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Mk1 concentrics ONLY:

derek

Well Known and Active Forum Website User
VOC Member
How many people have achieved a pilot air screw adjustment of the recommended 1 - 1 1/4 turns open. I ave spoken to a few people and they can only achieve, as I can, 1/2 - 3/4 turn open! Is this OK?
 

Howard

Well Known and Active Forum Website User
VOC Member
Do you mean you can only screw it out 1/2-3/4 turn, or the engine runs best at 1/2-3/4?

I usually start at 1 1/4 turns (plenty of adjustment) but I usually end up at 1/2-3/4 for best running. My carbs are 1970s vintage, so new ones may be different.

H
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Derek, I would have thought it depends on what jet you have, I had a 25 but find it runs better with a 30, Some old ones had no jet, The bikes are all different,But it sounds OK to me, If it runs well. Cheers Bill. One other point is the float level, I have just fitted the new Stay up float and a new bowl, the float was way to low but you can adjust the new ones, now it runs better than ever ! Cheers.
 

mr.hutch

Well Known and Active Forum Website User
VOC Member
Hi Dereck,
just adding to Bill and Howards comments, I fitted a new pair of MK2's some 15 years back at the original rebuild, purchased from Hitchcocks, a firm to be recommended for such items.
There are some small changes however, like the pilot jet is replaced by a pilot bush of fixed size.
My set up has been all these years, after some early trial and error, Pilot 25, Needle 106, Main jet 320, needle top notch.
Currently the pilot screws are 1/2 to 3/4 open, remember screw in ,to richen the mixture.
Having got a sort of nice tick over on full retard, (I have manual advance and retard) I check the plug colour after a short run and adjust as necessary, which seems to do the trick. Hope this may help.

Go carefully mr.hutch :)

Vincent outfit rides 101, BMW 44.
( they are my main transport )
 
Last edited:

derek

Well Known and Active Forum Website User
VOC Member
Thanks for all the help. Mine runs best at 1/2 to 3/4 turn open. The fuel level is recommended to be 0.170" to 0.240" from the top face of te bowl. I set up a bench test and mine was 0.310". I concluded that due to the location groove in the needle for the float tangs beeing too great the needle was seating under its own weight when the float lift to the needle became zero. A new needle was ordered and found to be the same. I therefore made a 8BA screw, drilled axially down the needle and fitted the screw adjusted to give minimum clearance to the float tangs. I again tested on bench rig with the following results, fuel level from top face of bowl 0.190" to 0.200", this is with in limits. The float is now positive as it opens and closes the needle valve. Having drilled out the pilot bush fitted, I have tried pilot jets 20,25,30,35, and now settled for pilot 25, needle 106, slide 4, and main jet 180. but have not yet run at max, as still running in. Thanks again Derek
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Derek, Unless your engine is very soft and you have a quiet exhaust, I would have thought the Main jet a bit small, But you don't say what size your carb' is. It might be better to go up a bit, you don't want to Nip the pistons. Good luck Bill.
 

derek

Well Known and Active Forum Website User
VOC Member
I have the vitro tipped needle and they still self seal; that is why I reduced the groove width in the needle to be a close fit on the float tangs. The float now controls the needle to a closer tolerance and also gives the correct level. Thanks Derek.
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
From Big Sid Biberman
There are a few easy modifications one can do to boost both air and fuel flow in these carburetors , taken from flat track , etc. racers which bring them up nearer to the performance of GPs . To increase the air flow you need to polish back the sharp step at the bottom edge just inboard of the inlet bell . Grind it back on a smooth slope into the throat and polish . This caused the incoming air to bounce off and upwards creating a blockage to flow , this plus a longer ram tube { the lovely tapering John Player Norton type probably the best ever } together will increase air flow and ram effect .
As to fuel flow - before setting the float height correctly - seldom ever correct even new ! - one must enlarge the float needle orifice and use a Viton tipped needle . The bore size std . varies between bowls from 100 to 125 thou . and in alky racing bowls is usually 156 thou . For gas I try for around 140 thou ., its tricky to get a clean sharp perfect edge here for good sealing . Set the float height after this mod . as it will change .
Next racer trick is to mill a passage interconnecting the needle passage above the seat interface into the main bowl area - a cross slot about midway up the needle bore . This takes away much of the flow turbulence affecting the needle action allowing much of it to enter the main bowl through the new slot directly , immediately it rises past the needle seat . Clean away all burrs both sides of the slot .
I find that starting is improved along with initial pulling power by slightly enlarging the tiny drilling just inboard of the mixture needles tip . This is the newer location for the idle jet orifice , deep within the body past the needles tip . Std . this is a 16 thou. hole , and often clogs with todays fuel . Using index drills I enlarge this size to 18 or 19 thou . Power is improved as is starting ease and clogging is rare . No worry with it being too rich as the mixture needle still works to control things .
Main jet sizes will depend largely on the use or not of air filters , the exhaust system employed and the muffler chosen . The more nearly open and unrestricted the larger jet sizes will be needed . Greater power will run pistons hotter thus need richer settings to prevent overheating .
The 32 mm Mk1 Concentrics employed on our Vincati and also on the hopped up Meteor sport all these changes and give very satisfying performance

Sid (bigsid@webtv.net) offers to answer other questions.
 

Howard

Well Known and Active Forum Website User
VOC Member
All this talk of carbs reminded me my MkI slides are a bit rattly in the bodies. I couldn't find the price for a body on the web site, so I emailed Burlen. The answer is £69 for a body or £85 for a complete carb (all plus VAT).

I'm thinking of sending £16 for everything but the body.

H
 
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