Lawnmower won't start when hot!

[Spelunx]

Not sure where to put this question, so thought I would start here....


I bought a secondhand lawnmower the other day, it is a 11 year old Masport 4 stroke. It goes really well, starts first time, every time, two pumps of the primer and pull the cord, away she goes. Has a bucketload of power, cuts my buffalo lawn with ease.

The problem I have is that it doesn't restart when hot. I have two lawns, and I can't mow one, turn the mower off, and move it to the next lawn, cause it won't restart! I have to idle the mower all the way down the side of the house to the front lawn.

I tested it again today. I pulled it from the shed, primed it, then pulled the cord, bang, away she goes! Let it idle for 10 minutes, turn it off, and it wouldn't start.

Any ideas as to what is wrong? Or and advice as to further tests I can do to establish what is wrong?

Cheers

[Blu_Rock]

At a guess, it is either a compression, spark or fuel problem, more than likely a compression or spark problem.
1. Compression - check the compression (with a compression guage) when it's cold and again when it's hot. If there is a dramatic difference, then it's stuffed and will need some work (new rings and maybe a rebore).
2. Spark - when hot, remove the spark plug lead from the plug, connect it to a spare plug and earth the body of the plug to the engine, using a suitable clamp to hold it in place. Pull the starter cord and if you can see a spark jump across the gap betweeen the electrode and the plug's body, then all is OK. If not, then maybe the coil/points/sender are buggered.
3. Fuel - unlikely, but if the above test OK, then maybe a fuel problem.

P.S. This post is so old, he's probably gone a bought a new mower by now.

[Sturdee]

Lawnmower won't start when it's hot!

Neither will mine. Mainly because I refuse to mow the nature strip when it's hot. I'll wait on cooler weather.


Peter.

[dazzler]

Valve clearance may be too tight and is holding the exhaust valve open when hot.

[joe greiner]

Valve clearance may be too tight and is holding the exhaust valve open when hot.

Similar problem with Briggs & Stratton, too. The valve guides get hammered into the block so as to prevent complete closure. Solution is to grind about a hair's width off the valve stem length.

Joe