[chrismahon]

Having seemingly endless problems with Stihl 2- stroke engines and have just replaced the third carburettor. The first was one that wouldn't draw fuel, the second wouldn't rev and now our leaf blower won't start. The first two resolved the problems but the third hasn't.


It hadn't been started for a while so I changed the fuel (which worked fine in the strimmer) but it became obvious it was flooding and had been running rich for a long time. Also the fuel tank was becoming pressurised, so something was very wrong. Fitted a new carburettor which seems to be fine- no flooding, no tank pressurisation and delivering fuel mixture. But still the same issue- it fires once and then nothing. So suspecting the spark plug that was changed but, even with an improved spark, still it fires once then nothing. Pulling it over with the spark plug out gives a regular spark and I've compared the intensity of the spark to our strimmer and it's the same. There is good compression. I must be missing something, but what?


Can anyone help?

[Puddlebrook]

Definitely sounds like a fuel issue. I wonder if the fuel tank isn't venting and your getting a vapour lock. Try running with the fuel cap loose to allow air in.

[arobwk]

Don't ya just love 2-strokes !  Did Puddlebrook's suggestion help chrismahon - iow was it a fuel tank venting issue ?

[bazzais]

Most have got a small filter inside the tank - check thats not got muck in it or the internal pipe isnt perished.

An double check your fuel filler cap def has its vent hole unblocked.

Sometimes you can see a spark on the outside on a plug but when its under compression and the resistance is far higher it might not spark - always try a new plug.

[chrismahon]

Sorry for the late reply- computer problem.


It did sound like a fuel issue Puddlebrook and the first thing I suspected was the new carburettor not drawing fuel, having checked the fuel tank vent. Problem is how do I prove that? So I tried putting neat fuel into the air filter, which resulted in it still only firing once. Strange thing though-shortly afterwards the remaining fuel in the filter started to vaporise?


Now the fault finding lists on the internet mention a blocked spark arrestor, which is an American fitment and isn't on this unit. But thinking about it further, a blocked exhaust could answer the problem with hot gas blowing backwards? Perhaps the running rich had blocked it with soot? So i took the exhaust off and tried blowing through it- definitely blocked.


Now this isn't a fault that appears in any chart i've seen! We have a species of solitary bee here which spends its life finding places to lay single eggs. Favourite seems to be holes in wood about 6-8mm diameter, as all the adjustment holes in my scythe handle had been filled with pollen, an egg deposited and the hole sealed with clay. Of course the outlet pipe of the exhaust box is 8mm and sure enough it was blocked solid. The pipe has a taper on it so the clay plug couldn't blow out and had to be knocked backwards. This dedicated little bee caused me so much stress, but I had to laugh about it. Thanks for the replies.

[arobwk]

Don't ya just love solitary bees !?   I'll add it to my fault-finding check list    Glad you're sorted.

[chrismahon]

Here we are 4 years later. Recently purchased a Stihl MM56 mini-tiller after the bearing broke up on the MM55 and seized the motor solid. It's just run-in and has been starting first pull every time and was used last week. So today it wouldn't start. Tried choke, half choke and no choke but nothing. The fuel is new so the very first thing I checked was the exhaust pipe. Sure enough there were flecks of yellow dust around it. Being new and no soot it was easy to see the pollen deep inside and there was an awful lot came out, so that little bee must have been working flat out for quite a few days!


Perhaps I need to fit some plastic caps?

[Kiran]

You can get packs of assorted brightly coloured bungs from Amazon, if I had that issue once I think I'd be tempted to bung the exhaust once it had cooled down just to stop the stress. Nothing worse than going to do a job and getting no life from your kit 🤦‍♂️