Hi,
A cold walk with Barley this morning Logan.
Frost here too and -1C when I set off to the supermarkets; I've just returned in a sweltering 3C.
The mower I mentioned in a previous post is now fully sorted; the symptoms definitely looked like fuel starvation and with dozens of YouTube videos all saying the same my chum David spent ages even to the point of buying a new carb but the mower refused to run for more than 5 minutes before dying. I decoked it and ground in the valves knowing this would help but not cure the fault at least I then knew compression was fine. I quickly found the magneto at fault breaking down as it became hot but I wonder how many otherwise excellent mowers have been scrapped just for the sake of a £28 magneto in this case and the magneto is a very easy fix taking less than five minutes. YouTube videos are generally very useful but it pays to have an open mind and exercise common sense.
Kind regards, Colin.
Cylinder head decoked.
Inexpensive valve grinding tools.
Valve seats nicely reground. The original head gasket was reused.
The faulty magneto; I suspected the SCR (silicon controlled rectifier) which is embedded with a capacitor etc was breaking down when hot then when it cooled the mower would fire up and run as normal again for a short period. I'm familiar with SCR's and have used them whilst making home made bench power supplies but they need a decent sized heat sink to dissipate heat. All the symptoms though of fuel starvation when in fact the fault was ignition; this mower tried to torment me the way it had tormented David. When I had it home last Saturday I fired it up and it ran for five minutes before dying but I had already taken resistance reading of the plug lead to ground whilst it was cold but now very hot the resistance reading was just the same; I quickly checked for spark having bought a new spark indicator and sure enough the spark was still present; then I poured neat petrol into the carb intake and the mower roughly fired up all this indicating fuel starvation and the many YouTube videos to be correct? At first I was puzzled so I browsed the web and found what I was looking for; a circuit diagram of one of these type of magnetos and as soon as I saw it had an SCR I had the problem solved. When it comes to electronics I don't trust them one little bit even as in this case having tested the ignition; good try mower but your card is now marked. David is a top guy and did a good job with the fuel so I knew I didn't need to follow what he had already done however David simply hadn't the time to play further so he bought a new Honda mower; I've now got a spare mower that I'll keep.
Above is a generic magneto circuit diagram; not specifically for this mower but plenty good enough for me to understand what most likely would be embedded in this faulty magneto. What's this got to do with weather; well my life was a lot sunnier once I found the solution.
