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To make my 250 a bit more tractable and improve pick up i am fitting a less wild cam and would like to speed up the airflow and venturi effect in the carb. It currently is fitted with a 29mm SSI. I understand that the 27 and 29 share the same body but have different choke tubes . has anyone any knowlege of this and know if its possible to sleeve the 29 down ?.

SSI carbs are beautifully made and look great, but are a pain to live with on a "daily" bike.
They cannot idle reliably or operate smoothly at low revs, in my experience. I tried several times!

I now have a VHB. It's not much to look at, but behaves very well.

When rebuilding my 200 SS, I was fortunate in finding a SSI 27 A, In as new mechanical condition. I also run a remote SS2 float chamber, (with new float valve) which facilitates the easy setting of the correct fuel level. After some initial issues, plug wetting etc. When it had covered a few miles, and things had freed off , I was able carry out some serious experiments with the settings, needle position, main jet size, etc. The final solution being an increase in the slide cutaway, which involved making a new slide, as altering the original being a one way street, in the event of needing too refit it.
It starts o/k . Idle perfectly and responds well to the throttle when up to temp. But as said the Carburetter itself is in very good condition, perhaps I've been lucky ?
And yes the SS carb certainly looks the business, akin to the GP on my Goldie. Which will undoubtedly feel "considerably" heavier than it did last year, when it comes out of winter hibernation.

Brian

Hi All , not going to use another carb. The SSI is in first class condition and has a rubber mounted SS2 ? fully remote chamber. Its just too big for good power , together with a 40 mm inlet valve. Might have to go to another head ,Monza ?.

Bob, fitting a Monza head with the small inlet port and valves will only make a real mess of your ssi carburation......(unless you use a low compression piston and the 24mm carb to suit) a more pragmatic thing to do is fit a milder camshaft with less overlap......maybe the 250 scrambler cam?. Cheers, George

I would try using a Monza (lower compression) piston first. I have a friend with two Mark 3's with your head and carb and that's what he did and it worked well for him.