Hey MarkT does your Ranchero have a 429 CJ shaker with hideaway headlights I always liked them when I worked at Butler’s he had a Ranchero 

1. Choke on down position.

If you cannot flip the choke lever up relatively quickly adding more air to the mixture you possibly have a bad left crankshaft seal allowing air in the combustion chamber - then up choke lever _ more air … I had a 69’ DT1 this year only ran choke down. Owner was a cheap millionaire so he got the Yama-Boot Home. That was when I purchased a NOS OEM left crankshaft seal from my colleague EN. It’s in my parts box. They are rare …

Do a bubble test with the stator removed - if you’re blowing bubbles with soapy water while kicking the bike there ya go… ? 

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Yeah true what you say about the crank seal, but just thinking if its gone hard & doesn't seal on cold start the heat softens it & gets a bit oily & then seals ok untill next day. Probably not 'that' so don't read too much into it. Just guessing. Without having seen a carb kit for a while i'd guess it only has the choke plunger & spring. Plunger doesn't wear out anyway. Pad on the end might get indented but only makes it seal better when closed. If the pad was leaking it'd only make it idle rich & not be able to lean the mixture screw off satisfactoraly. On my 250 i had weird lean condition getting up through the range & wouldn't take throttle without flicking the choke. But was stale fuel deposit built up inside the needle jet & had grown in there over a long hibernation & was hugging the needle. Took me ages to find that.

Sounds to me the starting procedure is wrong.

Mikuni VM's use the enricher to add extra fuel after the air valve, it also adds more air.

Very little throttle will disable the enrichment part off idle with no load(neutral).

cliff

Brzn:     So the rebuild kit you get from Niche cycle has parts for the enrichment circuit ?   I thought the enrichment jet
              was fixed in the float bowl and not able to removed ? 

Hmm... Sorry, as RT325 states, looks like the carb rebuild kit I'd purchased just has the plunger and spring for the choke. I replaced the left side crank seal in my DT2 earlier this year with one out of a kit I bought off eBay and it's holding just fine. The NOS Yamaha ones (if you can find one) will be pretty old, and possibly hard and brittle, since they've been unavailable for quite some time.

Thanks to those that responded : 

The  temperature was about 60 degrees

I will try the bubble test    consensus seems to the left side crank seal   

My other DT3  starts oin the choke just fine and revs considerably higher while warming up 

What brand  repro seal should I get  ?  thx 

The Oil Seal kit I'd purchased was from dream305 on eBay for $2499 plus tax and shipping. The kit included six seals; kicker, right and left crank, clutch, shifter, and primary gear. I've replaced all except for the right side crank seal (it seems to be okay) and all but the primary shaft seal are holding back the oil as they're supposed to. I took a chance on a NOS primary shaft seal and replaced the leaking one from the kit with it, and it's holding fine.

I don't think there's anything wrong with your seals since you have a stable idle and idle is on the low side. Air leaks usually raise the idle. Can never hurt to check seals though.

At 60 degrees F a little warm up doesn't sound unreasonable. If I were to look anywhere it would be for a partially clogged starter jet, partially clogged pilot jet, or someone installing an aftermarket or smaller size than stock pilot jet.

If you want to buy the left side crank seal or for that matter any seal for these Yamaha's check out bikebuzzard on e-bay. just message him if you don't see it listed. He will have them plus most any gasket. Be sure to know what part number you need. Look it up 1st before contacting him. He's easy to work with. I bought a couple sets for these DT's i have but as of right now there not needed. 

Check out item 175424106282 on e-bay. tried to copy link here but no go. Is it against forum rules now?
 
I agree, you should be able to give it some throttle with choke on and stone cold. I live here in Michigan, been 33 to 37 degrees the last week ( 74 degrees this weekend, Crazy! ) both the DT's i have start up quick and can be rev up right away with choke on. It doesn't matter if it's 20 degree out or 60 degrees, With choke on, engine should be extremely over rich and smoking. Same with my snowblower, lawnmowers or any other engine i am working on or starting up here. Only difference temperature makes is how long choke has to be on to get it up to operating temp so engine will run with choke off. 
Although the 71 DT1 is more cold blooded then the 70 DT1 by a small bit.  Both are identical engines and carburetors and jetted the exact same. Go figure. 
I would also start by making sure starter jet circuit is clean, pilot jet is correct, (Mikuni part or Yamaha), float level is good and choke parts are in good working order. But it could also be air leak on the intake side of things or seal but seams like you would have issues with the engine warm too?     

I might've mentoned seal as a last resort but more difficult than cleaning the carb, so just be sure with that first. Raise the needle for a try--drop the clip lower that is. Video would be great to hear & see what its doing--or not doing. Seal on mine is likely 40 years old but bike runs good, after i found the buid up on the inside of the needle jet. Fuel from the old days never caused problems like that.
I might 'regret spouting about my old seals' as in hindsight both my DT & AT are very cold blooded if left parked for weeks--not long enough for fuel to go off. But might be hard old seals that once running they soften up & get wet so start sealing again as they both run good once warm. Food for thought for me.