The Tecumseh 13 h.p engine (OHV130) in my lawnmower stops after about 20 minutes - after cooling down I can again run it for about 20 minutes. Any ideas from anyone about this problem - I've changed the spark plug without any improvement.

As soon as it quits, see if the prob is electrical or gas (check spark, check fuel flow.) Check for vapor lock in the fuel line.

Sounds like either the gas cap isn't venting... (take it off or leave it loose to check this) or the coil is bad (windings shorting out)

One thing you can look at is the routing of the fuel line. It could be heating up and vaporizing the fuel causing it to vaporize and not get through to the carb iin a liquid state. I never have seen this on a mower but have seen this on cars and motorcycles. If you find it close to the motor or exhaust you can either re-route the line or shield it with heat resistant wrap.