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?

[Mave]

Seems reasonable to me that either the HT lead, the connector on the lead, or the plug itself got physically damaged in the process of the lead getting yanked off.

[xstian]

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

[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!

[Peanut Gallery]

Have you stuck a circuit tester along the ht lead? - I have had the copper break in the middle, nightmare to find.

[Vanordinaire]

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

[Wacky Racer]

Yesterday I was trying to start my Flymo XL500 with a Honda four stroke engine for ten minutes, removed the plug, gap was correct, not wet, everything seemed fine.

Fitted brand new Champion spark plug, started first time.

Cool story bro etc...….

[dickymint]

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.

[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!

[dickymint]

WreckJohnB said:

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!

Compression really needs to be checked with a tester. From memory anything under 60psi and it wont start even though you can "feel" it. I read somewhere that Briggs and Stratton didn't use to publish actual figures but stated "turn the flywheel backwards by hand and if it bounces back it's OK !!

Yes that is how the kill switch works so bear in mind if you do start it with kill disconnected it wont stop easiest way to stop it is to give it full choke.

[jet_noise]

Peanut Gallery said:

Have you stuck a circuit tester along the ht lead? - I have had the copper break in the middle, nightmare to find.

Vanordinaire said:

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

I see you're doing the 2nd suggestion but have you also done the 1st?

A couple more diagnosing ideas:
Try it in the dark (oo er etc). If there's a crack in the plug or break in the cable or connector you'll sometimes see arcing that daylight conceals.
With the plug removed from the cylinder but held against the block with a lump of wood (or other insulator) give it a try.
Can you see a spark?
If not then that's a clue. If there is a spark it isn't wholly indicative that all is OK. It may be just weak. Many years ago I had a Bedford CF van that had a couple of ignition issues. First was a broken condenser - spark was weak but there. Second was a plug which would spark outside the cylinder but not in compression.

[curlie467]

It'll be connected to you ripping the wire out.
Have you tried for a spark with the plug cap removed, just hold the end of the lead near one of the head bolts and pull it over.

[jet_noise]

curlie467 said:

It'll be connected to you ripping the wire out.
Have you tried for a spark with the plug cap removed, just hold the end of the lead near one of the head bolts and pull it over.

Shocking!
You really don't want to be holding an HT lead. At least not without insulation

[independentLee]

Sounds like the spark plug cap has been pulled off the HT lead. This will interfere with the ability to spark.

Do as suggested. Remove plug, connect back to plug cap, rest against block to make an earth. Pull the starter cord and look for spark at the tip. Blue spark = good. If this is the case, then it's likely to be that you knocked the governor connection to the carburettor. Load of YouTube videos to readjust.

[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?