Monobloc jets

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Pharisee
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Monobloc jets

Post by Pharisee »

Another question... sorry. :(

I've stripped and cleaned the Monobloc carburettor, not that it needed much as it was very clean. The "Supplementary Instructions" leaflet for the '56 G80CS quotes a 440 main jet "for use with open exhaust pipe". My bike has the usual silencer and the carb has a 380 main jet. All other parts as per the book i.e. 106 needle jet, 30 pilot jet, No. 3 slide, needle in centre notch.

Is a 380 main jet about right for use with modern "super unleaded" fuel and a silencer?

On my Triumphs, I find they run better with a 10% increase in main jet size with unleaded, ethanol loaded fuel.
Any comments appreciated
Cheers
John


EDIT... I suppose I should have said "Any relevant comments appreciated" :D
I'm from the Fens.... Gimme six.
Trefor
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Re: Monobloc jets

Post by Trefor »

I have a 1952 G80s fitted with a monobloc which I bought from Amal jetted for the 1955 monobloc 500cc. I fiddled around with the main jet when I fitted the carb about 10 years ago and (I just checked) the main I have is 250 (standard is listed as 260 in the manuals). I have a K&N pancake type filter, the plug is NKG B5E2 and I run it on 91 unleaded with squirt valve seat preserver. The muffler is not an original type pattern internally, probably less baffled. It has run with these settings for as years, the plug looks brown. I have a Triumph 3TA which runs one main jet size up (with a K&N) and one grade hotter plug running on the same fuel.
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clanger9
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Re: Monobloc jets

Post by clanger9 »

One solution is to buy a range of jets and try them. Modern fuel is different, your bike is different, etc.
When a Monobloc is set up right, the jet only affects full throttle. Ride it flat out, then back off the throttle slightly. If it has more power just off full throttle, go up one jet size.
You want the smallest jet that gives maximum power at full throttle. You should be able to home in on the right jet fairly quickly...
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1993 Ducati 750SS
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