mystery miss

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Trefor
Member
Posts: 228
Joined: Fri Dec 31, 2010 10:02 am
Location: South Australia AUSTRALIA

mystery miss

Post by Trefor »

I have a '52 G80S fitted with a 389/1 monobloc. All was well until I had the magneto re-built. The bike starts and runs well, pulls strongly, but at a steady 50mph there is a from time to time a slight hesitation, like a half miss! The plug, with the re-built magneto has gone from dark brown to light grey (NKG B5ES), so I thought perhaps the mixture was a little lean with 250 main jet and went to the standard 260. This hasn't solved the problem perhaps made it more pronounced. I'm reluctant to fit a softer plug. Perhaps I'm barking up the wrong tree and looking in the wrong direction. Any suggestions?
Mick D
Posts: 2886
Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: mystery miss

Post by Mick D »

Hi

I'd be tempted to raise the needle one notch and see if things improve.

Regards Mick
ray.woolford
Member
Posts: 68
Joined: Wed Mar 09, 2011 3:53 pm
Location: WEST SUSSEX UK

Re: mystery miss

Post by ray.woolford »

If the carburation was all right before you had the mag done it will be the same now. Did you check the point gap when before you fitted the mag I made the mistake once of not checking the gap which was almost nil had the same problem. Good luck hope you solve your problem soon.
Triumph-Legend
Member
Posts: 476
Joined: Sat Oct 26, 2013 9:09 pm
Location: CAMBRIDGESHIRE UK

Re: mystery miss

Post by Triumph-Legend »

Hi, My 53 G80 fitted with new Monoblock and recon mag ran well up to around 50mph and them seemed to run out of puff. After a bit of experimentation it seemed to indicate that it was running weak at the top end. Fitted a larger main jet and now it pulls really well at all throttle openings. I put it down to modern fuels having a lower octain rating. Going up one or two sizes on the main jet may have little effect, I jetted the carb up a few sizes and worked from there.

I do agree with the comment regarding the checking of points and timing before making any other changes. If these are correct, then go for the carb jetting route. Main jets are not expensive and you may even be able to snaffle a couple from a friend to try before buying new.
Trefor
Member
Posts: 228
Joined: Fri Dec 31, 2010 10:02 am
Location: South Australia AUSTRALIA

Re: mystery miss

Post by Trefor »

Thanks for the suggestions. I'll try them and see how I go.
SPRIDDLER
Member
Posts: 8542
Joined: Wed Sep 01, 2004 1:00 am
Location: WEST SUSSEX UK

Re: mystery miss

Post by SPRIDDLER »

Notwithstanding the suggestions already made above and if you haven't already done so, the very first thing I would have done is to try a new plug, (not necessarily a softer one).

EDIT LATER.
Just noticed this:
Trefor wrote:.........from dark brown to light grey (NKG B5ES),......
In my 350 and 500 singles I've always used B7 or B8 ES in NGK or preferably Champion N5C (as Clive mentions below)
Last edited by SPRIDDLER on Thu Dec 10, 2020 10:03 am, edited 5 times in total.
'There is a tide in the affairs of men
Which taken at the flood............'
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: mystery miss

Post by clive »

SPRIDDLER wrote:Notwithstanding the suggestions already made above and if you haven't already done so, the first thing I would have done is to try a new plug, (not necessarily a softer one).
And forget NKG somehow our magneto bikes don't seem to get on with them. Try Champion N5C from a known reliable source such as Green Spark plug company.
clive
if it ain't broke don't fix
Reynard24
Member
Posts: 362
Joined: Mon Dec 16, 2013 2:27 pm
Location: CAMBRIDGESHIRE UK

Re: mystery miss

Post by Reynard24 »

clive wrote: And forget NKG somehow our magneto bikes don't seem to get on with them. Try Champion N5C from a known reliable source such as Green Spark plug company.
That's a rather sweeping statement Clive. I use NGK in all my bikes, both magneto and coil, and have had no issues whatsoever. I certainly wouldn't use a resistive plug or plug cap with a magneto but any genuinely sourced plug should be ok.
For my G80S I use a B7ES.
56G80S
Member
Posts: 3354
Joined: Wed Oct 12, 2005 10:45 pm
Location: N YORKS UK

Re: mystery miss

Post by 56G80S »

Always used the Champion N5.

Johnny B
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: mystery miss

Post by clive »

Reynard24 wrote:
clive wrote: And forget NKG somehow our magneto bikes don't seem to get on with them. Try Champion N5C from a known reliable source such as Green Spark plug company.
That's a rather sweeping statement Clive. I use NGK in all my bikes, both magneto and coil, and have had no issues whatsoever. I certainly wouldn't use a resistive plug or plug cap with a magneto but any genuinely sourced plug should be ok.
For my G80S I use a B7ES.
But this is my experience (and others who have been experiencing problems which go away when changing to Champion), sweeping or not, perhaps there are a lot of fake NGK around.
clive
if it ain't broke don't fix
Locked