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Bike ran okay 'till it was laid up for a few months, (nearer a year), and then it would only run with the choke on. Suspect idle circuit? Stripped and cleaned carb, some gunge around the needle valve was only problem found. Still wouldn't run with the choke off. Inspected fuel tap and found it partially blocked and rubber parts disintegrating. Bought a pattern replacement tap, cleaned out tank and tested with fresh fuel - new tap functioning perfectly in all 3 positions. Engine will still only run on choke. Stripped, cleaned and inspected carb again and checked float level. Would still only run with choke on. Had carb ultrasonically cleaned - no difference, still will only start and run with choke on. Tried tuning the mixture screw on the carb from the 1 1/2 turns out base setting as recommended by the manual and fiddling with the slide stop. Which I admit is not going to achieve a lot with the choke on, but I'm running out of options and am willing to try anything. With the bike running, (with the choke on), any attempt to lower the choke lever results in the engine 'dying'. But if I grab a handful of throttle as I lower the choke lever and hold it at around half throttle the engine revs up okay and will run at that speed until I back it off and the engine dies again. I would think that this shows that the problem is with the idle circuit? However the jets and carb drillings are all clear and besides having the carb ultrasonically cleaned and blowing through all of the orifices several times with a high pressure air line - the damn thing will still not run with the choke off. Any ideas would be welcome and appreciated.
 

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Might be worth posting this question over on the other forum, link to which I'm not allowed to post here. Google the name in my avatar, there are some carb experts over there.
 

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If the bike ran perfectly ok before it was laid up, I suspect a blockage in the main circuit, i.e. the main jet circuit. Check that there's nothing obstructing the main jet and that there's no build-up of deposits on the lower end of the needle.

You should be able to see straight through the main jet orifice if you hold it up to the light. Blasting it through with carb cleaner or acetone (nail polish remover) should clean it pretty effectively.

I got caught out many years ago trying to get home in my old Vauxhall Magnum after going to the pictures. It got there no problem, but wouldn't run without full choke going back home. Checking next day, I found a tiny fly slap bang in the middle of the main jet! How the hell it got there I don't know, as the big pancake air cleaner was firmly in place! Good for keeping out pancakes but not flies maybe??!!
 

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Have you checked for air leaks between the carb and the head?
 
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Poss air leak all so check valve clearance as you might have rust/carbon on the seat ? next is a compression test just to rule it out. Had the same fault on my C200 & it was tight tappets.
 
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