I have just purchased a fully restored DT250A . I also purchased a new Mukini carb with a new spec set of jets and neddles etc. I cannot get her to rev out. Will idle all day but claps and sputters when I turn on the throttle. I had a similar problem with a 74 360 I restored a few years ago. As we all know there’s no one I can find who knows how to tune them. An additional problem is I live in Australia. Does anyone know the solution because pulling both the carb that was on it and the new one apart and changing jets and adjusting needles is driving me mad. I owned one new in 74 and these carb problems didn’t exist then. I use 91 petrol. TIA

Try running it for a short while without the air filter case lid (and maybe even the air filter) and see if there is any improvement. Could also be a bunged up silencer.

I just picked up the same bike.The bike had been sitting for years and initially it would run on top but it wouldn't idle or mid. I replaced the o-ring on the needle jet setter valve which made it made it idle and run fine in the middle but wouldn't run on top. Well I forgot to replace the washer on the needle jet setter. Now it runs great. Thanks to this site or I would've been pulling my hair out trying to figure that one out. Now I wish I would've read the thread on not running it without the battery hooked up.

First thought I had is it's likely impossible to get a new Mikuni carb body these days that is the same as the original proprietary Yamaha version from 1975.

Anyway, even less likely that both carbs are doing the exact same thing... sounds like something else is wrong?

RT325's mantra of plugged exhaust has already been mentioned.

I'm wondering if it might be electrical... timing way off or bad condenser or something.

Any way you could upload a video to Vimeo or YouTube of it running and link it here?

Link a video would be good if pos. Std carb & jetting & clean--not restricted exhaust & should go good. Flywheel key is a thought to check because if that is broken then timing goes wherever it ends up. Has someone fitted a 360 cdi mag & it's faulty on the high speed part of the fangled pulser coil those early ones had. Try i minus the tail muffle--take it right off not just the baffle in the end. Seat not collapsing on the air box. Do those have the jet setter small Oring which is super important if not sealing.

Thank you for your advice gentlemen. I will try all the above and let you know.