Concentric carburetters

[old53]

Thanks Brian-  Like you I can not fit an air cleaner but I am  about to try a 30mm concentric and was interested in your main jet size i didnt have a big choice of jets but do have  a 220 and a 260 so I am now thinking of fitting the 220.

[andy2565]

oh the overtighting of the flange brings back memories of the slide sticking on my triumph metisse as i was going down hill at a hell of a speed,i had to jump off and watch my lovely bike bounce through the air,it was due to the previouse owner over tightening the flange nuts,i always fit a flat washer and then a spring washer,which keeps pressure without having to over do the nut,i would always prefer the concentric over the monoblock.due to the float chamber being on the side instead of underneath,it suffered from a fuel  surge on cornering,my first bike a 250 c15,if you leaned it one way,the revs went up,leaned the other way it cut out.

[Brian]

I have found if you throw away the "O" ring on both the Concentric and Monobloc and use a gasket instead then you dont have any trouble with the flange warping. I use a gasket with a thin smear of grease on it.

old53 has the inlet tract on your bike been enlarged, if not a 30mm wont work very well. The standard size is 1 1/16" which is 25.4mm.

[BSA_54A10]

I gave up on O rings years ago and never looked back.
You still can not tighten the carb too much as it has to be able t  move relative to the head to compensate for different expansions.
Eades in NSW started to stock a 1/8" composite gasket which works very well.

The iron heads don't need the insulator due to Cast Iron's poor thermal conductivity, but what you want to be very careful of is not to overtighten the flange nuts.  These will warp the carb body and make the slide stick.  I install split lockwashers and tighten them just to the point where the flatten out, no more.

Try that line on the side valvers and see how far you get.
The old iron headed bikes had long aluminium inlets principally to keep the fuel cool.
I have a 1" spacer ( off my WD B40 ) on my WM20, a 1/4" tuffnol gasket and I can still boil the fuel in a 626 concentric.

As to types of carbs.
The best by far are the monoblocks. They are a far more precise instrument than any of the others and offer far more tuneing oppertunities.

Concentrics are the most robust and easy to use, They will run ( badly) over such a wide range of maladjustments with foul fuel, blocked idle jets and ovalled needle jets it is not funny .
I have seen concentrics with slides so badly worn that the owners had jets 3 to 4 sizes larger to compensate . Don't even think of trying that with any other type of carby.

Where possible I do fit Concentrics to my bikes because I like to ride more than wrench and do not get in a tizzy if I am not getting 110% out of my Beesa, part are plentiful and cheap and they are easier to tune than just about any thing else.
Others I know fit Monos excluseivly and go to the extent of filing down slides to get 1/4 incriments instead of the usual 1/2's.
It is horses for courses

[Triton Thrasher]

The iron heads don't need the insulator due to Cast Iron's poor thermal conductivity,

Put your hand on an iron head and you'll see that poor thermal conductivity makes it run hotter than an alloy head, but does not stop it conducting lots of heat to things that touch it.

Having said that, I've never used anything more than a thick-ish gasket between carb and manifold.

[Triton Thrasher]

a reconditioned Type 76 sidebowl promptly started being a pain. .......  Just died of fuel starvation after about 5 seconds, until the throttle was closed a bit and all was restored except road speed. All for the want of fuel, obviously, but whether from a lack of float height (unadjustable basically although I tried and cost myself a few litres of precious liquid all over the magneto from the overflow hole), or for want of a larger main jet I do not know.

I've experimented with fully remote float bowls ages ago and found that passages from float bowl to main jet had to be a lot bigger than I thought would be necessary.