The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: RoyC on 03.03. 2019 19:47

Title: Misfire
Post by: RoyC on 03.03. 2019 19:47
Started the A7ss up yesterday, first time for two months, she started first kick but there is a misfire on the left side. what could be the most likely cause of this ?
They are new plugs. New carb.
Title: Re: Misfire
Post by: Triton Thrasher on 03.03. 2019 20:43
The new carb.
Title: Re: Misfire
Post by: Joolstacho on 03.03. 2019 21:20
The new carb.
So if it's the new carb why wouldn't it be missing on both pots?
Title: Re: Misfire
Post by: berger on 03.03. 2019 21:27
roy I would check pick ups on maggy and slip ring etc , apart from that swap plugs over and see if it misfires on the right pot
Title: Re: Misfire
Post by: Swarfcut on 03.03. 2019 22:09
Run it in the dark, looking and listening for odd sparks anywhere between pick ups and plugs. Even a bit of damp from atmospheric condensation will cause tracking if the plug leads are duff.

 Swarfy.
Title: Re: Misfire
Post by: metalflake11 on 04.03. 2019 00:36
Start at the top!.....Plugs, caps (I've had those rubber ones rust inside causing a misfire) Leads (I've had them fail too, and I spent good money 'fixing' things that weren't broke) Lead pick up to lead connection. Pick up. Brushes.

Start simple!
Title: Re: Misfire
Post by: RoyC on 04.03. 2019 08:33
The new carb.

It was doing the same on the old carb.
 was wondering if it could be a tight exhaust rocker on the left.
Title: Re: Misfire
Post by: morris on 04.03. 2019 11:33
What does it do exactly?
Is it like firing irregularely or is it backfiring or does it bang in the exhaust?
Could be due to an induction bias. Both mine suffer it, the plunger runs richer on the left pot and the SA on the right.
This can be heard at idle when there's like two or three combustion cycles that just seem to puff, and then a sharper combustion, after which again there's a couple of puffs.
Doesn't bother me too much at the moment but one day I'll dig in to that en get it solved... *roll* ... one day.
Title: Re: Misfire
Post by: Joolstacho on 04.03. 2019 12:36
You used to be able to get an induction bias spacer that was supposed cure that induction bias problem.
(But there's so many more likely causes of the problem eh?)
Title: Re: Misfire
Post by: RoyC on 04.03. 2019 16:08
What does it do exactly?
Is it like firing irregularely or is it backfiring or does it bang in the exhaust?
Could be due to an induction bias. Both mine suffer it, the plunger runs richer on the left pot and the SA on the right.
This can be heard at idle when there's like two or three combustion cycles that just seem to puff, and then a sharper combustion, after which again there's a couple of puffs.
Doesn't bother me too much at the moment but one day I'll dig in to that en get it solved... *roll* ... one day.


On tick over.
It doesn't backfire, bang etc. it sort of just misses a beat every few revolutions. When I blip the throttle it picks up fine without any misfiring.
Title: Re: Misfire
Post by: Triton Thrasher on 04.03. 2019 16:55
The new carb.

It was doing the same on the old carb.
 was wondering if it could be a tight exhaust rocker on the left.

Slightly enrich the idle mixture and see whether that makes it better or worse.

Do you get a blue spark at the plugs at kick over?