Carburettor choices

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
56G80S
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Carburettor choices

Post by 56G80S »

I've read lots of posts following a search on this. I'm currently still using the Concentric 930 that was on the Matchless when I bought it in the mid 1970's.

No, it's not a show machine I ride it as much as possible and it has been struggling to get an appropriate idle for a long time. I've dismantled, cleaned, new "repair" kit worked on the top of the float bowl to minimise bowing and the same with the mating to the head.

Once stationary it slowly increases idle speed until it's racing and I crash into 1st gear. This all came to a head after my trailer towing exploits on the way to the IJR when very tired and slightly dehydrated I took a wrong turn and rode through instead of round Rotterdam.

But reading the horror stories about new 389/1 monoblocs being poor, suggestions that the Concentrics are cr*p (although in the early days this one was OK) and better off getting an old carb reworked by someone like Martin Bratby I'm at a loss.

It's a significant outlay to only find it's not an improvement. And lately I seem to have been getting the riding / fixing balance the wrong way round. More of that to come to replace the broken headlight, ammeter and indicators following the trailer incident.

I know this has been chewed over before but any helpful comments would be really welcome.

Johnny B
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Duncan
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Re: Carburettor choices

Post by Duncan »

Johnny

In my experience I have tried to improve worn carburettor and then fitted new monoblocs to both twins and singles and have found they have transformed the bikes in the way they tick over and pickup, I would always advised striping them and cleaning them before fixing however. I have not had a carburettor refurbished so can not comment on that approach
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Re: Carburettor choices

Post by Digby »

I don't know whether this is of any help but I have an 16MS with a pre-monobloc 76 Amal and after a run I would always have an extremely fast tick over even though the slide was fully down. I decided the problem was probably cause by wear allowing air to leak around the throttle slide. I had it re-sleaved and it now works perfectly.
56G80S
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Re: Carburettor choices

Post by 56G80S »

Thanks for the comments. I've also had a chat with others and time to take the plunge - If I can bring myself to spend the money!

Johnny B
56G80S
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Re: Carburettor choices

Post by 56G80S »

Revisiting m,y previous thread.

Still not plucked up the courage to buy a new carb from the Spares Scheme. Duncan's suggestion to strip and clean and before fitting is a bit off putting.

I have less time now than ever before and just want to have the Matchless idle reasonably rather than either dying or having a gradually increasing idle speed in traffic to the point where engaging first is a tremendous "clunk" job despite the clutch disengaging well.

New, recent, voices on this welcome (and yes, I scrolled through the Search output first).

Johnny B
leswaller
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Re: Carburettor choices

Post by leswaller »

You almost certainly have a carb problem, probably due to wear and old age. If you haven't got the time or the courage to strip the carb, then get a new one from the spares dept, at least you will then know that the carb is not the problem. I know they cost a few bob but how much is your time and peace of mind worth?
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clanger9
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Re: Carburettor choices

Post by clanger9 »

Fast idle on an Amal is usually down to wear between the slide and bore. The clearance here is super-critical. The carb body tends to wear oval, so even buying a new slider doesn't always fix the problem.

Either spring for a new carb or get it re-sleeved (e.g. by Martin Bratby). Either way should resolve the problem.
I don't think there's much difference between a re-sleeved carb and a new one (provided it's done properly).
You could argue that Martyn's carbs are better, because he sleeves the slider with a new brass bush (instead of Amal's alloy-on-alloy design), which sorts out clearance and wear issues once and for all.

My bike went from having an annoyingly unpredictable and fast idle to *perfect* tickover after I got the carb refurbed.
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56G80S
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Re: Carburettor choices

Post by 56G80S »

Thanks for the input.

I've stripped and rebuilt carbs before and did fit e refurb kit into the Concentric but it is clearly worn out.

So it's the Spares Scheme and a new carb and for the first time since I bought the Matchless it'll have the "right" carb - 389/1 - 1 5/32"

I wonder if I'll be able to fit the pancake filter that I bought and took off the Concentric because it made it run too rich and sootied the plug? I'll certainly need to re-work the cable and the choke; never used the latter on the Matchless.

Thanks chaps, you've given me the support I needed.

Johnny B
56G80S
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Re: Carburettor choices

Post by 56G80S »

Bought the carb, from Jampot Spares, but haven't fitted it yet.

Sure I recall that someone advised stripping and checking all before fitting.

Not inclined to do this, partly because of time and also because I forgot to order a spare gasket set for it.

Any thoughts?

Johnny B
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Duncan
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Re: Carburettor choices

Post by Duncan »

Completely your choice, but I can not understand the reasoning, the gaskets will be fine/reusable and it will take about 20 minutes to strip, clean and reassemble :?:

Personally and from others experience I would strip and clean.
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