Forum

[Presto]

Thanks Thack, I understand your point. But if a carb 'works as it should' then it wont have flat spots, poor response, etc, etc. It will work as it should - and if it does then its a 'good carb'. If not then its not a good carb. An AMAL Mk 1 that works as it should is as 'good carb'. A Mk 2 Amal that doesn't work as it should on a particular bike is not a good carb. What I'm saying is that carburation is much more a matter of setting the instrument up correctly than theoretical aspects of design. That's what I meant by saying that standard carbs on Hondas, Suzukis, etc were 'good' carbs, even though the designs of these instruments were very different one from another. They worked exactly as they were intended to - and so were 'good carbs'.

[samj]

The Kei-Hins on my 1977 Honda 550/4,closely followed by any of a series of Amal Monoblocs.

[Thack]

Presto, thanks for your comments. I agree with you in one sense, but not another. A carb that is correctly set up is arguably a good carb, but a lot of analysis was done, and documented, on Amals back in the 70s, and it was clear that it is rarely possible to set them up correctly. That is, the compensation system of the Amal at mid-range throttle openings is poor, so if the mixture is correct at one engine speed, it is automatically wrong at another (at any given throttle opening, I mean). Also, the air feed to the mixing chamber just above the main jet is such that the fuel is too "bubbly" at the exit to the venturi, so the mixture varies from one stroke to another. Another problem with the Amal is that if you set the steady state mixture correctly (as near as you can) it weakens out when you ask for sudden acceleration. I would say that you may well be able to set the carburettor up such that it is perfectly drivable over the entire range, but at various points it will not be running the ideal mixture. Thus, although it may seem to work well enough, a different carb could probably do better: better acceleration, better fuel economy. Having said that, they are popular with racers of old Brit bikes because, when wide open, they are pretty much as good as any other carb. The niceties of part-throttle fuel economy and a smooth idle are of less interest to them!

[Beezabryan]

No matter what form of fuelling is applied to a vehicle by the designer and manufacturer there will always be some as will want to "improve" it.

[Mark M]

Thack, yes I know I can go to a richer slide but my attempts to iron out the flat spot have been partly successful, mainly by replacing the new Amal parts (that I fitted during the rebuild) with the original old ones and it's much better. Still not quite there though!
REgards, Mark

[simon]

The majority of carbs on bikes are copies or versions of an AMALS or Brown and Barlows and the other brands that amalgamated into AMAL. Personally I think Delortos are the best of them. Hard chromed slides and bodies of sufficient weight so as not to distort when they are clamped onto a machine. The later AMALs are so flimsy that they constantly go out of shape particularly if they are flange mounted. Their major problem was that the bike companies they sold to wanted them to be cheaper rather than better and so the metallurgy took a hit in the name of price. I've spent literally months tuning both the PHF Delortos on the Ducati and the mismatched 389 AMAL on the 350 Bitsa in order to get them to work properly and I've had pretty good success. I have come to the conclusion that the main issue is that the modern unleaded fuel is a far more finicky fluid than the old leaded fuel which was far more forgiving of both richness and leanness but particularly richness. For this reason when tuning the Ducati I turned off the throttle pumps and so I could work on the three complementary circuits idle, midrange and main jet without jets of juice shooting down the throat and confusing things. I may well turn them back on one day but not for now. I believe that the problems with plug fouling are exacerbated by the rather crude advance curve of the early ignitions which were also no problem with the more biddable leaded juice. My old Duc only has two advance steps and the bullet has bob weights but of course no vacuum override. So in a long winded way I tend to agree that a good carb is one that works. I've had AMAL 29's, 276's, 376's and 389's working to my total satisfaction but the Delortos seem to adjust more reliably and are built to last.

[Presto]

Thanks Thack - good case well made! I do agree of course that by design some carbs are clearly better than others. But in the final analysis it is the way the carb is set and used that is most important. Just one point, the sudden weaking of fuel mixture (due to the different specific gravities of air and petrol) when the throttle is 'suddenly opened' is as much a fault of riding as it is of carburation - opening the throttle suddenly is poor technique. Opening the throttle progressively not only eliminates that problem but will give far better engine response in any case. (Racing is a different situation, where low speed running is sacrificed for full throttle performance.)

[JTL]

Hi all Does anybody have experience with the Mikuni flat slide carburettor? Our host sell the round slide Mikuni (VM 32), but some of our american Bullet fans praise the flat slide (TM 32) instead. As I understand it because it doesn't have all these little "blind spots" when operating the throttle... Would love to hear an english side of the story.. Jacob

[Mark M]

I can't see that the improvement would be worth it on a simple road going Bullet. These carbs work best at high revs and that's not really a Bullet speciality! And if you want a flat slide carb why not get a British made Gardener?!
REgards, Mark

[JTL]

Hi Mark M I did just a brief search on the Gardner carb. It seems to be a racing carb; the TM 32 should be a road carb and not just a race carb. I believe it's the flat slide technology that makes them more even running at all throttle positions. For me it is not important where the carb comes from. I'm looking for a simple, functional and contemporary carb, that do not have "blind spots", but works smooth all the way when the throttle is opened. Right now I run both the stock 28 mm Micarb and an Amal MK2 30 mm. The 28 mm is simply not big enough, and the MK2 is giving me some backfiring problems at start. Only sometime in the future a Mikuni TM 28 may enter the picture Jacob