Is One Amal Enough?

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Howdy,
Still learning everything I can about my new 74 Commando. The original owner installed a one carb set up and used one of the original 32 mm Amals (I think they're called Mk-1) which was resleeved.

However, I just read in International Norton Association Tech Digest that, "A single Concentric will not flow enough fuel to run a Commando."

The bike runs great, but of course I have no frame of reference because I never had a Norton before. Just trying to understand if this is a good set up or should the carb be upgraded or put back to duals. Would the engine be negatively affected with this single Amal? Thanks,
Rich
 
Especially the British in the 70's would do everything to cut cost in their manufacturing process. If the Norton Commando engine would run properly on 1 Amal carburator, then the Norton factory woul have never installed 2 of them....
It would have saved them material cost and the carbs didn't have to be synchronized, etc.
So even without having the knowledge of the flow characteristics, I think it is still save to state that the engine will run a lot better on 2 amal carburators then just 1.
 
Respectfully, I have to disagree with deswartj, The 4 stroke parrallel twin motorcycle of any description runs very well on a single carburettor and most of the early iterations of these bikes were equiped with single carburettors as standard, the fittment of twin carburettors gave a slight advantage for sporting riding and racing, but was mainly a marketing feature of many of the sports models, generally speaking the same bike was available with a single carburettor under a different model designation.

Bikes with single carburettors benefit from better gas-mileage and often better low speed torque due to a more constant flow of the mixture, plus there is the benefit of not having to regularly synchronise two instruments.

By the time the Commando came along the market expected twin carbs on a sports bike in the same way that modern day purchasers expect racing bodywork, upside down forks, fuel injection etc.etc.

It would also seem that rmak has answered his own question by stating that "the bike runs great". If there were problems associated with a lack of fuel delivery from a single float bowl they would manifest themselves quickly in operation.

I personally like twin carbs on my Commando, they look good and they do the job, but if I bought a bike that had only one carb and performed well I would leave it alone.
 
"A single Concentric will not flow enough fuel to run a Commando."

Meaning: a single needle & seat assembly hasn't enough area? The area is larger than any main jet you would care to use. If that's not enough, use the alcohol No 622/054 float bowl (.l56”, 4.0mm) seating with the No 622/279 float needle; end of "problem".

Meaning: a single 32mm doesn't flow enough air? Vincent 1,000 Black Shadow: single 26mm carb.

The statement simply has no basis.
 
So you can run a 500cc single with a 26mm Amal on one cylinder of a Vincent but a single 500cc B-50 takes at least a 30mm to be happy.
Interesting the Norton rotary ended up using the 26MM before it made real power.
And the 26mm used on the Trident having a 250cc cyl size.
Also the twin 500 Triumphs used the 26 in a single carb form and seemed to suffer from it, some say a 28 works better.
 
Single Amal carb kits have been fitted to Commandos for years, which says it all, really.




norbsa48503 said:
And the 26mm used on the Trident having a 250cc cyl size.

Although Trident and BSA R3 carbs were "626" type, they were in fact a special 27mm size.
 
Hi there , I try too also to understand, why the advise size for single mikuni is 34/36 mm and for amal it runs well (normally) with a single 32mm ???
 
It's all down to flow rates through the carb. A small carb at low rpm will see a higher flow rate than a larger carb. At high revs the small carb may restrict flow. I reckon the major problem with the single carb on a Commando is the tight changes of direction within the manifold.
My mate runs a Mk2a on a single 32 no trouble other than the odd times he gives it a bit of stick it just runs out of puff earlier.
Cash
 
rmak said:
Howdy,
Still learning everything I can about my new 74 Commando. The original owner installed a one carb set up and used one of the original 32 mm Amals (I think they're called Mk-1) which was resleeved.

However, I just read in International Norton Association Tech Digest that, "A single Concentric will not flow enough fuel to run a Commando."

The bike runs great, but of course I have no frame of reference because I never had a Norton before. Just trying to understand if this is a good set up or should the carb be upgraded or put back to duals. Would the engine be negatively affected with this single Amal? Thanks,
Rich

sounds like the previous owner was "Amal-retentive"

sorry couldn't help it... :roll:
 
Cash, I think what you refer to as "flow rate" may be mixed up with air speed. At least in my mind.
Flow rate having more to do with the over all condition from air cleaner through to combustion chamber. So lets say you had a big air tank of a known size and you pumped it to a known pressure and than allowed this known amount of air to escape though a carb mounted on a head with it's valve open and timed the event that would be flow rate. How fast can a known amount of air pass through a given set up. This is how you would develop flat bottoms in the intake and exhaust ports but not how you might jet a carb.

So a smaller carb running the same volume of air builds more airspeed and therefore creates a stronger signal strength across the idle jet, needle jet, and main jet.
Suffice to say bigger is not better and Mr. Panic knows more than 12 of us about the direct applications that effect our bikes, His Amal booklet gets into some finner points. I went looking for it last night and I think I must have loaned it out.
 
Just adding my $.02

There is no reason why a single carburetor can't work as well as twins on a vertical twin with a 360 degree firing sequence.
The only advantage to two carburetors is a little better air flow at higher RPMs, when both are pretty much constantly flowing.
 
Might be different if both cylinders where on intake at the same time but they're not. 32mm is a bit smaller than the recommended 36 for an 850 though. If you're not a hot rodder and it runs good why worry about it?
 
Assuming that the Y adapter does not change the flow path (which is not true), a single carb provides slightly more flow to a single cylinder of a 360° twin than an individual carb, in both volume and pressure loss.

The difference is that an IR (individual runner) can only use what passes through the single throat, or is present in the volume between the slide and the intake valve.
A plenum (common carburetor, such as TR6, Thunderbolt) also has the larger adapter volume (small difference, but generally a plus), and also the volume from the adapter to the closed valve on the opposite cylinder. No, the gas doesn't flow backwards, but its volume reduces the pumping loss of the active cylinder.

In theory, a well-designed single carb can produce power greater than IR because the plenum volume is literally unlimited (V8 tunnel ram with a single 4 bbl. outperforms 2 × 4 bbl. in a conventional manifold), whereas an IR for all practical purposes cannot use a 50mm carb to feed a 34mm port. However, response, idle quality etc. are all fairly bad.

The big advantage to IR (and still current practice in EFI automotive engines preferring ITB instead of TB + plenum) is that the active cylinder is completely unaffected by the characteristics of the other cylinders, which means that high overlap, exhaust reversion, inappropriate wave returns, etc. are blocked from interfering with the active cylinder.
It's also far easier to tune to a specific length, since the math is simple (not Helmholtz flask resonance) and the results are more "pure" in that there are no super-imposed waves from other sources.

In summary:
Dual carbs do not "give better peak power".
Single carbs do not "give more torque'.
These are both propter hoc errors.
 
Thanks to everyone for all the input and expertise. It's obvious that even though I try to do as much as I can on my bikes, I'll be running to catch up with this crowd knowledge-wise.

The p/o did say he performed some porting on the manifold and carb work as a performance upgrade. He's on vacation, but I'll pick his brain when he gets back and post with anything relevant. I was just too excited when I was picking up the bike to focus on all the details.

I finally got plates and took my first long ride. I was very impressed and pleased with just about everything on the bike. My old Bonneville had so much more vibration. Even with one old carb it shot up to 70mph in what seemed like a heartbeat with plenty of throttle left. Great torque. The only thing that was not "unapproachable" was the braking, but I'm sure you all are aware of that already.

After a quick learning curve, started to enjoy the reversed shifting. Always tempting to power shift, but holding back. It can't be good on a 35 year old tranny.
 
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