Thanks, Steve. I have pulled the carb and am looking to order some jets to experiment with. I did order some smaller mains too but will wait to modify that until I get the idle corrected. And, yes, the orientation of the carb makes adjusting the front mixture screw a little tricky but it is doable. I have heard concerns expressed here before about the transverse application of this carb on the car, but I know that DCOEs and DCNFs have both longitudinal and transverse applications as well.I looked up the specs for a 38DGAS, and it appears the standard idle jet is a 45.
If it's anything like 36DCNVA setup (which also has two idle jets) and that carby definitely runs best with 47 or 50 idles... with the stock 40 idle jet there is no real change from winding the mixture screws back and forth, but a simple change to a 47 or 50 idle jet in both holders and the idle mixture adjustment screws will function as expected and the carb will hold a good idle.
At under 2k you are not on the main circuit, but with the throttle blade tipped at 2k revs you will probably be exposing the first progression hole to the airflow and at "fast idle" you are probably running off this.
Single simultaneous 2 bbl carby on a sohc, I recommend having a pair of 45,47,50,52 and 55 idle jets handy for tuning. The carb spends a LOT of it's time in the idle circuit for a street driven car so getting this metering right makes it much easier to set the carby up properly.
replace the idles with a pair of 50's and try again, leave everything else alone and make sure the float level is correct. The change may sound small, a 0.05mm change in orifice size, but viewed as a % increase of cross sectional area of the orifice (which is what matters) it equates to about 22.5% increase in the fuel volume available
and BTW that is a big carby for a small engine...I know you say your planning a 1.9 build and that's your justification, but it can't turn your sows ear into a silk purse no matter how hard you try...
38DGAS is just oriented all wrong for the application. including that it will put one of the mixture screws close to the motor right above ports 2 and 3... with a DCNF/DCNVA not only is the throttle shaft correct for application, but the idle jets are positioned equidistant from the engine, and positioned so the distribution of fuel is much more even at idle