Colder temps - below about 50 - require starter fluid. I took carb apart and checked the cold start circuit. I can blow air through all passages including the piece in the float bowl that the upper thin brass rod goes into. For reference, I had a problem when I tightened the carb and the throttle slide stuck in the low position unless torque was very light. Forum members noted it could be uneven mating surface to reed block or incorrect O ring size. I bought an OEM o ring and it appears to be 43mm OD by 3mm thick, old one is 46mm OD by 3.5mm. Old one had sloppy fit, new one drops in properly, so it may have been part of the problem. Before I install it, is there anything else I should check? Removal/installation is tight and tough, lol. Hope this is the last time I have to put the carb back on for a long time. I'd think this should be able to start in cold temps, I even did the trick where I put it in 2nd gear, rocked it back and forth a number of times to pull fuel in, but no luck without starter fluid. This trick worked wonders on my CR500s, but no luck here. Thanks!

My 3 RT3 and RT1B regular riders start on the third or fourth kick when it's cold. Carbs are clean, components are new and ignition timing is correct.

Check the choke components. The choke lever is actually not a choke but a fuel enrichment plunger. Pressing it down pulls the plunger up and allows more fuel to pass though. Make sure it's not plugged up. The throttle should be closed when starting.

Any air leak after the carb body or through the top cap gasket or cable boot will result in a lean condition.

Thanks. The carb was cleaned to address prior issues, but when reassembled it was much cooler and didn't start without starting fluid. I removed carb and verified the low speed circuit is clear. Plunger is squeaky clean along with rest of the carb, the Berryman cleaner did a good job. Jets are stock, but air screw was set at 1.0 turns out instead of 1.5, so I corrected that. It runs fine once started, but is grumpy to start in colder temps. I can try setting the air screw back to 1.0 to see if it has an effect. A friend suggested bumping up the pilot jet to help cold temp starts.

Should start in cold weather if its working as it should. I guess you have to decide if it need more choke or is flooded up--ie overchoked. But if start juice gets it going i'd say its not overchoked or flooded up. BUT if it 'is' flooded then it needs full throttle & no choke, or go have a cuppa & leave the plug out to evaporate some.

Think that's one long sentence below which i wrote first & nearly canceled as i'm sure you know all that--but i'll leave it there for now.

For the choke to suck proper the throttle needs to be closed so the suck goes through the hole in the carb mouth that lines up with the choke piston so with the choke piston lifted the motor sucking across where the piston lifts from & it draws fuel up from the choke jet in the bowl & exits out a hole the motor end of the carb.

My RT3 will not cold start with the chock completely closed. I'll set it at three quarters then the bike starts however there are times I have to feather the choke a few seconds to keep it running. Once it runs for a minute it's fine.

I'm assuming the bike is otherwise running good, when warmed up? If you checked the start circuit passages already, it would make me think the actual starter jet in the bowl is too small for some reason? I wouldn't alter any other jetting, if it runs well when warm. Mind you an air leak could be causing an issue as well, or perhaps a bad reed valve. Also ignition could cause issues when cold as well, if not in perfect tune. Sorry, lots of variables.

Also, ive found a helpful way to get these engines going cold, is by giving them 1 or 2 prime kicks, ignition off, choke on.  Usually fires 1 or 2 kicks for me every time after that.  Make sure to find top dead center.  Apologies if know that stuff lol.

Easy to have a habit of twisting the throttle when kicking without even realizing it.  RT325's advice about how the "choke" (enrichener) circuit works a couple of posts back explains why this can cause hard cold starts.

Indeed. Every bike has its own sweet spot. The 100 likes no throttle. The 175 likes the throttle cracked very slightly.  Maybe a couple of mm.

I never came across a two stroke with mikuni slide carb that started from cold with the technique of kicking it over furiously, as if the throttle hand arm was joined at the elbow to the kickstart leg knee with a length of string.

My 360 always appreciates the 1/8 to 1/4 throttle blip, AFTER the bike has initially fired. The extra throttle can induce a violent kick back if not careful, so no throttle till fire. Easily flooded so a good ear is key with it. Choke usually stays on a total of 5 seconds, even in 30 degree Fahrenheit temps. One or two choke blips if necessary, but it will usually idle fine. Mind you mine is a rt1mx top end currently. Imo the stock enduro top ends are much easier to cold start.

Thanks all! Slapped it back together and got it to start w/o starter fluid. Temps were in upper 20s overnight (unheated garage) and I tried starting when they climbed to upper 40s. Rocked it 15 times, it tried, but died. Rocked it 20 times it tried a little more, rocked it 25 times and it fired up and the world was a happy place, lol. My guess is the new, properly sized o ring helped, or blowing air through the starter circuit passages may have knocked something loose. I feel more confident in taking her out, leaving her sit for a while and then not worrying if she'll start when she cools down. Was nice to experience the two stroke aroma!