Running problems

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Ajaydave
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Running problems

Post by Ajaydave »

Hi folks, my 650 ajay twin doesn’t want to accelerate much above 50; I,ve given it choke to see if it’s weak but it wants to cut out, the plugs read ok, it’s running A 2 into 1 with a mega, puzzled, ????, any ideas, rgds Dave
SPRIDDLER
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Re: Running problems

Post by SPRIDDLER »

Ajaydave wrote: Mon Aug 22, 2022 1:58 pm Hi folks, my 650 ajay twin doesn’t want to accelerate much above 50; I,ve given it choke to see if it’s weak but it wants to cut out, the plugs read ok, it’s running A 2 into 1 with a mega, puzzled, ????, any ideas, rgds Dave
Is the bike new to you?
Is this issue something new or has it always been like it?
What carb and main jet is fitted?
It may be helpful to know from what year is the engine and the exact model of 650.

Have you checked the basics......
That the main jet is clear and unobstructed
That the ignition timing is correct
That the auto A/R moves freely from the Retarded (at rest) position to the Advanced (engine running) position
That there is no fuel restriction 'twixt tank and carb
:?
Last edited by SPRIDDLER on Mon Aug 22, 2022 3:26 pm, edited 2 times in total.
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Joker_Bones
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Re: Running problems

Post by Joker_Bones »

When you say 'doesn’t want to accelerate much above 50'
Do mean... above 50 it accelerates slowly or above 50 it will not go any faster?
Has it always been like that? or is this a new thing?
Ajaydave
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Re: Running problems

Post by Ajaydave »

1960 mod 31, running 928 concentric, 400 main jet, cleaned carb, needle in middle position, float level correct , checked points, plugs n5c,
Ajaydave
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Re: Running problems

Post by Ajaydave »

Trundles up to 50/55,smooth as you like, if I accelerate after that it bogs, if I give it choke to see if it’s weak it almost cuts out,
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Joker_Bones
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Re: Running problems

Post by Joker_Bones »

is that at wide open throttle it bogs? up to 3/4 throttle you are on the needle jet, main after that.
400 main is the size for a monobloc, if I remember correct you come down about 25% for a concentric.
i have twin concentrics on a G12, I have 220 main jets instead of 280 for monoblocs.
Ajaydave
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Re: Running problems

Post by Ajaydave »

I,m trying to find the correct needle jet size, anyone know, ?, for 928 concentric,
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Joker_Bones
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Re: Running problems

Post by Joker_Bones »

Quote from 'Jetting a Concentric Carb' thread:
'I recently ordered a Concentric from Burlen Fuel Systems (Amal) for a '56 model 20. They told me that to convert from a Monobloc to Concentric you keep all the settings the same (ie pilot jet, needle jet, cutaway) except for the main jet which you reduce by 20%.'

That's the same as I remember it from when I spoke to Burlen on the same subject, except I said about 25% for the main jet. 220/280 = about 22%.
Ajaydave
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Re: Running problems

Post by Ajaydave »

Thanks folks, it’s got a badly worn 3 slide in, Amal say a 3.5 so I have one on order,
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robcurrie
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Re: Running problems

Post by robcurrie »

Could also be that it needs more ignition advance or maybe it has too much, mine bogs down if I over advance the timing.

Rob C
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