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PR: Proprietary Items MK 1 concentrics


Well Known and Active Website User
VOC Member
Now I am no expert on these items TT ,GP, Monblocks, Mikuni Perhaps, but these.... However a mate of mine has a strange problem with a pair of these carbs. They were on the bike and working before it was in storage. They are on the bike and have been ultrasonically cleaned and the settings pilot etc are the same and importantly the slides are absolutely lifting in unison. I can see no leaks in the inlet system. The electrical system is OK the plugs sparks and have been swaped about and she starts OK however on tickover only the front pot runs untill about 1500 revs when the rear chimes in. Playing about with the pilot settings brings no joy and has been returned to the same as the front as original.
Now here is the strange bit which I am hoping will indicate to those who know what is the problem. On the front of the carb under the carb inlet are 3 small holes two smaller each side of the central larger one I guess they are part of the pilot system if I the engine is ticking over with the front lugging round the rear and I put a finger over this hole nothing changes but when I remove my finger after a couple of revs the rear chimes in and runs clean for a couple of seconds and dies again, this finger action can be repeated. it indicates fuel starvation but where? So concentric guys whats that finger fiddle indicate is wrong?

Nigel Spaxman

Well Known and Active Website User
VOC Member
Probably the pilot jet on the rear cylinder carb is plugged. It is usually not possible to un plug those without sticking a small piece of wire in the hole. I think when you plug the air inlet you may be causing a vacuum on the back side of the pilot jet that temporarily un plugs it, but after a little while the particles or gunk that were sucked away from the back of the pilot jet, get sucked back into the jet plugging it again. You will have to flush it a lot with carb cleaner in both directions to get it clean. http://www.jba.bc.ca/Bushmans Carb Tuning.html
Have a look at this link. New Vincent owner Jim Bush posted this many years ago it has all the info you need.