misfire at low throttle

25.05. 2012 02:05

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The BSA A7 & A10 Forum > Technical > Amal & Carburation > misfire at low throttle

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	Author	Topic: misfire at low throttle (Read 476 times)
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melvyndalby

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misfire at low throttle

« on: 03.09. 2011 17:31 »

My 1955 RGS rep misfires at speeds up to 40-45 mph.After that it performs as it should.Coil ignition has been fitted and the points are set at 15 thou.Can anyone help?
Hear is photo of my bike.
Mel

P8080036.JPG (125.22 KB, 640x480 - viewed 36 times.)
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t20racerman

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Keep it nailed!

Re: misfire at low throttle

« Reply #1 on: 03.09. 2011 20:00 »

Hi there

Need a bit more info to help out. Where is the misfire - in every gear? Are you saying it only runs above 45 MPH? What colour are the plugs? What is the timing set at? What advance and retard do you have?

Also, is this a new problem that has just arisen or one that it has always had?
Cheers

Adrian


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1961 A10 - somewhat modified
1980 TZ350 - lunatic Classic Race machine
1967 T20 Suzuki - heavily modified Classic Racer
1967 T20 Suzuki - pretty standard road bike
Ossa 250 and yet another T20 racer in bits both being built up

"If I had all the money back that I've spent on motorcycles... I'd spend it all on motorcycles!"

wilko

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Re: misfire at low throttle

« Reply #2 on: 03.09. 2011 20:39 »

Close your points gap a little.


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melvyndalby

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Re: misfire at low throttle

« Reply #3 on: 04.09. 2011 14:48 »

Hi Hadrian

I've had the problem since I aquired the bike in March.It has coil ignition and the points are set at 15 thou.The carb is Amal and the misfire occurs at about a quarter throttle in any gear.If I open the throttle fully the bike acclerates away smoothly in all gears and keeps on accelerating.The spark plug gap is set at 25 thou (I have trouble starting if they are set at 18-20 thou.The plgs are a grey/black colour.There is no adv and retard.
Cheers Mel


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muskrat

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Lake Conjola NSW Oz

Re: misfire at low throttle

« Reply #4 on: 04.09. 2011 15:32 »

No advance sounds like the culprit. At low revs the spark is too advanced and as the revs rise they come to the sweet spot in the timing. How is it to start?
Second opinion is a rich slide, and open the plug gap up a bit more
Cheers


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Only young once, immature forever. Now how can I make this go faster. '51 A7 plunger, '57 A7SS now A10, '71 A65 Lightning (gone to god) '76 XT500, '83 CB1100F, next project a '64 A65.

t20racerman

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Keep it nailed!

Re: misfire at low throttle

« Reply #5 on: 04.09. 2011 17:32 »

As a simple first step try weakening the mixture low down with the air screw and/or drop the pin down a notch. This is easy to do and might clear it up, but the advance retard needs looking at as if the timing isn't right you'll never get it to run right.
smile


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Triton Thrasher

Scotland
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Re: misfire at low throttle

« Reply #6 on: 04.09. 2011 23:53 »

Quote from: t20racerman on 04.09. 2011 17:32

As a simple first step try weakening the mixture low down with the air screw and/or drop the pin down a notch. This is easy to do and might clear it up,
smile

A rich needle position may well be the cause.

Seriously consider going through the whole Amal tuning procedure though. It's probably in your manual.


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melvyndalby

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Re: misfire at low throttle

« Reply #7 on: 18.09. 2011 18:47 »

Thanks for everyones advice.The problem was a faulty condenser which was found when I had it checked.
Mel.


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