mag sparks

[Guy Wilson]

I have a K2F mag on an A10 I've built up from parts... The mag is giving a very healthy looking spark both on a bench test spinning by hand and with drill as well as healthy spark while kicked over with the plugs out on the head.. however, its not starting or even firing... I read a while back on the forum an account of something similar with plugs sparking while out of the cylinder but refusing to do so when fitted..
I was both looking for that thread and any other ideas as to what could be the issue..
Thank you!
Guy

[Billybream]

Try swopping the ht leads over

[Guy Wilson]

thanks Billy!, I've tried that... still nothing... I seem to remember reading something about sparks looking healthy outside of the combustion chamber/ cylinder and then being unresponsive once fitted..

[KiwiGF]

thanks Billy!, I've tried that... still nothing... I seem to remember reading something about sparks looking healthy outside of the combustion chamber/ cylinder and then being unresponsive once fitted..

It’s possible the increased air density during compression could result in a spark being prevented but unlikely, if the bike won’t start and the mag when tested has a good spark it’s far more likely to be something else causing your firing issue.

Spraying “start yer bastard” (ether based liqued) down the carb can be revealing, the engine can fire without a spark as the ether ignites due to being compressed (like a diesel)

[Guy Wilson]

thanks Kiwi, I thought it was unlikely that there would be nothing - no back fire/ miss fire etc if there was a strong spark - the air density was, I think what I had read out on an earlier thread as a possibility but seems unlikely as you say... I'll go back to basics and check the static timing and the carb before returning to the mag!
best
Guy

[BSA500]

Yes I had no end of hassle and in the main it turned out to be the carb. I cleaned it twice using ultra sound and still no go. I dug out and old carb, wrong size plonked the jets into that and she started up right away. Still no idea why.

[sean]

put a SMALL amount off gas in plug hole see if it  tries to fire .......have you checked timing

[Guy Wilson]

I've changed the carb to one I know that worked and I'll try that first..after that the static timing ...
all part of the fun !
Guy

[CheeserBeezer]

Attach the HT lead to the plug without a plug cap so that the spark has to jump to the top of the plug off the HT lead. If the spark jumps outside the cylinder it must also be jumping inside. Are the plugs wet? They should be if you've kicked it over a few times. Try starting with the throttle slightly open, see if it kicks you back.

[limeyrob]

The Lucas manual says (somewhere..) that a static spark across a 1/4" gap should be good for a cylinder under pressure.  It seems you are sorted now but its worth checking the spark across the whole advance and retard range if you have manual A-R.  The best spark should be at full advance then get weaker as you retard. Not ideal but the mag has a sweet spot and that's set for full advance.

[Swarfcut]

 Two common mistakes...Plug leads on wrong cylinder and setting the timing after top dead centre. Always better to turn the engine forward to come up to the firing point, this removes backlash in the timing gears. Then it's a case of setting the maggy  with points just opening, manual maggy set to full advance and assembling the drive gear without moving the magneto armature.  With an auto unit wedge the ensemble fully open.  More Tricks, Tips  and Pitfalls in the Electrical Section.

 Just make sure you have good compression and there are no mechanical issues. A good spark in the right place should fire cylinders primed with fuel or start yer bast**d. Problems keeping it running then point more to a carb fault.

 Swarfy.

[Guy Wilson]

solved ( I think) ... as I mentioned it was a bag of bits.. the woodruff key on the camshaft timing gear was missing... can't remember if the camshaft was out and I missed it or if it was timing gear was on the camshaft and I didn't check it.. either way I should have spotted it/ checked it on assembly!
Building the bike with son, so it was a quite a good lightbulb moment when we got there!
thank you as ever!
Guy

[limeyrob]

As Swarfy says, come up to the timing the right way to get the back lash out.  Its surprising how much difference this makes, I've got very worn engines to run really well by taking time to get all the backlash out.  Its a right faff and a fair bit of trial and error but time well spent.

[groily]

solved ( I think) ... the woodruff key on the camshaft timing gear was missing...

Hope, if the valve timing was way out (or if the pinion slipped independently of the cam while kicking her over), that the valves are OK Guy?

[Guy Wilson]

yes Bill.. I have the same thought...the joys of motoring...I will check..
thanks
Guy