Misfiring

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Chessiegolf
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Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

Ok 1608,

You may well be right but I'd changed the plugs and coil this week and the HT leads a few weeks ago.

On my bike the right exhaust is 'blueing' more than the left so there may not be an even flow through the manifold and if the left cylinder is richer then perhaps one notch of the needle was all that was needed to bring the mixture back to a point where it satisfied both cylinders, at least a mid throttle openings.
I will keep experimenting with the carb for now as I've replaced everything ignition-wise and had the distributor overhauled by a reputable person so I do think its worth looking for other causes.
Interestingly the main jet that was in the old carb was a 380 and the manual lists a 390 so perhaps a previous owner had found that a finer jet was needed. The carburation was fine when I got the bike it was just everything mechanical that was shot and butchered!
Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Misfiring

Post by Mick D »

Hi

One exhaust blueing is an indication of something amiss with the exhaust valve or a bad cam ring in the mag - try setting the timing on the other cylinder.

Two exhausts blueing, look to retarded ignition timing.

Regards Mick

:oops: Just remembered you have coil ignition - forget the cam ring as a cause, - I'd still look to tappet clearance though - it's a bit difficult for me to see how it can be a carburation issue
mdt-son
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Joined: Wed Feb 01, 2012 1:22 pm
Location: Vestland NORWAY

Re: Misfiring

Post by mdt-son »

Hi CG,

I read of a similar symptom occuring on a Norton Commando. I suggest you concentrate on the electrics. There could be a problem related to the heat-up of the distributor. You say the condensers were new. Really new, or new old stock? Condensers have a shelf life and can develop an erratic behaviour.
The second path of investigation would be earthing. Check resistance between the frame earhing and the points plate by using an ohm-meter. Also check earting of the coil. Try swapping the coil - it may have a defect internally which shows up as it heats up (it's fitted under the petrol tank, above the cylinder heads - right?).

I take it the misfiring occurs on both cylinders, intermittenty.

Best of luck with your detective work,

Knut
g5wqian
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Location: wiltshire
Location: near swindon wilts

Re: Misfiring

Post by g5wqian »

i would check the electrolyte level in the battery and also check the battery takes a charge .

i had similar thing on my ariel and it turned out to be a low electrolyte level in the battery which meant it wouldnt charge properly and in turn after a few miles out on the bike id get a misfire .

the misfire was more evident when taking a left or right turn and using the indicators along with the led headlight on .

because these systems are not self generating like a magneto would , you tend to have to rely upon the state of the battery and on a motorbike there isnt a lot of reserve power if theres an issue with the battery .

my own probelm was probably down to me keeping the battery charged on a regular basis through the winter but i had charged it a few times and failed to keep a check on electrolyte level .

i would say top your battery up , give it a charge with a low current overnight , check levels again and then ride it for a time and see if the misfire occurs .

no doubt your alternator is charging , but perhaps check that out also with an analogue voltmeter .

other than that check the terminals inside the distributor cap for oxidation and give them a scrape with a knife , and check the carbon pick up brush in the cap is protruding enough to make contact with the rotor arm .
you can get new brushes from green spark plug company here .

hope you suss it out
ian
Chessiegolf
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Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

Thanks for the suggestions.

The battery was a MotoBatt gel battery which I have also replaced (no expense spared).

The distributor had been overhauled, at considerable cost, and a new coil and condenser (not new old-stock) had been fitted and neither made the misfire any different. Hitchcocks who I bought the carb from suggested that the cause could be that the bike had stood over the winter with ethanol petrol in it which apparently lays down deposits much quicker than non-ethanol petrol.

The carb has now been removed, all jets taken out, and every port cleaned with carb cleaner. The bike now awaits a good run - the problem only manifests itself when thoroughly warm.

Frustratingly I haven't been able to go for a decent run, as the newly overhauled speedo packed up (too much grease applied apparently which worked its way into the clutch). Once the speedo is back I'll try again and see if cleaning the carb has made any difference.

Thanks again so the suggestions, every one makes me think of the possibilities that could lie there.

John
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clive
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Location: LONDON UK

Re: Misfiring

Post by clive »

"Frustratingly I haven't been able to go for a decent run, as the newly overhauled speedo packed up (too much grease applied apparently which worked its way into the clutch). Once the speedo is back I'll try again and see if cleaning the carb has made any difference."

When greasing the cable do so sparingly. Just put a pinch of grease on your finger/thumb and run the cable through it. Do not grease the last 18 inches, what you have already run in will be sufficient to lubricate it and this prevents Archimedes screw action driving the grease into the speedo. Never oil the cable.
clive
if it ain't broke don't fix
mdt-son
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Joined: Wed Feb 01, 2012 1:22 pm
Location: Vestland NORWAY

Re: Misfiring

Post by mdt-son »

Hard to see how deposits would manifest themselves only when the engine heats up. Is your petrol tank devoid of rust? Fitting an external fine mesh fuel filter is a good idea, at least while the hunt for a fault goes on. And do the testing with fresh petrol.

I still think your problem has to do with electrics, and I suggest doing an external test of the left HT lead for sparks using a spare spark plug.

Have you doublechecked your timing?

Does the engine start readily after misfiring and subsequent cooling down?

One idea: Have you checked oil supply to the cylinder head? Are all four valve stems lubricated after a run? Is there a faint possibility the valves could be nipping up due to tight valve guides (esp. if you fitted custom made bronze guides)?
You claim compression is good on both cylinders, warm or cold. This should be checked using a compression tester the minute misfiring occurs. Spit-back can be attributed to a hanging valve and/or wrong timing.

-Knut
Chessiegolf
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Re: Misfiring

Post by Chessiegolf »

After a few months of disinterest in the bike with not being able to trace the misfiring fault, I thought I would have another go.

The carb jets and needle have been reset to the manual spec.

The bike started first try after a few months lay-off so encouraged, I went for a ride. The misfire returned as anticipated, once warm (13 - 15 miles running) but I've spotted that there is sparking across the LT and HT connections at the coil, not all the time but when it occurs then the bike misfires.

The coil is a new one, the plugs (N5C) are new, originally set at 25 thou, but now 21. The HT leads are new and so are the plug caps, these are 10K ohm. The distributor has been reconditioned by Distributor Doctor and the condenser has been replaced and I've done my best to make sure it has a good earth.

Why would sparking occur across the coil once the engine has warmed up - have I got the wrong plug caps at 10K ohm? Are some HT leads better than others, in which case what should I be fitting?

For information, the coil is tucked away under the tank, its dry, but I haven't sprayed it with WD40 or any other wonder cure.

Any help please as I'm getting thoroughly fed up with the bike!
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ajscomboman
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Re: Misfiring

Post by ajscomboman »

electrical current will always take the path of least resistance down to earth. I'd say that despite the coil being new it's faulty or the shielding from the HT lead is at fault. I can see no other reason for the spark to occur between LT and HT. Can you load a picture of the location and coil set up.
Mick D
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Location: Aberdeenshire

Re: Misfiring

Post by Mick D »

Hi

That's a result, arcing between the HT and LT at the coil means no spark at the plug. Have a close look at the coil for stray wire strands at the connections. Also look for cracks in the black insulation in which case you need to replace the coil, hopefully under warranty.

Supressed plug caps on coil ignition are OK.

Regards Mick
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