Diagnostic Tips

If your machine starts having problems, here are a few things to look out for:

A 2 strtoke engine on a moped

A two stroke engine should be more reliable than a four stroke. Sadly this is not the case. Despite the lack of 'things to go wrong', (valves, cam shafts etc.), two stroke engines have some problems all of their own.

Faced with a problem machine, start with the basics:

Starter Rope Jammed

If the Starter Rope on your chainsaw, brush cutter, leaf blower or whatever is "jammed", you can be fairly sure that the motor is seized. This will invariable be due to a lack of 2-stroke oil in the petrol. If you are hiring out the machine, this kit is a foolproof way to find who is liable. Experience will tell you why the engine seized but often the customer will not admit fault.

A simple two minute test with our kit will confirm "no oil". You can now charge for the repair, instead of having to absorb the cost of a warranty claim yourself.


Test for a spark with a testing plug, or, use a standard plug and remove the side electrode, you should get a nice fat spark. If there is no spark, disconnect any stop / ignition switch and try again, after this replace the coil or ignition module.

Is the spark at the right time? Check the key in the flywheel.


Is it really gas/petrol in the tank?

How old is the fuel?

Unleaded fuel goes bad a lot sooner than you might think. It will look OK, smell OK, but if you suspect it's been in the back of the garage for more than a couple of months, replace it.

In countries with a wide variation between summer and winter temperatures, the manufactures blend their fuel for the season, using summer fuel in the winter will make that cold engine even harder to start.

Contamination with diesel from the dregs in a can will look like petrol with a two stoke oil in, but may not run.

Unleaded petrol is "hydro scopic", it sucks water from the air. This water can form a "slime" on the fuel filter. This slime seems to filter out enough oil to cause accelerated wear without seizing the engine. If you suspect water contamination filter the fuel though a chamois leather, the water will stay on the surface of the leather. Replace the fuel if there is any doubt. You may just save yourselves a lot of time and trouble.


2 strokes need a lot of compression, don't rely on the feel of the starter, it is often quicker to remove the barrel and have a look.

If the engine will run without the air filter and the filter is clean, suspect a worn bore and piston.

Worn crankcase oil seals will tend to cause the plug to keep getting wet, or the engine runs but you find that the idle speed will increase as it warms up, and it will be hard to start when hot. Do not let a 2 stroke engine run lean. No 2 stroke should rev out cleanly. They should be restricted to the correct rev limit by the mixture screw.


If a machine is overheating check for blocked air ways, also pay particular attention to the gap between the ignition coil stator and the magnet. This gap will affect the spark timing dramatically. Some manufacturers give different settings according to the speed the engine will run at in a particular application.

Tinkering by the user. The user adjusts the mixture to overcome smoke and lack of power, and then does it again and again. Eventually the power is so low he investigates the problem and buys a new air filter, but then forgets to set the mixture back, result one badly over heating engine.

Piston Diagnostics

The piston in a 2 stroke engine is a fairly good diagnostic tool.

You will all have seen the usual scores on the exhaust port side from lack of oil. More rarely you may come across a piston where the rings and lands are badly worn. The rings will be moving up and down in the groves and will eventually snap. This is caused in our experience by water in the petrol.