Misfiring

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Chessiegolf
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Location: LINCOLNSHIRE UK

Misfiring

Post by Chessiegolf »

Can anyone offer any help please.

My 1960 G12 has developed a misfire. Its fine when cold but after 10 -20 miles it will start to misfire which gets progressively worse. For information, the bike is now running as 12v neg earth and has completed 830 miles since a complete engine rebuild.

Thinking the problem is electrical, I've had the distributor overhauled by Distributor Doctor, changed the coil, plug caps, condenser, HT leads, retimed the ignition as carefully as I can - everything 'ignition' has been changed! It has excellent compression a is good starter hot or cold. The plugs gaps have been checked and show generally a dry black with a brownish colour on the electrode tips.

Occasionally the bike will spit back through the carb when opening the throttle from idle but this is not regular enough for me to think its part of the misfire problem, but perhaps it is. The carb was replaced with a new (indian) Amal about a year ago, jetted as per the manual and I've not altered any setting on the carb since

The bike is run on best petrol.

Any suggestions as to why it misfires once warm are appreciated.

Thanks

John
SPRIDDLER
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Re: Misfiring

Post by SPRIDDLER »

As a first 'elimination' check (if you haven't already done so) new plugs would be my next step, plus giving the carb a thorough clean.
Also check that the the air vent in the petrol tank cap isn't blocked with fluff or polish.
'There is a tide in the affairs of men
Which taken at the flood............'
Chessiegolf
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Re: Misfiring

Post by Chessiegolf »

Ok Spriddler,

Thanks for that. A non-venting tank was a good bit of lateral thinking, unfortunately having rushed out to check, I can blow and suck easily through the hole, so that's not the problem.

The plugs are Champion N5C, which were replaced as part of the engine rebuild so they should be fine, but would be a simply elimination exercise I suppose - is there a 'better' plug than N5C?
SPRIDDLER
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Re: Misfiring

Post by SPRIDDLER »

N5C is fine. Best choice IMHO.
Had another thought - although you fitted a new condenser there's the outside possibility that it (or the new coil) becomes faulty when it warms up.
Most new points tend to have a protective coating that needs to be wiped off; drawing a petrol wet rag between them a few times when closed should do it.
You could try running with both petrol taps open to eliminate any possibility that there's a partial blockage in the main feed.
There could be a loose connection in the ignition circuit at the distributor or coil etc.
'There is a tide in the affairs of men
Which taken at the flood............'
Mick D
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Location: Aberdeenshire

Re: Misfiring

Post by Mick D »

Hi

Have you checked the valve clearances?

Regards Mick
Chessiegolf
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Re: Misfiring

Post by Chessiegolf »

I changed the coil and condenser as part of the elimination process but neither made any difference. I also changed the HT leads and plug caps but that didn't make a difference either, though I will recheck the terminations. I will replace the plugs and fit N5C again.

As to the valve clearances, I checked these at about 100 miles and then 500 miles following the rebuild (now 830 miles) and they had certainly closed up, I suppose as a consequence of head re-torquing. I will recheck those now as valve clearance could be the cause of the carb spitting and is a recent occurrence, however the misfire has been going on for a while now, lets say 500 miles and has come to the fore perhaps because the weather is warmer and I'm trying to ride for longer as confidence builds in the reliability!

I can't get out of my mind the cause being an HT breakdown due to heat. I can't see any rogue sparks around the plugs, distributor or coil but will try again at night when they may show up better.

I'm generally happy with the bike, it starts well, pulls strongly and with a balanced crank, conrods and pistons, seems to have quite a low level of vibration. Its just this annoying misfire...........
Mick D
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Re: Misfiring

Post by Mick D »

Hi

If it misfires whilst stationary you can do some diagnosis:

Get it hot and misfiring and remove one of the plug caps, (wear a glove ;) )

If the misfire stops you've identified the cylinder on which it's happening - if it continues refit the plug cap and remove the other.

If the misfire stops you've identified the cylinder on which it's happening - if it continues you have an issue common to both cylinders.

You can now refine your investigation.

Regards Mick
Chessiegolf
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Re: Misfiring

Post by Chessiegolf »

Well Done Mick,

I'll try that - always have misgivings about touching live HT leads but I'm getting desperate!
Chessiegolf
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Re: Misfiring

Post by Chessiegolf »

Update,

The tappets were checked - all ok.
I didn't have to pull off the HT leads with the engine running as a hand held behind the exhausts showed which cylinder was misfiring, the left hand one.
I read the tuning guide for a Monobloc 389 and thought that perhaps it was worth leaning the mid-range mixture so lowered the needle by one notch - success! The misfire has disappeared around the 1/3 throttle opening which generally corresponds to my 50-55 mph cruise.
The problem is its transferred the misfire or lumpiness to the bottom end, around 25-30 mph which is not pleasant in a 30 mph limit, however, we are getting there so now need to fiddle with the carb - main jet, needle jet and perhaps slide cutaway.
The new carb was jetted in accordance with the manual but I noticed that the main jet and slide from the original carb were different those of the factory spec so it appears a previous owner may also have been having carburation problems.
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1608
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Re: Misfiring

Post by 1608 »

I can't see how altering the mixture should affect one cylinder and not the other. Altering the mixture ( ie. weakening it ) may just have lessened the symptoms of an ign fault. Sounds more likely to be a dodgy plug, ht or mag.
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