Since the really no-one seems to want to answer this question, I explain what I have learnt in my limited experience with on-road engines.
Make sure that the engine is hot (working temperature), then let it idle at as low speed as possible without stalling. Then gradually lean the bottom out until the idling revs increase a little. Adjust the idling again to as low idling as possible, and give it a bit of throttle to clear the crankcase of fuel, then give the engine full throttle suddenly. The engine should not stall or four stroke at all if the bottom end is right, It could be too lean or too rich if it stalls and just let it idle to test that; It’s loading up the crankcase with a too rich setting if it slows down and stalls before about 10 - 20 seconds during idling, and the engine is too lean if the revs start increasing before stalling. It’s also too rich if it four strokes when given full throttle after idling for a while. The engine should be able idle steady for at least 10 seconds (and probably more) without any change in idling speed, and then be be able to handle a handle a sudden full throttle without four stroking or dying if the bottom is right.
After all that do the top end again, and this would probably be too rich or the glow plug too cold if the engine stalls on the track. Keep checking that it’s not getting too hot, and check bottom end again you’re when happy with top end. Also do the pinch test, and the engine should stall between 3 to 5 seconds when pinching the fuel line close to the carby inlet. The bottom is usually too rich if it runs longer that, and too lean if it stalls straight away. The idling should also not change when tilting the car forwards or backwards. There is probably some other problem like air leaks, dirty jets in carby or crook bearings if that doesn’t work. Very highly modified engines can be very hard to get right sometimes, and very minimal adjustments might be needed. You can check for air leaks by plugging air intake and exhaust hole with plasticine and fingers; or any thing that works, and blow into the fuel line with the engine submerged in a bucket of water.
It is important to realise that the fuel goes through the crankcase first in two-stroke engines, and that the crank will load up with fuel if the bottom is rich. It will cause slow acceleration with lots of smoke or stalling when powering on out of corners. Engine running on when backing off could be caused by this, but can also be caused by too lean settings (bottom or top) and a too hot engine.
However; the first thing to do whit engines that won't idle is to change the glow plug,
do that first, at least 50% of problems with idling are caused by glow plugs that aren't right. Don't worry about whether or not it looks O.K. just change it!
I’m still learning, and anyone that know better or disagree; please don’t shoot me, I’m willing to listen and learn. There is a very informative article on engine tuning with writing from Ron Paris at
http://www.wynnumrc.com/ . Click on “Technical Information” and you’ll find it. I recommend that reading to anyone with tuning questions.
Kindest regards,
Lars.