Which carb ?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Krishna
Posts: 17
Joined: Tue Jun 24, 2014 6:25 pm
Location: FRANCE

Which carb ?

Post by Krishna »

Hello and bonjour, Can anyone tell me which monobloc I can put on a 1952 18S and what jets etc etc so I can be as specific as possible when I order from Amal? I have spent hours messing with the float and it still continues to flood when tickled although it does stop doing so sometimes when the bowl is 'tapped' with a heavy object. I suspect the R300 928 that's on it is getting tired and whilst it may be that that is the one I need as we are surrounded and blessed with such august company in these forums it seemed logical to ask.
Merci et a bientôt and best wishes from deepest Burgundy.
Krishna
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Which carb ?

Post by Groily »

Sounds as if the float needle is suspect, or the seat or even the float itself. Not really a reason to swap the whole carburettor I wouldn't have thought, as needles with viton tips, and floats, and (I think) the valve seats, can be had for a lot less than a replacement instrument?
One thing I have found, several times on several different machines after a week or so of non-use, is that needles can be sticky if using 95 octane fuel - but that they don't seem to suffer the same way on the 98 we have in France. Not sure what's so different in the blend - but just a thought.
Best wishes from la Manche!
SPRIDDLER
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Posts: 8542
Joined: Wed Sep 01, 2004 1:00 am
Location: WEST SUSSEX UK

Re: Which carb ?

Post by SPRIDDLER »

As it happens, last week I recommissioned my '54 G3LS which hadn't run for 2 years. I had just cleaned out the petrol tank, cleaned the mag points and slip ring and rebuilt the 375 Monobloc using a genuine AMAL 'Stayup' float kit comprising the StayUp float, Viton tipped needle and gaskets/fibre washers.
(To digress for a moment, I found that as mentioned recently on the Forum the fibre washers were made of very much harder material than the originals).

Anyhow I refitted the carb and turned on the petrol tap and petrol poured out of the tickler boss. I removed the float chamber side cover and looking carefully as I lifted the float I noticed that it was touching the lower end of the tickler pin before touching and closing off the needle valve. I removed the fibre washer(the old one had been in place for the past c25k miles) from under the petrol inlet housing on the top of the float chamber and having reassembled the carb without that fibre washer she started third kick and ran beautifully with no further flooding from the tickler. I checked the petrol level with a piece of plastic tube fitted over the Pilot jet and it was spot on.

My conclusion is that the new Viton tipped needle valve is slightly shorter than the old brass one (by the thickness of the fibre washer). I could take the Viton needle out and compare it with the length of the old one to prove the point but I won't bother just yet. If it ain't broke.............so forth.
Last edited by SPRIDDLER on Tue Jul 04, 2017 10:49 pm, edited 1 time in total.
'There is a tide in the affairs of men
Which taken at the flood............'
Krishna
Posts: 17
Joined: Tue Jun 24, 2014 6:25 pm
Location: FRANCE

Re: Which carb ?

Post by Krishna »

I agree having ripped it all apart yet again, the Viton one is slightly shorter. I am going to change the float and continue hitting, sorry tapping, the bowl with a hammer, sorry light metallic object. Basically it's just one of those stupid nuisances.
SPRIDDLER
Member
Posts: 8542
Joined: Wed Sep 01, 2004 1:00 am
Location: WEST SUSSEX UK

Re: Which carb ?

Post by SPRIDDLER »

It has just occurred to me that your R300 928 carb is an AMAL Concentric (with which I am not especially familiar) and I'm not sure that the principle still applies vis-à-vis the Viton needle being shorter than the brass ones fitted in a Monobloc because the needle in the Concentric has a groove into which the 'ears' of the float pivot bracket fits and is pushed downwards as the float rises. The needle in the Monobloc is not physically connected to the float and is just a loose fit in its housing.
However, if yours is flooding due to the shorter needle I can't see how tapping the carb body will seat it unless the float is being prevented from lifting by some obstruction. To shut off the fuel supply the Concentric needle works in the opposite direction (it's pushed downwards) to that of a Monobloc needle (which is lifted upwards).
Have a look at these detailed instructions and illustrations for the R300 928 Concentric:

http://amalcarb.co.uk/rebuilding-mark-1 ... arburetter


Finally, I do recall reading on the Forums about the need to fit or not to fit the fibre washer under the needle valve housing depending upon whether it's an older or a more recent model of Monobloc carb but not with regard to replacing the original brass needle with the updated (possibly shorter) Viton tipped alloy one.
'There is a tide in the affairs of men
Which taken at the flood............'
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