Are 47 year old carbs worth rebuilding?

I replaced my perfectly running original Amals with a set of Premiers last year. The originals had been sleeved by the previous owner before I bought the bike in '06. So...if they were working fine, why did I buy new carbs? I don't know, I think it was 'because they were there!' Since I had no complaints about the original carbs, the new carbs didn't bring anything to the mix other than the improvement of the idle jet - which made no difference that I could tell in actual operation. Both the old and new carbs could idle smoothly as allow as 5-600 RPM though I set the idle at 1000RPM. Stay-up floats? Well, certainly easier to adjust but it's not like you have to adjust floats regularly - or, once set, ever again. FWIW, I have a long-standing prejudice against foam floats caused by the Rochester Quadrajet's famous "Stay Down" foam floats. OK, I admit they didn't call them "Stay Down" floats but that's what they did! I'm sure (well, I HOPE) current foam float material will actually float for the life of the carburetor.

One thing I did have to address with the original Amals when I bought the bike is that all carb mating surfaces were badly warped due, IMO, to over tightening . It wasn't difficult to correct with some 120/320/400 wet/dry paper and a sheet of 1/4" thick glass but MAYBE the warpage was accentuated by the sleeving, if boring out the carb body is part of the process - making the body walls thinner.

So I bought new carbs that I didn't need just because I wanted to! However, if I had a pair of clapped out Amals and I had to decide between restoring them or buying a set of Premiers, I would definitely purchase the Premiers as opposed to sleeving/restoring the old ones. The Premiers are externally identical so there is no issue that they look "wrong" though admittedly, the color of the metal is slightly different. Whether that's due to age or the metal itself, I don't know.

Fast Eddie said:

My issue with sleeving is that it doesn’t replace the jets, needles, floats, etc, etc and, worst of all, you’ve still got the shite original pilot bush set up.

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The parts for a "revival" (slides, seals/gaskets, needle, needle jet) probably come to somewhere around $50 USD for each carb, add Stay-up floats brings each carb up to about $65 USD. Some of you may be able to do better, some not on pricing; not worth arguing about here.

As Fast Eddie points out the effort still leaves you with the idle circuit fuel bushing of which much is written about with details and suggestions for how to expose the inner port and starboard sides of the idle circuit with drilling, tapping and plugging the newly created access points. Ultrasonic cleaning, sinking the carb bodies in a highly toxic mixture of caustic agent(s). Keep in mind that the idle fuel passage bore is so small that even an off-color thought can seal it.

If your carb bodies aren't warped from being over torqued and the slide bores aren't too badly galled you may win the day. Do keep in mind that your out of pocket costs are now flirting with the $100 mark for each carb, and they are still on your bench.

So, Norcoastal, do you feel lucky?

My numbers feel a bit high, but, still, can be looked at as somewhere between a 50% and an 80% discount on a set of Premiers or standard Mk1 Concentrics, respectively or a Mikuni conversion.

Plus, purchasing new carburation will keep your luck intact for uses more akin to romance...

Best

Here's a good reason to go for Premieres.
This is the slide at about 20,000 miles.
I can't tell it from new.

I've never liked the idea of putting aluminium oxide bits thru the engine as happens with the old Amals, Monoblocs too.
Aluminium oxide is the grit material for sandpaper.
Why worry about K&Ns vs paper etc when you have unfiltered al oxide going thru the engine? Is this the cause of all the shallow vertical scores one sees on these cylinders , even with good air filters? Seems likely.
Mikunis fix this with their chrome slides, Premieres with the anodized al.

But to be fair - a sleeved slide would provide the same improvement.

worntorn said:

Here's a good reason to go for Premieres.
This is the slide at about 20,000 miles.
I can't tell it from new.

I've never liked the idea of putting aluminium oxide bits thru the engine as happens with the old Amals, Monoblocs too.
Aluminium oxide is the grit material for sandpaper.
Why worry about K&Ns vs paper etc when you have unfiltered al oxide going thru the engine? Is this the cause of all the shallow vertical scores one sees on these cylinders , even with good air filters? Seems likely.
Mikunis fix this with their chrome slides, Premieres with the anodized al.

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I was about to ask about monoblocks when I saw you mentioned them. Well here's a question right alongside of it anyway. Were the concentrics that replaced the monoblocks a real step-up? And here's a general comment on the whole issue: If you just want to go for reliability and performance in general when you go out for a ride, why not just buy a brand new Japanese bike -- or even a slightly used one? I'm sure you can get a good deal out there. And the answer is because there is something about making an icon from the distant past whole and like new again. Can't put my finger on it, but it effects me too. Now, of course, there's a whole range of options in between totally original and just having the old exterior with all brand new super bike running gear, motor and all (like the "Fords" and "Chevies" at NASCAR. you pick your poison, it's your hobby. But one has to understand what someone else is after as well as they have to understand it themselves. Still, I'm wondering about monoblocks vs concentrics.

you can always purchase new bodies!!!!

Concentrics were designed to fix Monobloc fueling issues in a leaned over state.
I have a Monobloc on a Super Rocket and it doesn't seem to suffer from that, so perhaps that is just the maker's unfounded claim.
The Monobloc and the Concentric are both made from the same " Pot metal".

