Actually, a temp gun is very handy for setting your low speed needle. After everything is fully warmed up and you have the HSN set where you want it, make a few fast passes, bring it in and shoot the head temp. Count down 10 seconds and shoot it a second time. If it's dropped off 6~8 degrees your LSN is where it should be, if it's gone up or stayed close to the same, your LSN is too lean, and if it's dropped more than 8 degrees it's too rich. This is much more accurate than the pinch test. Think of all your adjustment needles as a clock face and never turn them more than an hour at a time when tuning, a little goes a very long way here. Also, make sure you get the chassis heat saturated before doing final tuning.

A good way to check your piston/sleeve seal is to bring the engine up on the compression stroke and hold it there, then put the exhaust port almost in your ear. If you hear a frying or gurgling sound, it's time to think about a new p/s set. The best way to check for a worn engine is to see if the piston will push through the top of the sleeve. If it won't you still have some pinch left and you should check for a good seal as described above. If the piston will push through the top of the sleeve, you have lost all pinch. Most engines will still crank in this condition, but will flame out once warm.

The dark spot on the piston crown is not actually carbon, it's more of a burnt oil glaze. If the fuel you use contains dye, the stain will be a darker shade of the same color. If your fuel does not contain dye, the stain should be a nice golden yellow color. If it's black you are running too hot a plug. If you have a blackish looking skid mark in line with your exhaust port you have run the engine too lean. This will also show as signs of metal transfer from the roof of the combustion chamber and edge of the piston. Metal transfer looks like the parts have been sandblasted with a corse media. The problem here is the places where metal has detached itself will become hot spots, and if it's bad enough will cause detonation.

Last but not least. If you have a mid range adjustment and after doing all the usual adjustments to the HSN, LSN, and idle screws you are still not getting what you want, remove the carb and looking in from the bottom adjust the mid range until the hole(s) in your spraybar are centered in the throat. Do this with the slide or rotory valve fully open. You will then have to retune everything else, but you should get a better idle, more crisp off idle response, smoother mid transition, and much better overall acceleration.