Carb tuning

Information relating to the Matchless G12 or AJS Model 31 650cc twin
McBob
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Posts: 151
Joined: Sun Sep 19, 2010 10:17 am
Location: AUCKLAND NEW ZEALAND

Re: Carb tuning

Post by McBob »

That article is well worth a read for anyone that hasn't. I bought the carb locally and put it aside for years till the rebuild required it, on fitting realised the rotters had flogged me the wrong size. Decided to open up the manifold to match the carb using a die grinder etc, as far as I can tell there is a smooth path for the fuel air mix, well worth checking because as noted in the pdf even a hairy gasket will cause problems!

When purchased the bike had a knackered concentric fitted which was used for 1000 odd miles after rebuilding the engine at which point everything else on the machine has been tackled. I've dug the old carb out, it didn't have this problem so will hopefully be useful for fault finding.

Yep same in all gears, like the idea of a tacho maybe able to use this with the bike in neutral to see what happens as well
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clanger9
Posts: 1165
Joined: Sat Jun 20, 2015 9:38 am
Location: Chester, UK

Re: Carb tuning

Post by clanger9 »

Good, that answers the manifold question then! Next step: try following the Amal "tuning wheel" guide:
https://amalcarb.co.uk/downloadfiles/am ... _guide.pdf

That'll take you through the right steps to get your Monobloc set up. There's no value in referring to the original carb specs, as your carb is a different size. If it dies on full throttle, it does sound to me like the main jet is too big...

PS: I assume you've confirmed that the float height is correct? That should be mentioned as "Step 0" in the guide. An Amal is untunable if the fuel level isn't spot on...
1989 Moto Morini Dart 350
1993 Ducati 750SS
McBob
Member
Posts: 151
Joined: Sun Sep 19, 2010 10:17 am
Location: AUCKLAND NEW ZEALAND

Re: Carb tuning

Post by McBob »

It was the air filter, thought I could just retune, but not so with the one fitted (it has foam as the filter material). Once removed the bike goes like a skinny hogget.

What works as filter or do folks not use one, just a bellmouth and wire gauze?

Will defiantly be following the tuning guide, float level is correct, now just need the time to do the rest.

Thanks for all the suggestions :beer:
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clanger9
Posts: 1165
Joined: Sat Jun 20, 2015 9:38 am
Location: Chester, UK

Re: Carb tuning

Post by clanger9 »

Glad to hear you got it sorted!
I just use a gauze bellmouth. Works great (and looks the part too).
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1989 Moto Morini Dart 350
1993 Ducati 750SS
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