Wow, guys. When it gets over 86F here, we consider it an exceptionally hot day.
So we head the 10 miles to the ocean, where it will be in a tolerable 70s again...
Platinum and iridium plugs have a very hard and fine center conductor that is not prone to wear.
Usually a bit more money goes into their construction also, so sometimes the insulator is more foul proof.
Their main advantage is longevity, although some claim fire advantages also.
Plug heat range (the number on NGK plugs) has to do with the heat transfer path out of the plug and what temperature the tip electrodes are kept. The ideal temp is about 500C. This is on the verge of glowplug like pre-ignition so that the spark has the "tipping point" energy to fire the mixture off. Too low a temp and it might misfire (often called "4-stroking"). Read it all here:
https://www.ngksparkplugs.com/about-ngk/tech-talk/spark-plug-basics
So how do you read if you have the right heat range?
Look at the insulator and the ground electrode. Here is an excellent graphic:
If you have the slant head on these engines, you have the option of facing the plug forward or back. This makes a difference, mostly in the heat range of the plug. If the plug faces forward, you will likely need an NGK 7 or 8 plug. If you face it to the rear, you will likely need a 6 or 7 plug. I found a slight power advantage with the plug to the rear but others have contradicted this, so don't worry if your frame limits you. Just check that the heat range is right.
The "R" in the NGK plug name is for "resistor".
I know we think "I don't want to resist any of the spark!" but it doesn't work that way.
The resistor tend to hold off the spark until it reaches a threshold energy. It sparks hotter, more Volts.
It also acts to dampen the after EMF oscillation which affect radio frequency (RF). This protects electronics.
In my crude testing I cannot feel a performance difference between resistor or non-resistor plugs.
Resistor plugs have a theoretical advantage and don't cost any more so I use them.
The "P" in an NGK plug name is for PROJECTING electrode.
It can make a big difference. It typically helps low speed operation by projecting out into the center of the mixture but certain high performance, high turbulence engines, or engine using high oil ratios (16:1, 24:1) can snuff out a "P" plug at high RPM. A standard or even recessed plug may be needed. Only testing will tell and this will affect top speed.
Regarding outside temperature and its effect on power, it is mainly jetting mixture related.
Colder temps lead to more dense air and tends to lean the mixture. COLD NEEDS RICH JETS
This is why engines often work better at cooler temps. It is also why 2 strokes often burn up at cold temps from leaning out.
Air pressure and humidity are factors as well, as well as cylinder cooling in hot and cold temps.
Sounds like a nightmare, doesn't it? You cannot figure it out, you have to learn to read the plugs.
Refer to the chart above (you may have to cut an almost new plug apart to see the smoke ring).
Just remember that temperature is the biggest variant on your jetting. Jet for the temps you run.
As for the carb leaking at the top? Don't worry too much about it.
It should be sealed to keep out dirt but unless the throttle barrel is very worn it does not let air in or affect mixture.
Same with the float bowl. These 2 areas are designed to be at ambient pressure, not vacuum.
So what plug is best for high speed? You have to test it yourself.
Find a consistent location you can test at and try to allow for conditions (temp, humidity, pressure).
Ideally your location would be a slight hill to test power under load and another steep hill also.
You need a consistent measuring tool, speedometer, tachometer, or time between points.
You cannot master what you cannot measure.
Steve