My 59 jolly 500 will start and run with the choke on, but as soon as I ease off the choke it dies. My throttle pull is not connected, but I try to press the throttle pedal with the choke on and I get a backfire through the carb and engine dies. Carb is a Weber 28.

I just disassembled it and put it through my ultrasonic cleaner. But forgot to remove the idle mixture screw. Could that be the problem? Needle and seat looked fine. Timing and point gap are set correctly.

I nominate warped base of carby. Pulling out the choke gives it some fuel but without it the mixture is too weak and it dies. Remove carby, 're-flat' the base flange and reinstall with new gaskets. Of course you will inspect the bakelite spacer to ensure thats ok.
I'm assuming the Jolly has a sedan motor because I can't remember the carby set up for the Gardinera.
Regards
Viv

When I say pulling out the choke gives it more petrol I mean it chokes off the venturi air and the fuel/air ratio made up of carby fuel and leaking flange air becomes more like a proper 'choked' mix and it will start.
Viv

Thanks Viv. I've unscrewed the idle mixture screw and plan to blow out that part of the carb with compressed air and spray some cleaner in there. Then will screw the mixture screw back in and then back it out about 1 and a half turns. I'm hoping that that will do the trick. If not, then I would agree with you that the base must be warped and air is leaking in. So I'll remove the carb and carefully file down the base to ensure it's flat. Will let you know what happens. For now, my Jolly isn't living up to it's name.

Hi Jollyboy,
I guess once you have the carby off you can check for flatness on a straight edge or known flat surface but if you do have to try 'flatting' the base flange don't use a file. I recommend, as others on the forum have before me, using emery paper attached to a flat surface. I use a sheet of glass and stick on say, 600 grit paper. The hardest part is ensuring the carby is held perpendicular as you swirl the carby around the emery paper.
Cheers
Viv

Thanks. Well, it's not the idle mixture setting and cleaned it up as well. No difference. So the carb will come off tomorrow and I'll check flatness of the base. I have a good 26 IMB off my 59 Nova 500 that I will install temporarily.