Forum

[Adrian]

Have you tried our hosts to see if they have a suitable rubber flange in their stash of unlisted parts?



If they can't help with this one, Allens the Mikuni specialist might have an off-the-shelf part that will match carb to head. Have a look here under Plate Manifolds, about 2/3 of the way down the page.



http://www.allensperformance.co.uk/carb ... tion-page/



A.

[stinkwheel]

Cheers for that Adrian. Yes, Allens do one to fit a VM28 carb to a 2" hole centre manifold which is exactly what you need to put a 500 bullet mikcarb onto a 350.



You have no idea how long I've spent scouring the internet for that exact thing!



Sorry for the thread hijack.

[Presto]

The choke on a Mk 1 Concentric does do something. Whether it helps starting or not is another question. With some it does, with some engines it doesn’t. Some need tickling, some don’t. Some need throttle, others don’t. These differences don’t mean that the Mk 1’s are problematic but that engines differ.


Everyone to their own but Mk 1 Concentrics have been fitted and used on countless engines, road and competition, with perfectly good results. Are they better than Mikuni or Mikcarb? No one could tell the difference riding a bike if the fitted carburettor is set-up correctly. And the Mk 1 Concentric is as easy to set up as any – and easier than most. And cheaper.



By the way our hosts have all that’s needed to fit these various carbs to 350 and 500 Bullets – take a look in their catalogue or website.

[another Allan]

My 625 Bullet which has a 32mm Mk1 Concentric will not start from cold without full choke.

I also have a Mk3 Norton Commando with two Mk1's and it also needs full choke from cold.

[John L]

Presto - The voice of common sense and reason - That's a very fair and balanced evaluation of Amal carbs and the effectiveness of their chokes. My Triumph TR7V (30mm. Mk.I Concentric) would be impossible to start without full choke - AND a good flooding from the tickler....Don't ask, etc.......

[Leon Novello]

If the bike starts and runs from cold, without needing choke, something throughout the carburettor's fuel system is set too rich. Which means that when at running temperature, the mixture is way too rich.

Confucius.

[Alan R]

Hi Guys---------Leon, that's not quite correct..If your bike is experiencing a rich mixture over the full rev range then the cause will be something that effects the full rev. range---typically an air filter that needs changing etc.......However if the bike can cold start without the need for choke and/or tickler and then run normally the primary circuit is likely to be at fault........Hello Mark M....Don't forget that the primary jet on the Mk 1 Concentrics used to be a pressed-in bush....Invariably some swarf would be generated and form a partial blockage.........The 90 degree flow-line didn't help either.....Initially a mod was developed by drilling and tapping from the opposite side of the bush and a removeable jet fitted.........Later the jet was moved to a position at the join face of the body and bowl where the primary fuel is drawn up from within the bowl.....Yes, that slowly slipping choke lever can catch you unawares sometime......

[Alan R]

------forgot to add that when I was building and racing grass-track BSA C15 and B40 bikes back in the 1980's I ran Mk1's on Methanol with no probs that I can remember ( ACU--Southern Centre, No. 176 )...Tadley & District Sporting Motorcycle Club......

[Presto]

As for the engine starting without need of choke when ‘cold’. How ‘cold’ is cold? What throttle position is being used by the rider starting the bike? What fuel is being used? Has the tickler been used or not? The more I have experience of carburation the less it seems to me to be a science and the more it seems to be an art. But I’d agree that according to ‘the book’ the pilot setting at least would seem to be too rich if the choke is not required at starting from cold.