Jet Size

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Jet Size

Postby cornishjock » Mon Aug 21, 2017 8:59 am

After 18 months of ownership I have at last enjoyed one half decent run on my Model 30. Long story short the bike is a bit of an oily rag. I have recently found that the new NGK B7es were duds and changed to NGK B6es. Bike running a little better but with the throttle just open stuttering and plugs still sooty. The carb is stamped 376/40 (Tiger 110 or Tr6) should be 376/058 (so will never win a concours prize). Main jet was 280, should be 260 according to the blurb. I fitted a 230 I found in a draw and the bike is running much better but still a bit of soot on the plugs. So is the much smaller jet size due to modern fuel or am I missing something else? Are others running smaller jet sizes?

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Re: Jet Size

Postby Rob Harknett » Mon Aug 21, 2017 9:27 am

There could be other reasons why the plug is running a bit sooty carb parts or settings ( needle and slide cut a way ) burning oil ( worn rings, guides, incorrect oil control screw setting in head ) Is this really a problem? When the bike was new it got a lot more soot that it gets now. With 50's / 60's fuel you had to de coke the engine almost every 2, 000 miles. Also scrape carbon off a plug with a penknife. You cannot even clean a plug now or it will further damage the now unglazed central electrode. Also had to clean sooty oily blocked exhaust silencers.

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Re: Jet Size

Postby cornishjock » Mon Aug 21, 2017 10:36 am

Hi Rob. No not really a problem, more is this familiar nowadays. Fuel vapours differently etc. I have not looked at the bores as the bike is not smoking at any point so rather not disturb. My son has followed it for miles observing this for me.

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Re: Jet Size

Postby Mick D » Mon Aug 21, 2017 11:29 am

Hi

If the problem is with the throttle just open changing the main jet will have little to no effect - the main jet regulates the fuel when the throttle is close to or fully open.

If it stutters at a very early position, (close to closed), try screwing the pilot air adjusting screw out a little, (try a quarter of a turn), and see if that improves things.

If the problem appears later try dropping the needle down a notch.

This is a good guide to carb set up:

http://amalcarb.co.uk/downloadfiles/ama ... d_Tips.pdf

Regards Mick

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Re: Jet Size

Postby cornishjock » Tue Aug 22, 2017 8:08 pm

Hi Mick
I was thinking about dropping the needle one and still may just for the fun of it.
The dropping of the main jet alone cured the woes strangly which is possibly due to the plug being cleaner while out of town. Pick up is not bad now either. At the end of the day the it is now only 30 down on the recomended size. I was more wondering if this is something others are experiencing as the size is goodness knows how old?
Thank you for the download link. This is the book I can't find following a house move last year so a good help.
I was through Aberdeenshire yesterday on two wheels, Cairn o Mount was stunning.

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Re: Jet Size

Postby clive » Mon Aug 28, 2017 9:48 pm

I have been having problems with my plugs sooting up in town riding. (G11CS 58 with air filter fitted). Tried first dropping the main jet down from the book 280 as far as 240 but it just affected top end performance. Then I reduced the pilot jet from 30 to 25. Still sooting up on when throttle just up from tickover. Then it started misfiring when at 30 in third gear ( and similar revs in other gears) this could be ridden round but was annoying. Then I dropped the needle from slot 3 to slot 2. Misfiring sorted.
clive
if it ain't broke don't fix

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