How to troubleshoot a McCulloch string trimmer MT780

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The McCulloch MT780 string trimmer has a two-cycle gas-powered engine. Small engines such as this one operate using three basic components: fuel, spark and compression. If any of these crucial pieces is missing, the trimmer will not function properly.

There are quite a few individual parts that make up each system, so finding one small flawed part can be challenging. Isolating the problem into one of the three main areas first will help you isolate the broken part or parts, and will make fixing it easier.

Compression check

Move the choke to the half-open setting. Remove the four screws holding the air filter/muffler cover in place. You cannot remove this cover if the choke isn't set.

Take off the air filter/muffler cover. Remove the air filter from the engine and the spark arrester screen from the muffler. Clean these with the brush in water and washing up liquid. Check inside the muffler for any damage or clogs around the exhaust port. Put everything back on.

Pull the trimmer up from the ground by the starter handle. If the trimmer sags under its own weight, your compression is low. Fill the trimmer with fresh, properly mixed gas at a 40:1 fuel-to-oil ratio.

Start the trimmer (if you can) and adjust the idle speed on the carburettor. If this doesn't solve your compression problems, you should perform a compression test. Most two-cycle engines need around 620 kPA (90 PSI) to function properly.

Check the bore, piston rings and cylinder for any cracks or damage.

Spark check

Pull off the rubber spark plug attachment boot. Remove the spark plug. Clean it with a little petrol, a rag and a brush. Check the gap to make sure it is 0.5 mm (0.02 inches). Replace if it's old.

Put the spark plug back into the attachment boot. Hold the two near a metal point on the engine. Pull on the starter cord and check for a spark across the metal points. If there's no spark, replace the spark plug and repeat test.

Work back from the boot attachment to the wire, the coil, the magneto gap and the starter to figure out where the spark is getting lost.

Check the electrical wiring for the "On/Off" switch and the "Blue EZ Start Lever."

Full system check

Empty the gas tank and spray a little carburettor cleaner into the tank. Wrap the rag around the brush and scrub the inside of the tank clean, while checking it for any warps or dents.

Check the fuel exhaust release valve on the petrol cap to make sure it's not blocked. Remove the old fuel filter and fuel line from the engine and replace.

Remove the spark plug again. Pour a teaspoon of petrol into the spark plug hole. Spray a little starter fluid into the carburettor's throat. Try starting the trimmer. If it runs for a few minutes and then dies, your carburettor needs to be serviced.

Remove, disassemble and clean the entire carburettor. Check the diaphragm or bowl, whichever your carburettor uses, for damage.

Replace any worn seals and gaskets. Make sure the carburettor's mounting screws are properly seated. Install a carburettor kit before replacing the entire carburettor.