BXProject

[mat_the_cat]

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!

[Way2go]

Usually the problem with points is that the gap is not initially adjusted to be wide enough. After a short period of use material migrates from one contact to the other which partially or completely shorts them out.
If it is more involved than that, have you checked the lift on the rotor cam to ensure the points are being driven apart and closing again correctly and that the timing rotation adjustment is correct? Are the lobes on the cam worn down perhaps (less likely). What about the integrity of the electrical connections to the magneto coil, also checking to see if there are any stray "whiskers" that could be shorting out the action of the points. Does it have a capacitor that has gone intermittent s/c?

[mat_the_cat]

The points are changed well before any visible pits and pips appear, and are set to the correct gap. Electrically (with the low voltages a handheld multimeter uses) they always appear fine, but after a storage period *something* happens that until now has been cured by a new set of points.

This winter I started it in December, no problems after a couple of months rest. Come to March, try starting it and no spark (even when not under compression). Replace the points, I get a spark but not one (it appears) strong enough to consistently spark under compression...

All wiring connections are clean and good - the capacitor/condenser is what I'm now wondering about (even though it was renewed) as it's cheap and easy to change.

[Father Ted]

Flymo and a VERY long extension lead - jobbed.

[docchevron132]

if the sparks weak under compression it usually points to a lack of elektrikery. So I'd guess back towards the magnet?

[mat_the_cat]

So I'd guess back towards the magnet?

But…two flywheels showing exactly the same problem, the last one unused? Possible, but I'm hoping not as that took a lot of searching and cost £40!

[Philhod]

Does it have an advance? If it has, check the earth connection baseplate to body. That's the only thing I can think of that would be specific to your machine. I think you covered everything else. OH check the choke is adjusted correctly.

Back in the days, if you were leaving your car for say, a couple of weeks or so, you put a piece of cig packet between the points.
Nobody could nick it and the faces were kept clean and dry.

[docchevron132]

um, you've obvioulsey changed the plug right? I mean, the only other thing I can think of here that gives a weak spark under compression across the gap would be a wank plug, but, if you're checking the spark pre plug then I guess actually it cant be..

Does the LT side of the contacts have a plastic housing that slots into anything metallic? It's not unknown for voltage arc through shite plastic.. unlikely, but I cant think of anything else!

[Vanny]

Have you tried turning it off, and turning it back on again?

[Philhod]

[mat_the_cat]

Have you tried turning it off, and turning it back on again?

Just as soon as I've managed to turn it on, I'll try that trick!

No advance mechanism, and no path to earth that I can see via the LT side of the points. Points are genuine Kohler and not pattern parts. Plug is new also!

So it's not just me that's puzzled! Strange how something so simple could be causing such head scratching. I'm sure it's a new component that's failed, and I'm suspecting hoping it's the capacitor/condenser.

The other thing to note is that the plug gets black quickly - but if it's only firing every third firing stroke (say) then it will obviously have 3 times the fuel it should. Or could it be somehow running so horrendously rich that the plug barely sparks when it's in the cylinder? Still think it's weak ignition though, as I struggle to get a decent spark gap.

[Way2go]

So it's not just me that's puzzled! Strange how something so simple could be causing such head scratching. I'm sure it's a new component that's failed, and I'm suspecting hoping it's the capacitor/condenser.

Just a thought here....have you observed the points with the capacitor disconnected?

The purpose of the capacitor is to maintain the energy in the resonant circuit by preventing arcing across the points on the LV side.

ergo...you should see arcing at the points without the capacitor and if you don't then either (1) The points aren't breaking (2) the coil is defective or (3) the magnet is seriously misaligned to the points cycle. (ps The engine won't run properly without the capacitor but even if arcing is seen then at the points it does not say that the capacitor is healthy)

With regard to magnet alignment, is there a single woodruff key for the flywheel or is it possible to fit in the wrong position relative to points-break?

[Bx Bandit]

I was gonna say capcitor mate (if it had one on a lawn mower...)

I often had ign problems on the Morris and that damn capacitor seemed to always be the culprit.

Having said that, my Briggs and Stratton went to bed one winter in fine fettle and point blank refused to start in the spring despite a good going over. My only thought was that perhaps the laminated doobery had de-laminated (moisture and freezing perhaps) and so magneto no sparkio enoughio.

Can't add more than that me old tater. Presumably (as you don't do things by half) your lawn mower is made by Caterpillar?

[mat_the_cat]

Presumably (as you don't do things by half) your lawn mower is made by Caterpillar?

Erm, it's a (very old) one of these, but I paid less than 1% of the new price... (Who'd pay that for a fucking mower?!!! )

[docchevron132]

I suppose it's possible the condensor is breaking down under load?

[Philhod]

(Who'd pay that for a fucking mower?!!! )

Someone who wants one that works....presumably

1 there should be an earth return from the baseplate and 2 my first two cars had no capacitor

[Vanny]

Someone who wants one that works....presumably

Damn it, beaten to it,

[Bx Bandit]

30" of grass sword! Be jesus that's big! mind you, with a name like Condor Hydro it's not going to dissappoint is it?!

[Philhod]

Hayter were always one of the best makes, but they are expensive.
I cut the son in laws grass last summer, over in Sweden. He sells all this type of stuff and brought a Husqvarna home for me to try. It was one of those with a seat and steering wheel and you just drove it round. Brilliant, really hard work...not
Mind you the grass area of their garden is about 100 meters by 50

I think it was priced at 12,000 Krona, which is about just over a grand.

[Bx Bandit]

you can get 10 bxs for a grand!