Author Topic: Misfire  (Read 435 times)

Offline RoyC

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Misfire
« 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.
My bike is a 1958 A7SS
Staffordshire UK

Online Triton Thrasher

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Re: Misfire
« Reply #1 on: 03.03. 2019 20:43 »
The new carb.

Offline Joolstacho

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Re: Misfire
« Reply #2 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?

Online berger

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Re: Misfire
« Reply #3 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

Online Swarfcut

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Re: Misfire
« Reply #4 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.

Online metalflake11

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Re: Misfire
« Reply #5 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!
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Offline RoyC

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Re: Misfire
« Reply #6 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.
My bike is a 1958 A7SS
Staffordshire UK

Offline morris

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Re: Misfire
« Reply #7 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.
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'52 BSA A 10 Plunger
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Offline Joolstacho

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Re: Misfire
« Reply #8 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?)

Offline RoyC

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Re: Misfire
« Reply #9 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.
My bike is a 1958 A7SS
Staffordshire UK

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Re: Misfire
« Reply #10 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?