tear down of the engine.  Also, a leaking head gasket (I mentioned this earlier) can cause a poor or erratic idle or a fast idle speed.

Please keep me posted as to what you find.

Herr Professor Nuzanbolts

To my three loyal readers, Wits End got back to me. When the gas tank support was removed, a decal was found indicating that the engine had ACR.  Also, the lawn tractor is a hydrostatic-drive machine which requires a higher (1,500 RPM) idle.  After finding an old time mechanical tachometer and re-setting the idle speed to where it should be, the engine ran fine.