Are you talking about one engine your having a problem with or more then one? Cause you saying engines.

Here’s some things to look for especially if more then one engine.
fuel tank
fuel tank lid tight
replace all fuel line
pipe gaskets
engine gasket to pipe
springs tight on pipe to engine
springs tight on pipe

As said above If tune is changing when you screw your high speed needle in and out is there like no resistance? Very easy to turn. You may need to take out the high speed needle completely and replace gaskets as said above. Another thing is check your backplate make sure it’s tight. Check shims on head button if there all in good shape. Head button on tight. Is carb on tight with pinch bolt is there a gasket in there good.
Linkage good and smooth and opening and closing properly the same every time?
Mend points set
I’m not sure of your experience so I’m trying to see what you know etc
Is fuel old?