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I'll preface this by saying that I know my way around bikes and carbs and am typically pretty good at tuning them, but this issue has me stumped.
The bike: 1986 Suzuki RG250 two stroke oil injected twin that I imported from Japan. It has Mikuni flat slides with pod filters and stock exhaust. In Japan it was at similar elevation and was someone's daily rider that got put in storage for a few years until I bought it. I got the bike here, fully cleaned and rebuilt the carbs (not changing anything) and went through everything else on the bike (everything you'd do on a new-to-you or fresh from storage bike).
The bike runs and rides MINT. Idles perfect, doesn't foul plugs, no flat spots, zero hesitations...it's awesome. EXCEPT it doesn't cold start for shit. I'm talking choke on, cranking the throttle, 500 kicks and it's not even thinking about starting. A squirt of ether and it stumbles to a start and if you can keep it running for ~30 seconds you're good to go for the rest of the day.
With the fact that it was a regular rider previously, I've resisted changing anything, thinking it might be a problem that would fix itself. I've now gone through this process a bunch times and put about a tank of fresh gas through it with no improvement. Because it runs so well everywhere else on the spectrum, I'm scared to change anything. And because it's a cold start issue only, it makes trial and error a pain (you basically get to try one theory per day).
Anything jumping out at anyone? My next plan of action is to fatten it up a tiny bit and see if that makes any difference.