idle jet change.

[oldscooterbloke]

My idle jet in the shb 19 carb is   a 38. starts well revs and returns to idle ok.has a slight stumble not smooth heading towards 1/2 throttle
was thinking to be safe should i go to a 40 size .how can i tell what size is needed.
i only have a 42 next size , put it in today, started fine revs well returns to idle quick, but struggling to idle, went to lean it on the mixture screw and it stalled just about when i started turning it. ( the engine may have been a little cold)
can't need to be richer turned out more can it?
you thoughts before i have another go.
plug is beige  colour on the 38 idle and 95 main that i have.
130 polini and sip banana , vape road ign. 19/19 carb std filter. spot on 2% fuel mixture.

[jbcollier]

You can't adjust the idle mix until the engine is warmed up. I would put the 38 back in, warm it up and then richen the setting by a 1/4T at time until it pulls smoothly. Now check how many turns you are out from lightly seated. Above 2.5T? Fit the next up idle jet and try again. It should be in the 0.75T to 2.5T range

[oldscooterbloke]

Hi, thanks for your reply, warm it's out just over 2 turns and a tiny bit.on the 38. have now ordered a 40 jet will try that.
My plug has a mid grey/almost brown electrode , and ceramic insulator and wet dark soot on the outer ring with a 42 idle jet. got it to idle now with the 42.didn't seem too bad on the 42 overall. just needed you to see the plug picture really.
can't seem to load a picture on hear to show you
just trying not to be lean on cruise.

[jbcollier]

Plug readings can be misleading, especially with modern gas formulations. On stock and mildly tuned engines, I still set them up by feel/ear/experience. On heavily tuned engines I now use a wide-band O2 sensor and an A/FR gauge. It makes finding transient lean spots very easy.

[oldscooterbloke]

J B Collier. Thanks again for all your help and regular posts, thanks