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I've just finished rebuilding my 1967 250 Mk3, and started it for the first time today. The engine has been fully rebuilt by Craig Ridgely, and it has Electrex world self generating ignition system fitted. The SS1 29D carb has been rebuilt and I've got it set to the spec I found in Ian Falloons book - main jet 118 and an 80 slide. It started first kick, but wants the idle screw almost fully in, then will tick over and rev crisply in neutral, but try and ride it and it won't pull at all much past tick-over, as soon as there is more than a tiny throttle opening it spits and misfires and won't rev. I don't think it's getting anywhere near the main jet. Any suggestions please on where to start - should I try raising the needle?

Are you sure that you're not confusing the mixture screw with the idle screw?
The idle on an SSI is adjusted by the knurled screw on the top of the carb, not the screw at the side, which is the mixture adjustment.

Jon, yes you're right, I mean the mixture screw at the side.

I love the looks of the SSI carb, but I also had a difficult time living with it on my road 350.
Just about every possible configuration was tried but improvements in one area only moved the bad running elsewhere.
I guess it's really a racing carburettor, and easy starting and idling was not the priority when it was designed.
A VHB square slide carb is now installed for many years, and it does everything well.

The fuel level is often the culprit. Probably too low . and varies with the angle of the bike , A remote float made all the difference but that may have been because it was new. They often start best when flooded and with an open throttle . The mixture screw works the opposite way to usual ,out is rich. The cable operated device that appears to be a starting choke IS NOT ! , and needs to be raised for normal running . Sometimes problems go away with a less restricted silencer. I expect you know all this stuff anyway ,looking at the great resto job. PS that silencer is good for a 125 , no good for your bike . Mach 1 etc came with a much bigger outlet hole and a shorter internal tube. I am currently running a Norton cheroot silencer which allow revs up to 10,000 ! Expect a big bang anytime soon. Try a 60 slide and real petrol. Esso do one.

Spitting back just off idle is the slide having too large a cutaway.

Fit a 60 slide as Bob suggests.

I have bikes running both Amal and Mikuni carbs, and they are reluctant to pull away normally when cold, but a few hundred yards down the road they work properly.

Fuel doesn't work the same as when the bikes were made, so some adjustments are necessary.

I also run a 67 NC Mark 3, The SSI can be made to work ,some lucky devils have little trouble. Its a bit of a balancing act and unless you have lower gearing , a 27mm carb, a mild cam and sensible valve sizes its always going to be an exciting ride , best away from traffic , even better on the IOM !!.

PM sent.

Thanks all for the input. BevelBob, I think you're right about the slide, possibly fuel level also contributing. I had the float bowl rotated back close to the frame spine so as high as I can get it. Running Shell VPower petrol. It was so difficult (and expensive) to find a new 80 slide that for now I've bought and fitted a new Amal Premier concentric. Guessed at a main jet from other posts on here, fitted it, took a couple of minutes to get the idle mixture set, runs like a dream. At some point I'll have another go with the Dell'Orto, take 2mm off the 80 slide, and check the fuel height in the float bowl, but for now I'm going to enjoy riding the bike.

My 250 runs with a 60 slide. If you reduce the slide height dont forget that this also drops the needle height and you will need to raise the needle a notch or two in order to compensate. Watch out for air leaks at the rubber manifold gasket , holes develop at the lowest point and are hard to spot.