[MikeK]

Originally Posted by Mad Mac

Why?

I am about to do a compression test and don't understand why the throttle must be wide open.

And do I shut the fuel valve off first? If not, am I not just introducing a lot of raw fuel into the cylinders and diluting the oil?

And why must the engine be hot?

You open the throttle because you want to measure compression vs. atmospheric pressure. Cranking with the throttle closed will present a partial vacuum, yielding an erroneously low reading.

If the throttle is wide open, cranking will not provide sufficient flow velocity through the carb venturi to draw fuel. With the throttle closed, cranking does draw raw fuel from the idle port. The idle port delivers nothing when the butterfly is open.

Hot gives you a better indication of how the rings perform when heated and expanded to their normal operating position. They almost always leak more when cold. Since you run hot, that's the compression reading that matters most.