Odd problem with ignition

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Odd problem with ignition

Postby rajeshnair » Thu Jul 19, 2018 12:34 pm

Hello all
My 1952 M20 has a very odd problem,only one cylinder fires,and when I disconnect the working cylinder plug cap the other cylinder fires up.wonder if this due to induction bias,i have checked the magneto,plugs are god plug caps and wires are new.Timing is correct.Would anybody know of a similar problem.Thanks in advance,cheers!

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Re: Odd problem with ignition

Postby SPRIDDLER » Thu Jul 19, 2018 1:07 pm

rajeshnair wrote:My 1952 M20 has a very odd problem, only one cylinder fires, and when I disconnect the working cylinder plug cap the other cylinder fires up.

You're lucky having a twin (I didn't think I'd ever say that ) :o
Did it run o.k. before you fitted those new parts? Try swapping the leads and caps side for side (by disconnecting and reconnecting them at each other's point on the mag, not just swapping them over!), and see if the same cylinder fails to fire. If no different swap the plugs side for side and check again.

........the magneto, plugs are good plug caps and wires are new.

Precisely for that very reason try the checks above. Many new parts can be faulty.

BTW - the plug caps must not be type with integral resistors.
I poke badgers with spoons.

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Re: Odd problem with ignition

Postby Rob Harknett » Thu Jul 19, 2018 2:59 pm

Do your new leads & old leads have wire? If your new are fibre I would used wire leads.

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Re: Odd problem with ignition

Postby aobp11 » Thu Jul 19, 2018 4:08 pm

IIRC this could be caused by a dirty slipring. The "spark current" prefers to flash over the slip ring surface to the opposite cable brush since the non-active cylinder has low compression at that moment.

Albert
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Re: Odd problem with ignition

Postby cbranni » Thu Jul 19, 2018 6:51 pm

Is it a big spoon Nev?
only dead fish go with the flow

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Re: Odd problem with ignition

Postby SPRIDDLER » Thu Jul 19, 2018 7:24 pm

cbranni wrote:Is it a big spoon Nev?

It's heresy for me to praise a twin. I've never had one, ever, as I adore the thump of a 350 or 500 long stroke single.

However, I recently dropped in on a pal who was just finishing the total resto of a '51 G9 and who had been struggling for several days with an identical problem. The engine was rebuilt meticulously. The frame and all tinware hand-painted. The mag had been rebuilt by a well known specialist in Wiltshire but one cylinder wouldn't fire.The mag had been rebuilt and returned complete with new copper-cored HT leads and plug caps.

2018-03-09 Frame panted studio shots 009.JPG


2018-04-09 013.JPG


Peter came round with a spare mag and to add his two-penn'orth and we read all the manuals (again).......
2018-05-29 RTFM! M20 003.JPG


We did the checks I mentioned in my earlier reply (after re-stripping the carb but before taking off the head to check the valves) and after swapping the leads the other cylinder fired but not the first one. We tried Peter's spare mag and it ran perfectly. Before we sent the rebuilt mag back we refitted it to the bike with old HT leads and caps from under the bench and it ran perfectly on both pots. A thorough check of the new HT leads and caps revealed one had a very high resistance - almost open circuit. Hence my new parts cynicism.
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I poke badgers with spoons.

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Re: Odd problem with ignition

Postby cbranni » Thu Jul 19, 2018 8:09 pm

Looks a well sorted rebuild Nev, very impressive, I went to help a guy with a twin few months back would only run on one and they had checked the plugs, so I went back to basics, checked valve timing, tappet clearance, timing, clean carb, still same, changed both plugs and ran fine doh doh :headbang:

I was alluding to your signature "I poke badgers with spoons" where did you find that?

Colin
only dead fish go with the flow

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Re: Odd problem with ignition

Postby SPRIDDLER » Thu Jul 19, 2018 9:25 pm

cbranni wrote:Looks a well sorted rebuild Nev, very impressive, I went to help a guy with a twin few months back would only run on one and they had checked the plugs, so I went back to basics, checked valve timing, tappet clearance, timing, clean carb, still same, changed both plugs and ran fine doh doh :headbang:

Yes, Ray and I could've kicked ourselves at overlooking such a (now) obvious basic thing. But we do take a long time to do much; just bursts of random activity in between Thursday morning cups of shed coffee and Hobnobs and reviewing Mr Trump's outbursts, the Euro exchange rate, operations, and the washing on the line of the young blonde next door.
Usual sort of stuff for 73 yr olds........
StatlerandWaldorf(2).jpg


I was alluding to your signature "I poke badgers with spoons" where did you find that?

That comes from an Eddy Izzard routine. He's seeking forgiveness of that sin in the Confessional box at church. It appeals my ridiculously childish sense of humour.
It's the sort of pastime I might find fulfilling when too old to ride any more.
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Re: Odd problem with ignition

Postby Groily » Thu Jul 19, 2018 9:37 pm

Sounds suspiciously like 'soft brush syndrome' to me. Poor brushes are still out there. They coat the slipring in carbon dust. Misfires, firing both sides at once or trying to ...it's not good. Suggest replacing brushes and cleaning slipring very thoroughly. Might be all that is needed - and is a very common problem due to poor parts continuing to be sold despite plenty of info on the consequences!

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Re: Odd problem with ignition

Postby JimFitz » Thu Jul 19, 2018 10:15 pm

I have a similar problem on a Super Rocket with a K2F mag. Starts up on one sometimes and the second pot comes in after about 100 yards. Can you clean the slip ring sufficiently through the pick up holes with a rag and a little switch cleaner while slowly turning the engine (using rubber gloves of course) or does it all have to come apart.
I am going to have a look at it tomorrow and do the drawing on paper test on the brushes first.
I will also try a new set of plugs when they arrive.

Jim
Too old to Rock and Roll but too young to die.

1952 G80 rigid, 1960 G12 DL / Watsonian Monza, 1954 G80S.

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