Magneto problems
Posted: 16 Jun 2011
It's on my mower engine, which has a magnet in the flywheel passing a laminated steel armature, on which is mounted a coil. As I understand it, as the magnet passes the coil, the points open and close, and you (should) get a spark.
A bit of background - bought about 3 years ago now and didn't have a spark. Replaced the coil, condenser, HT lead and spark plug with new parts (points seemed electrically OK) and still no spark. All wiring renewed as a matter of course. Clutching at straws I replaced the points and it fired up second pull, well chuffed!
Since then I have had to replace the points every time it isn't used for a few months, otherwise there is no spark. I bought an electronic ignition kit from the engine manufacturer, which did away with the points and condenser, but this did not work. I took it back to the supplier, who proved that it worked by putting it on another engine - he still gave me a refund even though it wasn't faulty though.
So at this point I'm thinking there is something slightly marginal in the ignition system, so that if the points are not in top notch condition (simply cleaning the contact faces does no good) there is no spark...
Until this spring when I try and start it, fit new points and all I get are a few coughs. It's sparking fine out of the cylinder, so I think it must be a fuelling problem. Spark plug is wet with fuel, so I suspect overfuelling - a strip down of the carb doesn't show any problems (sticking float, blockages etc) and the adjustment screws are similar to the default base settings.
So I put a strobe lamp on the HT lead, to see what the spark is like under compression. I compared the flashes with the plug out the cylinder to when it was in, and I guesstimate that under compression I'm only getting a spark 30% of the time (it's flash - - rather than flash flash flash when pulling the engine over at similar speeds).
So back to whatever it is that could be marginal in the ignition system. I found a new old stock flywheel (40 years old!) thinking that maybe the magnet could be weak. No change. Fitted a replacement (off a running engine) laminated coil armature, no change. Fitted the old (genuine) coil back on in place of the new pattern jobby (both check out fine with a meter but I wondered if one could be breaking down under high voltage). No change.
Does anyone have any more ideas, it's not doing my back any favours cutting large areas of grass with a strimmer!
A bit of background - bought about 3 years ago now and didn't have a spark. Replaced the coil, condenser, HT lead and spark plug with new parts (points seemed electrically OK) and still no spark. All wiring renewed as a matter of course. Clutching at straws I replaced the points and it fired up second pull, well chuffed!
Since then I have had to replace the points every time it isn't used for a few months, otherwise there is no spark. I bought an electronic ignition kit from the engine manufacturer, which did away with the points and condenser, but this did not work. I took it back to the supplier, who proved that it worked by putting it on another engine - he still gave me a refund even though it wasn't faulty though.
So at this point I'm thinking there is something slightly marginal in the ignition system, so that if the points are not in top notch condition (simply cleaning the contact faces does no good) there is no spark...
Until this spring when I try and start it, fit new points and all I get are a few coughs. It's sparking fine out of the cylinder, so I think it must be a fuelling problem. Spark plug is wet with fuel, so I suspect overfuelling - a strip down of the carb doesn't show any problems (sticking float, blockages etc) and the adjustment screws are similar to the default base settings.
So I put a strobe lamp on the HT lead, to see what the spark is like under compression. I compared the flashes with the plug out the cylinder to when it was in, and I guesstimate that under compression I'm only getting a spark 30% of the time (it's flash - - rather than flash flash flash when pulling the engine over at similar speeds).
So back to whatever it is that could be marginal in the ignition system. I found a new old stock flywheel (40 years old!) thinking that maybe the magnet could be weak. No change. Fitted a replacement (off a running engine) laminated coil armature, no change. Fitted the old (genuine) coil back on in place of the new pattern jobby (both check out fine with a meter but I wondered if one could be breaking down under high voltage). No change.
Does anyone have any more ideas, it's not doing my back any favours cutting large areas of grass with a strimmer!