A peculiar problem

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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A peculiar problem

Postby Mollbhan » Sun Oct 13, 2019 6:23 pm

I have a 61 G12 mag ignition that often sits idle for a week or more, I take it out, petrol on, flood the carb, choke on, ignition retarded and four times out of five it will start first kick, if not it usually kicks back but will start within four kicks. I dare not stop it, doesn't matter whether it runs for two minutes or an hour nothing will induce it to start again, no splutters, no farts just nothing. There is a good blue spark but the plugs will be dry, I have to leave it for a period of up to an hour depending how hot it was and it will be back to a first kick start again. I have seen plenty engines dour to start when warm but never one as extreme as this.Anybody seen this before? Thanks in advance for any help.

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Re: A peculiar problem

Postby 1608 » Sun Oct 13, 2019 6:37 pm

Apart from the mag being suspect there are some other reasons that make warm starting difficult. Too richer mixture, what colour are the plugs, could be flooding. Next time try a wide / full throttle opening. Could also be fuel evaporation, is there a 'Tufnol' spacer ( normally 1/4" on G12) fitted btn carb and head.
Not commonly known is that compression is greater on a hot engine, so the mag might no be up to creating a spark under these conditions.

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Re: A peculiar problem

Postby Mick D » Mon Oct 14, 2019 4:11 am

Hi

Do you tickle when warm/hot, does the carb still flood? Eliminate a blocked breather by removing and replacing the petrol cap and try starting.

Regards Mick

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Re: A peculiar problem

Postby MalcW » Thu Oct 31, 2019 9:15 am

Have you solved this? This sounds just like my Model 31 used to behave.

If not, when you try the restart, try it on full advance, with a good swift kick on the starter. Another change I made is to fit genuine Champion N5C plugs ( I got mine from The Green Spark Company). I had NGKs, which I read elsewhere don't mix well with magnetos - I have no idea why. Lastly, my bike seems to be very sensitive to the magneto points gap; make sure that is spot on.

I can now (touch wood) go out on my bike without having to be afraid to stop the engine.

Malc

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Re: A peculiar problem

Postby Rob Harknett » Thu Oct 31, 2019 9:41 am

I have to almost fully retard my G12 CSR to start it from cold. All it will do is kick back if not almost fully retarded. When warm it has to be fully advanced to start it. You say the bike kicks back when retarded. Could the ignition be set too far advanced or points gap too wide.
If my bike does not want to start and everything seems OK , I have to fiddle with the spring tension at the end of the mag.
Can someone advise why this happens, I have little experience with twins.

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Re: A peculiar problem

Postby Groily » Thu Oct 31, 2019 12:03 pm

Dunno quite what you mean about the spring thing Rob (earth brush and spring for engine stop, or spring on AR plunger maybe?) - but if the hot mag will only start the engine at full advance, it says it is probably starting to fade. Strongest spark is at full advance . . . which is why sometimes it's worth risking a bit of kickback by doing as MalcW says.
I've heard it said often that NGKs aren't that great, but not had any problems with B7ES myself (or Champion 4s and 5s for that matter). A lot of counterfeit plugs out there, they say?

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Re: A peculiar problem

Postby Rob Harknett » Thu Oct 31, 2019 12:34 pm

Groily wrote:Dunno quite what you mean about the spring thing Rob (earth brush and spring for engine stop, or spring on AR plunger maybe?) - but if the hot mag will only start the engine at full advance, it says it is probably starting to fade. Strongest spark is at full advance . . . which is why sometimes it's worth risking a bit of kickback by doing as MalcW says.
I've heard it said often that NGKs aren't that great, but not had any problems with B7ES myself (or Champion 4s and 5s for that matter). A lot of counterfeit plugs out there, they say?

True about alowing a bit of advancement from cold, I discovered about how much on my bike. Its at about the point it could kick back. Not rode the twin for a while. The spring in question I need to pull out a bit so it makes good contact. Does that tell you what I am fiddling with?

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Re: A peculiar problem

Postby Groily » Thu Oct 31, 2019 2:37 pm

Tells me you're probably talking about the brush and spring for the engine stop Rob.
It only needs to make contact to stop the thing though, not to start it!
If a tweak's needed, then the carbon brush or spring is touching summat it shouldn't. The only thing it should touch (except the insulated sleeve it's in) is the head of the centre screw on the cb. If it touches some other bit as things whizz round in, or there is damage to the insulated bit, then it might short out the low tension side. Happens quite often in fact. Only becomes a pain when the tweak doesn't sort it!

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Re: A peculiar problem

Postby Rob Harknett » Thu Oct 31, 2019 3:06 pm

I think you're spot on there. I have not seen it for ages. I'm sure there was a brush.

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Re: A peculiar problem

Postby bitza » Thu Oct 31, 2019 4:59 pm

Mag fault when hot??? Condenser breaking down but fires up when cold ???.

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