Leaking Carburettor

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Leaking Carburettor

Postby Tigermoon » Mon Apr 24, 2017 1:07 pm

I would much appreciate some advice with regards the carburettor which leaks fuel as soon as the tap is turned on. The leak appears to be coming from side cover, although it could also be from the main jet cover at the base. Are these symptoms likely to be due to something amiss inside the carburettor or is it probable that the gaskets need replacing? If it is possibly the latter, is a kit of gaskets available to do the job?
Many thanks.
Peter

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Re: Leaking Carburettor

Postby Mick D » Mon Apr 24, 2017 1:23 pm

Hi

Dry everything completely, get some good light on the carb and turn on the fuel and observe. As soon as you see fuel turn off the tap. You should be able to positively identify where the source of the leak is - if not repeat the process until you do.

Float chamber cover plate - replace the gasket after having cleaned and inspected the mating surfaces for defects.

Main jet holder - strip, clean, examine, re-assemble with new gasket, From memory the jet cover seals with a taper so no gasket.

The other 'culprit' is a punctured float or sticking float needle valve - both of these faults will manifest as fuel coming out of the tickler - take the float out and shake it, (you will hear fuel inside it if it is leaking), replace the float needle valve if the float is not punctured.

Replacement gasket sets, and all parts are readily available - all you have to do is diagnose what you need.

Regards Mick

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Re: Leaking Carburettor

Postby Rob Harknett » Mon Apr 24, 2017 2:16 pm

Other problem is mono block float chamber covers. They can bow. I have also come across two types of these covers. The script on them should read horizontal when fitted. If its at an angle it's the wrong cover. To find the correct cover you need to compare where the 3 screw's are positioned on the cover. One with screws in a different position, compared to the script, when fitted, the scrip will read horizontal. Perhaps there are different covers for single, twin, lightweight or year, model. I have never checked this out.

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Re: Leaking Carburettor

Postby Janet » Mon Apr 24, 2017 2:33 pm

The writing on Vesta's float chamber cover isn't level. It fits. It doesn't leak. Apart from spoiling the perfect image of my bike, where's the problem?
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Re: Leaking Carburettor

Postby clanger9 » Mon Apr 24, 2017 5:29 pm

Janet wrote:Apart from spoiling the perfect image of my bike, where's the problem?


None. Apart from if you ever want to check the float level, the little bump on the cover that tells you where the fuel level is supposed to be will be in slightly the wrong place.

Some of us fret about such things ;)
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Leaking Carburettor

Postby Janet » Tue Apr 25, 2017 11:20 am

See, there's something I never knew. I wasn't aware that there was a bump on the cover. If I'd seen it I wouldn't have known what it was for. Having found it lurking in the shadow under the letter M and then measuring from it vertically to the top of the cover and then from the bump to what should have been the top, my measuring equipment couldn't detect any difference. Since I'd have to replace it with something transparent anyway to see the fuel level and I definitely wouldn't have the skill to adjust the float height to a thou or two, I'll leave it as is. With my level of ineptitude, 'near enough' has to be 'good enough'.

I had a look at my G80 carb that's currently on Glyn's M18 and the cover on that is angled, too.
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Re: Leaking Carburettor

Postby Tigermoon » Tue Apr 25, 2017 11:35 am

Many thanks for the ideas and information. All very prompt and a great help.

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Re: Leaking Carburettor

Postby Tigermoon » Sat Apr 29, 2017 6:21 pm

I have now had chance to take on board the suggestions and strip the carburettor down. I have replaced the gaskets, the needle seating and needle, the filter gauze and the banjo bolt (previous one a bodge). There was evidence of quite a lot of red gasket cement around the filter gauze. I think I have now solved the original leak(s) but fuel now pours out of the tickler (Dave mentioned this in a previous reply. However having stripped this part down a second time I couldn't find any evidence of a punctured float. Do I need to put the old plastic needle back instead of the brass one that came with the new needle seat, or could it be something else?
Thanks again.

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Re: Leaking Carburettor

Postby 1608 » Sat Apr 29, 2017 11:14 pm

You should replace the new needle and seat together. The old nylon needle is probably worn. When replacing the float make sure it is not up side down ( it is possible to fit it the wrong way up ). Ensure that the small brass spacer tube is fitted to the float spindle which prevents the float chafing and sticking on the inside of the cover.

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Re: Leaking Carburettor

Postby shandi » Fri Nov 30, 2018 5:08 pm

Any update on this topic? Did it get fixed?

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