Hello ,  it is  just as should be ....  
The hot and humidex make your mixture too rich because carbs have less air .
if you go mountain high , with same temp and humidex  , your car will go worst than now.
make mixture lean reducing main jets a little or go to a shop to set the carb properly .
or 
wait a weather change and try again with average humidex , if goes better without touching the carbs you know the guilty and redo the first step 
hope it helps

ChrisH IA wrote:

Sounds like it could be carburetor icing up. Cold engine and humid air with no heat source can do this.

m1 mustang refers as 30 degrees celsius  not farenight . 
icing up with hot seems hard to me

zero  degrees  c =  30  f  
30  degrees c  =  90  f   
90  degrees   c=   190 f
100 degress c =   water boiling point

Last edited by Alessandro (6/01/2018 1:34 PM)

6sally6 wrote:

Ahhhh yeah........!!!
Welcome to my world
Try turning the distrib a little to advance the timing some?!
More long term fix may be to build some type of ram air using OUTSIDE air instead of hot under hood air.

"When the go'in gets tough.........the tough go inside with the AC and drink beer!"
6sally6

yes !    i cant say it better !!!    italy and florida has same hot and mist !  
Now here we have 90 f noon time expectiing  more next months  ,  and ac is damn sufferin right now !  the engine bay is a perfect howen!   
Sparks plugs are clean as ever ( maybe too clean , need to enrich )
i think i need new radiator and hood with fresh air intake for next project !