Forum

[Mark M]

I'm not saying the Mono is better for competition! Give me strength! I've listed the reasons why a NEW Monobloc would, in my opinion, be better for Bandit's Bighead as a roadster. And the numerous road bikes it was used on didn't have a choice, the Monobloc ceased production when the Conc was introduced so that's not an endorsement either!

REgards, Mark

[Bullet Whisperer]

We used a Monobloc very successfully on our 350 racer for many years, until a larger TT carb came along, but this has recently been replaced by a 1 1/2" GP carb, as the latest engine is a Big Head, short stroked to a 350. The only downside with a Monobloc is they never made them any bigger than they did, but they went up to the equivalent of about 32mm. There was a MK1 Concentric on a Crusader racer we have, when I bought it, but I took it off and fitted the redundant Monobloc from our 350, which is still there to this day. For the Big Head, I would either use a Monobloc or a MK2 Concentric of suitable size. There is a Fury Big Head 'out there', wearing a 38mm MK2 concentric. Regards, Paul.

[Presto]

It’s OK Mark. I completely understand your reasoning. And respect your views. But it’s simply a difference of opinion in terms of the practical reality. I recognize that in theory the Monobloc is (or at least is supposed to be) a better carburetor, but I believe that in use the Concentric is at least as efficient, if not more so. I’d always opt for the latter – unless cosmetics was my priority.

[Mark M]

Presto, don't worry! I was trying to keep the conversation on Bandit's original theme, I'm not averse to a proper debate though! I've only done about 1,000 miles on the new Mk2 fitted with the new Premiers so far, a long term report will be a while off. Back in the day magazines complained that the Concentric was thirstier than the Monobloc, I seem to recall figures of up to 20% worse being quoted? Anyone else seen these sort of numbers? I suspect that the Premiers are a good deal better judging by the slide to body fit but when riding I still get the impression that I'm using more fuel than my Mk1 Interceptor fitted with re-sleeved Monoblocs. (It being difficult to judge accurately as neither bike has a trip meter.) Anyway, poor old Bandit asked for opinions, he's now got plenty to think about!

REgards, Mark

[Dennis C]

Just one example, I bought a Velocette Venom as the owner had had his fill of the bike it was fitted with a new concentric which was set up as recommended, bad starting, poor unreliable tick over, awful to ride due to very mixed performance.

I fitted a monobloc and the bike was transformed, easy starting (for a Venom), good idle and a pleasure to ride, and as Mark says much improved fuel consumption.

PS I still have the concentric if anyone wants to make a sensible offer.

[Presto]

That 20% heavier fuel consumption figure is interesting - inasmuch as the main jet size for a Monobloc should be reduced by 20% for a Concentric.

[Mark M]

I agree. Many owners of Concentric equipped bikes report much improved fuel consumption when the carbs are re-sleeved, Trident owners for instance, as a friend of mine discovered. I'd be interested to know if the new Premiers had the same effect. I will try and do a comparison between my Mk1 (389 & 689 Monoblocs) and my Mk2 (Premiers) when the weather improves in the new year. Normally I wouldn't be too bothered, a few MPG is a small price to pay for so much fun but both bikes have 2.5 gallon tanks and spirited riding can see them empty very fast!

REgards, Mark

[simon]

For me the answer is Monobloc. The bike is 59 and a monobloc is both an excellent carb and the right equipment. Concentrics are a flimsier carb but arguably a better design. My B44 has one which I interestingly got off a friend of mine who took it off his 56 Venom to replace it with a 389.

[Chris Tindal]

Interesting thread, let us know what you decide Bandit12. I have a new monoblock on my BH bullet, short bell mouth/gauze and 250 main jet , goes really well. There's something about the BH bullet, it would be nice to know how many survive.

[Adrian]

Someone with more knowledge than me will know how many Big Head roadster Bullets were built in the first place between 1959 and 1961, but fewer than 200 Furies were produced, apart from another batch of 20 badged as Indian Westerners. Since our hosts produced their own Big Heads I think there will have been a few conversions joining the surviving originals, would Allan care to mention how many they sold? They do look a bit odd when fitted in the pre-56 style or Indian Bullet frames, though. Having failed to come up with an impressive enough offer to tempt the owner of an original Fury a while ago, I bought one of these heads and a '61 Bullet frame and started building my own Not a Fury project from there (using a Dell'Orto rather than an Amal carb, in case anyone was wondering).



Mark M's point about the fragility of the bottom end for a tuned engine is a very good one, especially if making the most of that ported head is part of the plan. A steel con-rod is recommended as a priority if the P.O. hasn't already fitted one, or possibly one of our hosts' performance crankshafts.



A.