Hedge Trimmer stalls, backfires then won’t start again!

[fieldscience]

I have a hedge trimmer with a Kawasaki TF 22 engine. It runs fine for perhaps 15 minutes then it stalls with a backfire. It then won’t start for a while then runs as normal……until it does it again. Any ideas please?

[adw]

If you haven’t already,  change the spark plug, if no better then unfortunately it is the ignition coil, many of the these engines also have an electronic tablet connected to the coil, check the connection, or this may be the faulty component.

[Mark Carter]

More likely a whisker on the plug , clean air filter and exhaust including the port and widen the plug gap, then try again

Edited August 30, 2021 by Mark Carter

[adw]

When a plug is whiskered the machine will not start, this is a heat related issue in my humble opinion, hence ignition issue.

[daveatdave]

check the exhaust and spark arrester is not carboned up  

[spudulike]

I think ADW is correct. The backfire would appear to be the ignition unit sparking too late and causing a backfire similar to when the flywheel key has failed but seeing this runs for a period, it won't be the key.

I would close the coil to flywheel gap - you usually set to 0.3mm but try setting it with a piece of printer paper or similar so the gap is much closer.

If this doesn't do it then it looks like the coil is failing when it gets hot and is showing up as the ignition timing going wrong.

I had a MS460 once fail after 20 mins running and found this gap was too big, once adjusted....no issues.

He is also correct about the spark unit and the connections - it may need a new one if it has one (it probably does - but make sure the connections are clean and good - a bit of a going through may save £££ in parts you don't actually need.

The spark unit looks like this - the little metal box on the end of the wire....the big lump is the coil. The box needs earthing : -

 

Edited August 30, 2021 by spudulike

[fieldscience]

Thank you for the replies. However, I may have sorted it! All my Stihls run on 50:1 and I failed to notice that the cap on the TF22 clearly states in LARGE numbers “25:1”

I expect running it in 50:1 has made it run lean or have I got that the wrong way round?

[spudulike]

The mix is dictated by the quality of the oil and not the machine. When the machine was manufactured, most oils had to use a 25:1 mix as the oil was of a lesser specification than the oils of today where most oil manufacturers now stipulate a 50:1 mix.

Look at the oil you are using and see what the manufacturer recommends. Stihl and Husqvarna recommend 50:1 and some like Almsoil recommend 100:1. Your oil from your local garage is probably pretty low spec!

It is very unlikely that this is the issue!

[fieldscience]

I made up a special 25:1 mix using Stihl 2S oil (which would normally be mixed at 50:1) are you saying that the mix, with twice the oil content per litre, does not ‘burn’ differently within the engine?

[adw]

I think you are barking up the wrong tree with this.