Briggs Stratton ignition issue?

[WreckJohnB]

Hi all,

Long time reader, new poster. I'm at a loss with my lawnmower (Bigg's Strap-on 6hp 12H807), it's always been a 1-2 tug starter but whilst mowing earlier this week the spark plug wire got caught behind a twig and pulled loose so naturally the engine cut out. I reconnected it, wouldn't start, I figured the engine might have got flooded so I let it sit for a while and tried again, still no luck.

Before you ask, yes it's got fuel, plenty (and fresh). I took the spark plug out and checked for a spark, no spark as far as i can tell (visually, haven't got a tester). I've taken the guarding, shields etc. off to check wires but nothing looks out-of-sorts. The magneto gap looks OK but reset it anyway to business-card thickness. I've also checked the killswitch wire from the magneto to the brake, no obvious breaks there.

From my limited understanding of these engines and seeing that it suddenly cut out after the spark plug wire got pulled out, the only thing that I can think of is somehow the magneto/spark wire is buggered and needs replacing, does this sound right? It's a £50 secondhand mower, so before I shell out on a magneto/repairs I wondered if I'm on the right path with this repair?

[WreckJohnB]

xstian said:

The HT lead is just screwed into the plug cap. have you checked its not just come away?

Had another look, yes it's intact.

I think it'll be an ignition coil replacement, pricey little part at £40!

[WreckJohnB]

Vanordinaire said:

Have you tried a different plug? It's pretty easy to crack the insulation on a modern sparkplug if you bump it.

Thanks, I've now got a new one on the way. I figured I'd best try the cheaper solution before sticking in a new ignition module at £50. Are Farmertec parts any good? Considerably cheaper than OEM parts!

[WreckJohnB]

dickymint said:

First thing I always do when fixing motors for friends is check the compression (with a guage) . If it's too low I normally give it straight back as I don't have time to change rings or piston barrels.

I would also disconnect the kill switch as they fail to off.

Haven't checked this but it had been running fine before the HT lead got pulled off. There was quite a difference in resistance on the pull cord with spark plug on and off, so I guess compression is OK or would this not be a reliable indicator?

I'll give disconnecting the killswitch a go. Am I right in thinking this shorts the ignition to stop sparking?

Please bear with me on this, I can turn a spanner etc. but before this engine c**k-up I'd never done any work on combustion engines so still learning... Keen to learn though!

[WreckJohnB]

Thanks for the suggestions and apologies for the radio silence! I tried a new spark plug, no joy. Ordered a new magneto which took ages to arrive (hence the radio silence), but it looks like this was the issue. Or at least sort of. The mower starts again, briefly, sometimes surges a bit, had one backfire, and then dies. I've not touched the carburettor at all and had no issues before the ignition problem, is there any chance this surging/dying could still be an ignition issue? The magneto I got (Farmertec) looks like the 'contact' area has a slightly different radius to the flywheel so I set the spacing at 0.01" at the narrowest bit. Is this correct?