Both have slide and body wear concerns over time.


As far as why not buy a new bike intstead,well many do just that.
I say why not replace problem areas on the old bikes with some updated and uprated components? Then you can really use it for long range travelling and get reliability from it. On the Commando, especially the later bikes, there's not that much that needs changing. The original character of the bike can be left fully intact, if so desired.
If you aren't faffing about with carbs and other worn out bits, you can go riding!


Glen

Joe Schlaberdowski said:

I was about to ask about monoblocks when I saw you mentioned them. Well here's a question right alongside of it anyway. Were the concentrics that replaced the monoblocks a real step-up? And here's a general comment on the whole issue: If you just want to go for reliability and performance in general when you go out for a ride, why not just buy a brand new Japanese bike -- or even a slightly used one? I'm sure you can get a good deal out there. And the answer is because there is something about making an icon from the distant past whole and like new again. Can't put my finger on it, but it effects me too. Now, of course, there's a whole range of options in between totally original and just having the old exterior with all brand new super bike running gear, motor and all (like the "Fords" and "Chevies" at NASCAR. you pick your poison, it's your hobby. But one has to understand what someone else is after as well as they have to understand it themselves. Still, I'm wondering about monoblocks vs concentrics.

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Most consider the monoblock to be a better carb than the concentric. The concentric was ‘good enough’ though, whilst much cheaper, which is what BSA, Triumph, et al were primarily interested in.

TheFrib said:

you can always purchase new bodies!!!!

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I asked my doctor about that and he said no. Now, on the other hand, in the trucking business (I was an owner operator hauling all that steel, that once was, out of Pittsburgh) we had a saying, "Old truckers never die. They just get a new .......... ".

Joe Schlaberdowski said:

I asked my doctor about that and he said no. Now, on the other hand, in the trucking business (I was an owner operator hauling all that steel, that once was, out of Pittsburgh) we had a saying, "Old truckers never die. They just get a new .......... ".

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Eagle glider kit?

MexicoMike said:

But to be fair - a sleeved slide would provide the same improvement.

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My sleeved slides are better than the original Amal slide for wear, but still show considerable wear. That metal has gone thru the engine.
The Premieres, with a greater mileage, do not show any visible wear to the slide and just a miniscule amount of wear on the body, just a bit of polish.

Glen

Norcoastal said:

I have some old 932’s that I was going to rebuild for my 850. Should I be just buying new carbs?

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Here's my advice: Use what you have and decide after the bike is running. Needle jets are cheap. Clean the carbs the best you can, and start with default jets and adjustments.

It's highly unlikely that worn carbs will prevent you from starting and running. Once you're on the road if you find yourself twiddling idle settings all the time because your slides are worn, then you can decide on new or sleeved.

Good info Glen! Makes my happier that I bought the Premiers I didn't "need."

concours said:

Eagle glider kit?

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Not quite. Now there are only so many truck types so I'm sure you'll stumble on it pretty soon. Here's a clue. It's just a joke, a play on the name. But what was a surprise was the answer about monoblocks vs concentrics. I'd heard something of the sort, but wasn't sure. I'd like to read more about it if anyone knows a source

Wanted to add this question. If the old monoblocks were better than the concentrics that replaced them, was that a design issue or an issue of the material from which they were made or just the economics of manufacturing? And if it was a design issue, was that design deficiency corrected in the newest latest concentrics? And if not, just better material for the most part, maybe I should rebuild a pair of monoblocks? Now, I'm joking about that -- but only halfway joking.

Fast Eddie said:

Most consider the monoblock to be a better carb than the concentric. The concentric was ‘good enough’ though, whilst much cheaper, which is what BSA, Triumph, et al were primarily interested in.

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I was hoping someone could tell me why your statement might be true about the monoblock being a better carb.

From Wikipedia
The Concentric design with non-protruding float chamber was both slimmer and shorter, enabling easier mounting to two-port cylinder heads. Left or right sided carburettors could be produced from basically the same die-casting, and the design was compatible with up to 40 degrees of downdraught.

Joe Schlaberdowski said:

And if it was a design issue, was that design deficiency corrected in the newest latest concentrics? And if not, just better material for the most part.

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As has been mentioned already, stay up floats and adjustable floats are two design improvements. The main design improvement is replacing the old pilot jet arrangement, from one that is not removable or cleanable, to one that is.

Biggest material change is the use of an anodised slide, which as already mentioned, reduces slide and body wear considerably.

Fast Eddie said:

As has been mentioned already, stay up floats and adjustable floats are two design improvements. The main design improvement is replacing the old pilot jet arrangement, from one that is not removable or cleanable, to one that is.

Biggest material change is the use of an anodised slide, which as already mentioned, reduces slide and body wear considerably.

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Having said that Nigel, you can fit Stay-Up Floats with viton needle and anodised aluminium slides to the old carbs - will definitely improve things.
I did that to my '64 376 monoblocs with very good results.
I definitely agree with replacing needle and needle jet due to them being wear-prone

Fast Eddie said:

Better materials (like brass slides IIRC). And the throat / bore is a ‘smoothbore’ at WOT.

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My 376 monoblocs (1964) came to me with those old pot-metal slides.
I suspect, but don't really know, that brass slides were an owner retrofit.