Did you check the muffler and spark arrestor good for carbon blockage? Pull the muffler and you should see in the cyl and if it's damaged/scored.

I would check the valves and make sure they are adjusted properly. BTW, removing the muffler will not allow you to see inside the cylinder since this is a 4-cycle engine.
I'm not sure what would be causing the extreme heat, but I wouldn't continue to run it until you find out what is going on.

Hopefully, you are storing it in the horizontal position.
If the attachment is removable remove it and see if it still overheats.
Is the oil correct.
Older 4 cycles use 30wt Hd. newer ones use 10w30.
And I am using 20w 50 synthetic.
After it overheats remove the plug ad see if it pulls easy. If it does not pull easy I think you have problems.
So, before you pull out your hair try that first.
I am also glad to see someone adjusts the valves on 4 cycle trimmers.

Problem Persists..

I changed spark plug, rebuilt carburetor, cleaned muffler, changed oil but trimmer continues to overheat within few minutes of using it. I have always used full synthetic oil, storing it horizontally all the time.

I am guessing increased friction b/n cylinder & piston/rings. I guess it is close to it's end of life - will post if it dies soon.

Thanks for all replies.

Will the cylinder pull fairly easy for inspection? If so this could end some speculations. I tried to find a youtube that should show this. If you know of a youtub on rebuilding please share. It could show an easy inspection or not. Pulling 100 pieces w/n be easy to see a scored cyl.

I am confused by you term overheating?? does it just stop and you can't pull the rope? or does it die and won't start? Is the lower shaft still spinning during the lost of RPM or when it died?

"I am guessing increased friction b/n cylinder & piston/rings" I take it you remove the shaft to iltimate any problems down the road??????

First I would make sure there is no grass around the cylinder head cooling fins (you may have to take the plastic housing off to ensure this)

Next the problem could be in the clutch, upper drive shaft cable, lower drive shaft cable, or the gearbox down at the business end. All of this can be troubleshot by remove first the lower end shaft. run the string timmer and see if you notice any binding or lower of RPM or sound like it's bogging down. It should sing like sewing machine with the lower shaft assy. off.

If it don't them you have to take the upper shaft off do the same. next would be the clutch assy. Compression check, VALVE adjustment and another compression Compression check?

"about 2 years" New or second hand??? Mine 12 years old and still runs like new, starts on second pull. I had to replace the fuel lines, upper drive shaft, adjust the valves, and regrease the lower gearbox. Course if I had never done any of that it would not be working.

An exhaust valve gap out of spec would possibly do it as well as burn the valve. Remember someone suggested to not run it w/o finding the cause and that could be a major reason.

Are adjusting the overhead valves something that is needed yearly? Sounds like my 1968 and 70 VW every 6,000 mi or Etc and the specs were changed (larger) and they needed it. TX Heat is hard on air cooled equipment IMO. I saw a youtub that recommended once a year overhead valve adjustment on Briggs Engines in the video (at the address below). That is a lot of stretching but I have seen it on some air cooled engines resulting to burnt valves (exhaust mainly).

Adjusting Briggs Overhead Valve Engine.

http://www.youtube.com/watch?v=NjzX-PUjKmo

Here is a link that might be useful: Adjusting Briggs Overhead Valve Engine

I just checked the warranty on my Cub Cadet 4 cycle which I purchased a little over a year ago. It is covered for 3 years from date of purchase. I would check my warranty and if in is within or a couple of months older I would take it back to the place where you purchased it, or to a dealer with proof of purchase.

It does not seem that a 4 cycle should die within two years.

This post was edited by knuttle on Wed, Jul 17, 13 at 19:24

Properly adjusted and gapped intake and exhaust valves transfer heat much more readily when in spec. This is especially true of the exhaust valve.
Heat WILL build up in the engine until some type of failure occurs if the valve lash is incorrect even on these smaller engines.