Make sure when you make adjustments to the low end you only change it 1/8th of a turn at a time. It's easy to set the high-end back to a running postition but get the low end goofed up and it can be a whole day of frustration.

Some OS engines (did "od" mean odd or old?) have an air bleed screw for low end adjustments. The air bleed channel could be clogged with dirt or old oil varnish.

Squeeze the fuel line tight enough to block it at low idle. If it stops dead you are lean (back out the low idle screw 1/8 turn). If it runs faster after a few seconds and then dies the low idle is good. If it runs for more than six or eight seconds you are too rich (turn the low idle screw clockwise 1/8th turn). Start the engine after adjustments and take it to half speed for a few seconds and then back to low idle and repeat until you get it set. Then take it to wide open. Set the needle valve to maximum RPM and then richen (counter-clockwise) two or three clicks. Now you have to set the transition. If you can go from low idle to full throttle with no catch or only a slight stutter you are good. If it konks try leaning out the low idle a TINY bit at a time (like 1/16 of a turn or 20º or thereabouts).

If the fuel is more than six months old ditch it and start with fresh. And the first thing I do before futzing with a glow engine that has suddenly become a problem child is to replace the fuel tube from the tank to the carb. A tiny hole or tear in this - very common - will make tuning impossible.