The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: Brucie64 on 01.07. 2013 09:33

Title: Misfire when hot - A10 Spitfire
Post by: Brucie64 on 01.07. 2013 09:33
Hi All,

I read somewhere a while ago that a misfire or spluttering when opening up the throttle (with a hot engine) normally means that the magneto could be struggling to cope, I have this very same problem on my A10 Spitfire. A quick fix, other than a mag rebuild would be to close the spark plug gaps from 18 thou to around 10 thou. I did this on saturday (a lovely warm day here in SE UK) and took the bike out for a spin, sure enough the misfire under load seems to have vanished and the "bitch" behaved impeccably.

My question is, should closing the spark plug gaps by this amount cause me any concern? (other than reducing the gap for the sparks to jump across, what else does it do?)

Title: Re: Misfire when hot - A10 Spitfire
Post by: groily on 02.07. 2013 18:40
Think the only concern I'd have Bruce would be 'how far from civilisation am I going to be when the thing packs up completely?'!
Small plug gaps won't kill anything else, but it's a temporary fix, if the problem is really in the sparks department. When it misbehaves again, down to 8 thou maybe, and then  . . .? I'd not go TOO far right now till I knew what was up with it! Good luck, Bill
Title: Re: Misfire when hot - A10 Spitfire
Post by: Brucie64 on 03.07. 2013 09:17
Hi Bill,

Thanks for the input, I'm tending to go the other way. Open them up again until it starts the splutter again the nip back a little. If I can get the gap back to something reasonable like 14 thou plus then I might just let it go at that. Odd thing is I gave the bike a good blast last night and again it starts first kick (never has done in the past), it never missed a beat even up the steepest hill with the throttle wound fully open.

Ps. Always travel with my AA card and mobile in my pocket just in case  *smiley4*