comma or no comma before 'because'

Hi,

I know many people who did badly on the test because they didn't have enough time to finish it. 

I know that the semantics of the sentence changes with the use of a comma, but not sure if the sentence is correct according to what I'm trying to say.

Well, first you need to tell us what you are trying to say.

Clive

Hi,

I know many people who did badly on the test because they didn't have enough time to finish it. 

I'm saying that I know people who did badly on the test, and the reason that they did so was because they did not have enough time. 

You don't need a comma.

Best wishes, Clive