mystery miss

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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mystery miss

Postby Trefor » Tue Dec 08, 2020 3:07 am

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?

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Re: mystery miss

Postby Mick D » Tue Dec 08, 2020 8:14 am

Hi

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

Regards Mick

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Re: mystery miss

Postby ray.woolford » Tue Dec 08, 2020 3:00 pm

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.

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Re: mystery miss

Postby Triumph-Legend » Tue Dec 08, 2020 8:22 pm

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.

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Re: mystery miss

Postby Trefor » Wed Dec 09, 2020 11:05 pm

Thanks for the suggestions. I'll try them and see how I go.

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Re: mystery miss

Postby SPRIDDLER » Thu Dec 10, 2020 9:17 am

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.
I poke badgers with spoons.....from 2 metres.

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Re: mystery miss

Postby clive » Thu Dec 10, 2020 9:37 am

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

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Re: mystery miss

Postby Reynard24 » Thu Dec 10, 2020 1:20 pm

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.

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Re: mystery miss

Postby 56G80S » Thu Dec 10, 2020 2:27 pm

Always used the Champion N5.

Johnny B

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Re: mystery miss

Postby clive » Thu Dec 10, 2020 9:23 pm

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

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